深浅模式
AbstractQueuedSynchronizer
更新: 1/28/2026 字数: 0 字
过程
final int acquire()方法
参数(Node node, int arg, boolean shared, boolean interruptible, boolean timed, long time)
node 当前线程对应的队列节点（已经入队）
arg 要获取的资源量（ReentrantLock 永远是 1）
shared 是否共享模式
- false → ReentrantLock
- true → Semaphore / CountDownLatch
interruptible 等待时是否响应中断
- lock() → false
- lockInterruptibly() → true
timed + time 是否限时
- tryLock(time) 才会用
这 5 个参数，本质上只决定三件事：
用 tryAcquire 还是 tryAcquireShared
park 时要不要检查中断
park 多久
java
for (;;) {
// 1. 判断有没有资格尝试获取
// 2. 尝试获取（tryAcquire / tryAcquireShared）
// 3. 成功 → 出队，return
// 4. 失败 → 决定要不要 park
// 5. park / parkNanos
// 6. 醒来继续循环
}text
入队
↓
是不是 head 后继？
↓ 否
是 → tryAcquire → 成功 → 出队
↓
失败
↓
前驱设为 SIGNAL
↓
park
↓
被唤醒
↓
回到循环版本对比
一、始终不变的"灵魂"（核心原理）
无论在哪个版本，AQS 的核心架构永远由这三根支柱组成：
- 原子状态（State）：使用一个
volatile int state来表示资源状态（如锁的占用次数）。 - 双向链表队列（CLH Queue）：当线程获取资源失败时，将其包装成节点放入队列中。
- 自旋 + 阻塞（Park/Unpark）：线程在队列中通过自旋观察前驱状态，并在合适的时候阻塞，等待被唤醒。
二、发生剧变的"肉体"（实现差异）
在 Java 21 中，Doug Lea 对 AQS 进行了重构（主要在 Java 9 - 17 期间完成），目的是适配现代多核 CPU 和更高并发的需求。
1. 节点定义与状态处理
- Java 8：使用 Node 内部类，定义了明确的常量 SIGNAL, CANCELLED 等。判断状态主要靠
if (ws == Node.SIGNAL)。 - Java 21：引入了更抽象的 Node 实现，大量使用位掩码（Bitmask）。比如你之前看到的
status < 0，它不再只是一个简单的常量，而是通过位运算来快速判定多个状态，执行效率更高。
2. 指针操作的底层工具
- Java 8：全篇充斥着
unsafe.compareAndSwapInt。这直接操作内存偏移量，虽然快，但不够优雅且维护困难。 - Java 21：切换到了 VarHandle。这是 Java 9 引入的增强版变量句柄，它比 Unsafe 更安全，且支持更细粒度的内存语义（如 acquire/release 语义），这让 AQS 在现代架构 CPU 上能更好地处理内存屏障。
3. 队列清理机制（Scrubbing）
- Java 8：取消节点（Cancelled）的清理是"被动"且"点对点"的。通常是在排队过程中顺便跳过前驱的取消节点。
- Java 21：引入了更强大的
cleanQueue等方法。它不仅在入队时清理，还会在检测到竞争或特定状态时，更彻底地"擦除"掉链表中的废弃节点。这有效缓解了在高并发、频繁中断场景下的内存占用问题。
4. 线程唤醒的控制
Java 21 的实现更加"激进"。它减少了不必要的 CAS 操作。比如在处理 head 指针移动时，新版代码通过更精妙的逻辑确保了在极高并发下，唤醒动作（Signal）能更准确地传导，减少了线程"无谓醒来"的次数。
源码
AbstractQueuedSynchronizer
java
package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import sun.misc.Unsafe;
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
/**
* Creates a new {@code AbstractQueuedSynchronizer} instance
* with initial synchronization state of zero.
*/
protected AbstractQueuedSynchronizer() { }
/**
* Wait queue node class.
*
* <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
* Hagersten) lock queue. CLH locks are normally used for
* spinlocks. We instead use them for blocking synchronizers, but
* use the same basic tactic of holding some of the control
* information about a thread in the predecessor of its node. A
* "status" field in each node keeps track of whether a thread
* should block. A node is signalled when its predecessor
* releases. Each node of the queue otherwise serves as a
* specific-notification-style monitor holding a single waiting
* thread. The status field does NOT control whether threads are
* granted locks etc though. A thread may try to acquire if it is
* first in the queue. But being first does not guarantee success;
* it only gives the right to contend. So the currently released
* contender thread may need to rewait.
*
* <p>To enqueue into a CLH lock, you atomically splice it in as new
* tail. To dequeue, you just set the head field.
* <pre>
* +------+ prev +-----+ +-----+
* head | | <---- | | <---- | | tail
* +------+ +-----+ +-----+
* </pre>
*
* <p>Insertion into a CLH queue requires only a single atomic
* operation on "tail", so there is a simple atomic point of
* demarcation from unqueued to queued. Similarly, dequeuing
* involves only updating the "head". However, it takes a bit
* more work for nodes to determine who their successors are,
* in part to deal with possible cancellation due to timeouts
* and interrupts.
*
* <p>The "prev" links (not used in original CLH locks), are mainly
* needed to handle cancellation. If a node is cancelled, its
* successor is (normally) relinked to a non-cancelled
* predecessor. For explanation of similar mechanics in the case
* of spin locks, see the papers by Scott and Scherer at
* http://www.cs.rochester.edu/u/scott/synchronization/
*
* <p>We also use "next" links to implement blocking mechanics.
* The thread id for each node is kept in its own node, so a
* predecessor signals the next node to wake up by traversing
* next link to determine which thread it is. Determination of
* successor must avoid races with newly queued nodes to set
* the "next" fields of their predecessors. This is solved
* when necessary by checking backwards from the atomically
* updated "tail" when a node's successor appears to be null.
* (Or, said differently, the next-links are an optimization
* so that we don't usually need a backward scan.)
*
* <p>Cancellation introduces some conservatism to the basic
* algorithms. Since we must poll for cancellation of other
* nodes, we can miss noticing whether a cancelled node is
* ahead or behind us. This is dealt with by always unparking
* successors upon cancellation, allowing them to stabilize on
* a new predecessor, unless we can identify an uncancelled
* predecessor who will carry this responsibility.
*
* <p>CLH queues need a dummy header node to get started. But
* we don't create them on construction, because it would be wasted
* effort if there is never contention. Instead, the node
* is constructed and head and tail pointers are set upon first
* contention.
*
* <p>Threads waiting on Conditions use the same nodes, but
* use an additional link. Conditions only need to link nodes
* in simple (non-concurrent) linked queues because they are
* only accessed when exclusively held. Upon await, a node is
* inserted into a condition queue. Upon signal, the node is
* transferred to the main queue. A special value of status
* field is used to mark which queue a node is on.
*
* <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
* Scherer and Michael Scott, along with members of JSR-166
* expert group, for helpful ideas, discussions, and critiques
* on the design of this class.
*/
static final class Node {
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueuing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() { // Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
/**
* Head of the wait queue, lazily initialized. Except for
* initialization, it is modified only via method setHead. Note:
* If head exists, its waitStatus is guaranteed not to be
* CANCELLED.
*/
private transient volatile Node head;
/**
* Tail of the wait queue, lazily initialized. Modified only via
* method enq to add new wait node.
*/
private transient volatile Node tail;
/**
* The synchronization state.
*/
private volatile int state;
/**
* Returns the current value of synchronization state.
* This operation has memory semantics of a {@code volatile} read.
* @return current state value
*/
protected final int getState() {
return state;
}
/**
* Sets the value of synchronization state.
* This operation has memory semantics of a {@code volatile} write.
* @param newState the new state value
*/
protected final void setState(int newState) {
state = newState;
}
/**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a {@code volatile} read
* and write.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that the actual
* value was not equal to the expected value.
*/
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
// Queuing utilities
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices
* to improve responsiveness with very short timeouts.
*/
static final long spinForTimeoutThreshold = 1000L;
/**
* Inserts node into queue, initializing if necessary. See picture above.
* @param node the node to insert
* @return node's predecessor
*/
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
/**
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
/**
* Sets head of queue to be node, thus dequeuing. Called only by
* acquire methods. Also nulls out unused fields for sake of GC
* and to suppress unnecessary signals and traversals.
*
* @param node the node
*/
private void setHead(Node node) {
head = node;
node.thread = null;
node.prev = null;
}
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}
/**
* Release action for shared mode -- signals successor and ensures
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
*/
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
/**
* Sets head of queue, and checks if successor may be waiting
* in shared mode, if so propagating if either propagate > 0 or
* PROPAGATE status was set.
*
* @param node the node
* @param propagate the return value from a tryAcquireShared
*/
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus either before
* or after setHead) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*/
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared();
}
}
// Utilities for various versions of acquire
/**
* Cancels an ongoing attempt to acquire.
*
* @param node the node
*/
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
if (node == null)
return;
node.thread = null;
// Skip cancelled predecessors
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
// predNext is the apparent node to unsplice. CASes below will
// fail if not, in which case, we lost race vs another cancel
// or signal, so no further action is necessary.
Node predNext = pred.next;
// Can use unconditional write instead of CAS here.
// After this atomic step, other Nodes can skip past us.
// Before, we are free of interference from other threads.
node.waitStatus = Node.CANCELLED;
// If we are the tail, remove ourselves.
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
int ws;
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL ||
(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
/**
* Convenience method to interrupt current thread.
*/
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
/*
* Various flavors of acquire, varying in exclusive/shared and
* control modes. Each is mostly the same, but annoyingly
* different. Only a little bit of factoring is possible due to
* interactions of exception mechanics (including ensuring that we
* cancel if tryAcquire throws exception) and other control, at
* least not without hurting performance too much.
*/
/**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
*
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in exclusive interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in exclusive timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return true;
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared uninterruptible mode.
* @param arg the acquire argument
*/
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return true;
}
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
// Main exported methods
/**
* Attempts to acquire in exclusive mode. This method should query
* if the state of the object permits it to be acquired in the
* exclusive mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread. This can be used
* to implement method {@link Lock#tryLock()}.
*
* <p>The default
* implementation throws {@link UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return {@code true} if successful. Upon success, this object has
* been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in exclusive
* mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this object is now in a fully released
* state, so that any waiting threads may attempt to acquire;
* and {@code false} otherwise.
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to acquire in shared mode. This method should query if
* the state of the object permits it to be acquired in the shared
* mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread.
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return a negative value on failure; zero if acquisition in shared
* mode succeeded but no subsequent shared-mode acquire can
* succeed; and a positive value if acquisition in shared
* mode succeeded and subsequent shared-mode acquires might
* also succeed, in which case a subsequent waiting thread
* must check availability. (Support for three different
* return values enables this method to be used in contexts
* where acquires only sometimes act exclusively.) Upon
* success, this object has been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in shared mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this release of shared mode may permit a
* waiting acquire (shared or exclusive) to succeed; and
* {@code false} otherwise
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Returns {@code true} if synchronization is held exclusively with
* respect to the current (calling) thread. This method is invoked
* upon each call to a non-waiting {@link ConditionObject} method.
* (Waiting methods instead invoke {@link #release}.)
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}. This method is invoked
* internally only within {@link ConditionObject} methods, so need
* not be defined if conditions are not used.
*
* @return {@code true} if synchronization is held exclusively;
* {@code false} otherwise
* @throws UnsupportedOperationException if conditions are not supported
*/
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
}
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
/**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
}
/**
* Attempts to acquire in exclusive mode, aborting if interrupted,
* and failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquire}, returning on success. Otherwise, the thread is
* queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquire} until success or the thread is interrupted
* or the timeout elapses. This method can be used to implement
* method {@link Lock#tryLock(long, TimeUnit)}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquire(arg) ||
doAcquireNanos(arg, nanosTimeout);
}
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
/**
* Acquires in shared mode, ignoring interrupts. Implemented by
* first invoking at least once {@link #tryAcquireShared},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquireShared} until success.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
*/
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
/**
* Acquires in shared mode, aborting if interrupted. Implemented
* by first checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted.
* @param arg the acquire argument.
* This value is conveyed to {@link #tryAcquireShared} but is
* otherwise uninterpreted and can represent anything
* you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
/**
* Attempts to acquire in shared mode, aborting if interrupted, and
* failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquireShared}, returning on success. Otherwise, the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted or the timeout elapses.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
/**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
// Queue inspection methods
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations due to interrupts and timeouts may occur
* at any time, a {@code true} return does not guarantee that any
* other thread will ever acquire.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there may be other threads waiting to acquire
*/
public final boolean hasQueuedThreads() {
return head != tail;
}
/**
* Queries whether any threads have ever contended to acquire this
* synchronizer; that is if an acquire method has ever blocked.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there has ever been contention
*/
public final boolean hasContended() {
return head != null;
}
/**
* Returns the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued.
*
* <p>In this implementation, this operation normally returns in
* constant time, but may iterate upon contention if other threads are
* concurrently modifying the queue.
*
* @return the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued
*/
public final Thread getFirstQueuedThread() {
// handle only fast path, else relay
return (head == tail) ? null : fullGetFirstQueuedThread();
}
/**
* Version of getFirstQueuedThread called when fastpath fails
*/
private Thread fullGetFirstQueuedThread() {
/*
* The first node is normally head.next. Try to get its
* thread field, ensuring consistent reads: If thread
* field is nulled out or s.prev is no longer head, then
* some other thread(s) concurrently performed setHead in
* between some of our reads. We try this twice before
* resorting to traversal.
*/
Node h, s;
Thread st;
if (((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null) ||
((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null))
return st;
/*
* Head's next field might not have been set yet, or may have
* been unset after setHead. So we must check to see if tail
* is actually first node. If not, we continue on, safely
* traversing from tail back to head to find first,
* guaranteeing termination.
*/
Node t = tail;
Thread firstThread = null;
while (t != null && t != head) {
Thread tt = t.thread;
if (tt != null)
firstThread = tt;
t = t.prev;
}
return firstThread;
}
/**
* Returns true if the given thread is currently queued.
*
* <p>This implementation traverses the queue to determine
* presence of the given thread.
*
* @param thread the thread
* @return {@code true} if the given thread is on the queue
* @throws NullPointerException if the thread is null
*/
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
return true;
return false;
}
/**
* Returns {@code true} if the apparent first queued thread, if one
* exists, is waiting in exclusive mode. If this method returns
* {@code true}, and the current thread is attempting to acquire in
* shared mode (that is, this method is invoked from {@link
* #tryAcquireShared}) then it is guaranteed that the current thread
* is not the first queued thread. Used only as a heuristic in
* ReentrantReadWriteLock.
*/
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
return (h = head) != null &&
(s = h.next) != null &&
!s.isShared() &&
s.thread != null;
}
/**
* Queries whether any threads have been waiting to acquire longer
* than the current thread.
*
* <p>An invocation of this method is equivalent to (but may be
* more efficient than):
* <pre> {@code
* getFirstQueuedThread() != Thread.currentThread() &&
* hasQueuedThreads()}</pre>
*
* <p>Note that because cancellations due to interrupts and
* timeouts may occur at any time, a {@code true} return does not
* guarantee that some other thread will acquire before the current
* thread. Likewise, it is possible for another thread to win a
* race to enqueue after this method has returned {@code false},
* due to the queue being empty.
*
* <p>This method is designed to be used by a fair synchronizer to
* avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
* Such a synchronizer's {@link #tryAcquire} method should return
* {@code false}, and its {@link #tryAcquireShared} method should
* return a negative value, if this method returns {@code true}
* (unless this is a reentrant acquire). For example, the {@code
* tryAcquire} method for a fair, reentrant, exclusive mode
* synchronizer might look like this:
*
* <pre> {@code
* protected boolean tryAcquire(int arg) {
* if (isHeldExclusively()) {
* // A reentrant acquire; increment hold count
* return true;
* } else if (hasQueuedPredecessors()) {
* return false;
* } else {
* // try to acquire normally
* }
* }}</pre>
*
* @return {@code true} if there is a queued thread preceding the
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty
* @since 1.7
*/
public final boolean hasQueuedPredecessors() {
// The correctness of this depends on head being initialized
// before tail and on head.next being accurate if the current
// thread is first in queue.
Node t = tail; // Read fields in reverse initialization order
Node h = head;
Node s;
return h != t &&
((s = h.next) == null || s.thread != Thread.currentThread());
}
// Instrumentation and monitoring methods
/**
* Returns an estimate of the number of threads waiting to
* acquire. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting to acquire
*/
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.thread != null)
++n;
}
return n;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
public final Collection<Thread> getQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
return list;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in exclusive mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to an exclusive acquire.
*
* @return the collection of threads
*/
public final Collection<Thread> getExclusiveQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (!p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in shared mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to a shared acquire.
*
* @return the collection of threads
*/
public final Collection<Thread> getSharedQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* Returns a string identifying this synchronizer, as well as its state.
* The state, in brackets, includes the String {@code "State ="}
* followed by the current value of {@link #getState}, and either
* {@code "nonempty"} or {@code "empty"} depending on whether the
* queue is empty.
*
* @return a string identifying this synchronizer, as well as its state
*/
public String toString() {
int s = getState();
String q = hasQueuedThreads() ? "non" : "";
return super.toString() +
"[State = " + s + ", " + q + "empty queue]";
}
// Internal support methods for Conditions
/**
* Returns true if a node, always one that was initially placed on
* a condition queue, is now waiting to reacquire on sync queue.
* @param node the node
* @return true if is reacquiring
*/
final boolean isOnSyncQueue(Node node) {
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
if (node.next != null) // If has successor, it must be on queue
return true;
/*
* node.prev can be non-null, but not yet on queue because
* the CAS to place it on queue can fail. So we have to
* traverse from tail to make sure it actually made it. It
* will always be near the tail in calls to this method, and
* unless the CAS failed (which is unlikely), it will be
* there, so we hardly ever traverse much.
*/
return findNodeFromTail(node);
}
/**
* Returns true if node is on sync queue by searching backwards from tail.
* Called only when needed by isOnSyncQueue.
* @return true if present
*/
private boolean findNodeFromTail(Node node) {
Node t = tail;
for (;;) {
if (t == node)
return true;
if (t == null)
return false;
t = t.prev;
}
}
/**
* Transfers a node from a condition queue onto sync queue.
* Returns true if successful.
* @param node the node
* @return true if successfully transferred (else the node was
* cancelled before signal)
*/
final boolean transferForSignal(Node node) {
/*
* If cannot change waitStatus, the node has been cancelled.
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false;
/*
* Splice onto queue and try to set waitStatus of predecessor to
* indicate that thread is (probably) waiting. If cancelled or
* attempt to set waitStatus fails, wake up to resync (in which
* case the waitStatus can be transiently and harmlessly wrong).
*/
Node p = enq(node);
int ws = p.waitStatus;
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
LockSupport.unpark(node.thread);
return true;
}
/**
* Transfers node, if necessary, to sync queue after a cancelled wait.
* Returns true if thread was cancelled before being signalled.
*
* @param node the node
* @return true if cancelled before the node was signalled
*/
final boolean transferAfterCancelledWait(Node node) {
if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
enq(node);
return true;
}
/*
* If we lost out to a signal(), then we can't proceed
* until it finishes its enq(). Cancelling during an
* incomplete transfer is both rare and transient, so just
* spin.
*/
while (!isOnSyncQueue(node))
Thread.yield();
return false;
}
/**
* Invokes release with current state value; returns saved state.
* Cancels node and throws exception on failure.
* @param node the condition node for this wait
* @return previous sync state
*/
final int fullyRelease(Node node) {
boolean failed = true;
try {
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
node.waitStatus = Node.CANCELLED;
}
}
// Instrumentation methods for conditions
/**
* Queries whether the given ConditionObject
* uses this synchronizer as its lock.
*
* @param condition the condition
* @return {@code true} if owned
* @throws NullPointerException if the condition is null
*/
public final boolean owns(ConditionObject condition) {
return condition.isOwnedBy(this);
}
/**
* Queries whether any threads are waiting on the given condition
* associated with this synchronizer. Note that because timeouts
* and interrupts may occur at any time, a {@code true} return
* does not guarantee that a future {@code signal} will awaken
* any threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final boolean hasWaiters(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.hasWaiters();
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this synchronizer. Note that
* because timeouts and interrupts may occur at any time, the
* estimate serves only as an upper bound on the actual number of
* waiters. This method is designed for use in monitoring of the
* system state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final int getWaitQueueLength(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitQueueLength();
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this
* synchronizer. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitingThreads();
}
/**
* Condition implementation for a {@link
* AbstractQueuedSynchronizer} serving as the basis of a {@link
* Lock} implementation.
*
* <p>Method documentation for this class describes mechanics,
* not behavioral specifications from the point of view of Lock
* and Condition users. Exported versions of this class will in
* general need to be accompanied by documentation describing
* condition semantics that rely on those of the associated
* {@code AbstractQueuedSynchronizer}.
*
* <p>This class is Serializable, but all fields are transient,
* so deserialized conditions have no waiters.
*/
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
/** First node of condition queue. */
private transient Node firstWaiter;
/** Last node of condition queue. */
private transient Node lastWaiter;
/**
* Creates a new {@code ConditionObject} instance.
*/
public ConditionObject() { }
// Internal methods
/**
* Adds a new waiter to wait queue.
* @return its new wait node
*/
private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
/**
* Removes and transfers nodes until hit non-cancelled one or
* null. Split out from signal in part to encourage compilers
* to inline the case of no waiters.
* @param first (non-null) the first node on condition queue
*/
private void doSignal(Node first) {
do {
if ( (firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);
}
/**
* Removes and transfers all nodes.
* @param first (non-null) the first node on condition queue
*/
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
Node next = first.nextWaiter;
first.nextWaiter = null;
transferForSignal(first);
first = next;
} while (first != null);
}
/**
* Unlinks cancelled waiter nodes from condition queue.
* Called only while holding lock. This is called when
* cancellation occurred during condition wait, and upon
* insertion of a new waiter when lastWaiter is seen to have
* been cancelled. This method is needed to avoid garbage
* retention in the absence of signals. So even though it may
* require a full traversal, it comes into play only when
* timeouts or cancellations occur in the absence of
* signals. It traverses all nodes rather than stopping at a
* particular target to unlink all pointers to garbage nodes
* without requiring many re-traversals during cancellation
* storms.
*/
private void unlinkCancelledWaiters() {
Node t = firstWaiter;
Node trail = null;
while (t != null) {
Node next = t.nextWaiter;
if (t.waitStatus != Node.CONDITION) {
t.nextWaiter = null;
if (trail == null)
firstWaiter = next;
else
trail.nextWaiter = next;
if (next == null)
lastWaiter = trail;
}
else
trail = t;
t = next;
}
}
// public methods
/**
* Moves the longest-waiting thread, if one exists, from the
* wait queue for this condition to the wait queue for the
* owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
public final void signal() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first);
}
/**
* Moves all threads from the wait queue for this condition to
* the wait queue for the owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
public final void signalAll() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignalAll(first);
}
/**
* Implements uninterruptible condition wait.
* <ol>
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* </ol>
*/
public final void awaitUninterruptibly() {
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean interrupted = false;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if (Thread.interrupted())
interrupted = true;
}
if (acquireQueued(node, savedState) || interrupted)
selfInterrupt();
}
/*
* For interruptible waits, we need to track whether to throw
* InterruptedException, if interrupted while blocked on
* condition, versus reinterrupt current thread, if
* interrupted while blocked waiting to re-acquire.
*/
/** Mode meaning to reinterrupt on exit from wait */
private static final int REINTERRUPT = 1;
/** Mode meaning to throw InterruptedException on exit from wait */
private static final int THROW_IE = -1;
/**
* Checks for interrupt, returning THROW_IE if interrupted
* before signalled, REINTERRUPT if after signalled, or
* 0 if not interrupted.
*/
private int checkInterruptWhileWaiting(Node node) {
return Thread.interrupted() ?
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
0;
}
/**
* Throws InterruptedException, reinterrupts current thread, or
* does nothing, depending on mode.
*/
private void reportInterruptAfterWait(int interruptMode)
throws InterruptedException {
if (interruptMode == THROW_IE)
throw new InterruptedException();
else if (interruptMode == REINTERRUPT)
selfInterrupt();
}
/**
* Implements interruptible condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled or interrupted.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final long awaitNanos(long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return deadline - System.nanoTime();
}
/**
* Implements absolute timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean awaitUntil(Date deadline)
throws InterruptedException {
long abstime = deadline.getTime();
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (System.currentTimeMillis() > abstime) {
timedout = transferAfterCancelledWait(node);
break;
}
LockSupport.parkUntil(this, abstime);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean await(long time, TimeUnit unit)
throws InterruptedException {
long nanosTimeout = unit.toNanos(time);
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
timedout = transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
// support for instrumentation
/**
* Returns true if this condition was created by the given
* synchronization object.
*
* @return {@code true} if owned
*/
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
return sync == AbstractQueuedSynchronizer.this;
}
/**
* Queries whether any threads are waiting on this condition.
* Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
*
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final boolean hasWaiters() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
return true;
}
return false;
}
/**
* Returns an estimate of the number of threads waiting on
* this condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
*
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final int getWaitQueueLength() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int n = 0;
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
++n;
}
return n;
}
/**
* Returns a collection containing those threads that may be
* waiting on this Condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
*
* @return the collection of threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final Collection<Thread> getWaitingThreads() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION) {
Thread t = w.thread;
if (t != null)
list.add(t);
}
}
return list;
}
}
/**
* Setup to support compareAndSet. We need to natively implement
* this here: For the sake of permitting future enhancements, we
* cannot explicitly subclass AtomicInteger, which would be
* efficient and useful otherwise. So, as the lesser of evils, we
* natively implement using hotspot intrinsics API. And while we
* are at it, we do the same for other CASable fields (which could
* otherwise be done with atomic field updaters).
*/
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long stateOffset;
private static final long headOffset;
private static final long tailOffset;
private static final long waitStatusOffset;
private static final long nextOffset;
static {
try {
stateOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
headOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
tailOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
waitStatusOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("waitStatus"));
nextOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("next"));
} catch (Exception ex) { throw new Error(ex); }
}
/**
* CAS head field. Used only by enq.
*/
private final boolean compareAndSetHead(Node update) {
return unsafe.compareAndSwapObject(this, headOffset, null, update);
}
/**
* CAS tail field. Used only by enq.
*/
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}
/**
* CAS waitStatus field of a node.
*/
private static final boolean compareAndSetWaitStatus(Node node,
int expect,
int update) {
return unsafe.compareAndSwapInt(node, waitStatusOffset,
expect, update);
}
/**
* CAS next field of a node.
*/
private static final boolean compareAndSetNext(Node node,
Node expect,
Node update) {
return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
}
}java
package java.util.concurrent.locks;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RejectedExecutionException;
import jdk.internal.misc.Unsafe;
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
/**
* Creates a new {@code AbstractQueuedSynchronizer} instance
* with initial synchronization state of zero.
*/
protected AbstractQueuedSynchronizer() { }
/*
* Overview.
*
* The wait queue is a variant of a "CLH" (Craig, Landin, and
* Hagersten) lock queue. CLH locks are normally used for
* spinlocks. We instead use them for blocking synchronizers by
* including explicit ("prev" and "next") links plus a "status"
* field that allow nodes to signal successors when releasing
* locks, and handle cancellation due to interrupts and timeouts.
* The status field includes bits that track whether a thread
* needs a signal (using LockSupport.unpark). Despite these
* additions, we maintain most CLH locality properties.
*
* To enqueue into a CLH lock, you atomically splice it in as new
* tail. To dequeue, you set the head field, so the next eligible
* waiter becomes first.
*
* +------+ prev +-------+ +------+
* | head | <---- | first | <---- | tail |
* +------+ +-------+ +------+
*
* Insertion into a CLH queue requires only a single atomic
* operation on "tail", so there is a simple point of demarcation
* from unqueued to queued. The "next" link of the predecessor is
* set by the enqueuing thread after successful CAS. Even though
* non-atomic, this suffices to ensure that any blocked thread is
* signalled by a predecessor when eligible (although in the case
* of cancellation, possibly with the assistance of a signal in
* method cleanQueue). Signalling is based in part on a
* Dekker-like scheme in which the to-be waiting thread indicates
* WAITING status, then retries acquiring, and then rechecks
* status before blocking. The signaller atomically clears WAITING
* status when unparking.
*
* Dequeuing on acquire involves detaching (nulling) a node's
* "prev" node and then updating the "head". Other threads check
* if a node is or was dequeued by checking "prev" rather than
* head. We enforce the nulling then setting order by spin-waiting
* if necessary. Because of this, the lock algorithm is not itself
* strictly "lock-free" because an acquiring thread may need to
* wait for a previous acquire to make progress. When used with
* exclusive locks, such progress is required anyway. However
* Shared mode may (uncommonly) require a spin-wait before
* setting head field to ensure proper propagation. (Historical
* note: This allows some simplifications and efficiencies
* compared to previous versions of this class.)
*
* A node's predecessor can change due to cancellation while it is
* waiting, until the node is first in queue, at which point it
* cannot change. The acquire methods cope with this by rechecking
* "prev" before waiting. The prev and next fields are modified
* only via CAS by cancelled nodes in method cleanQueue. The
* unsplice strategy is reminiscent of Michael-Scott queues in
* that after a successful CAS to prev field, other threads help
* fix next fields. Because cancellation often occurs in bunches
* that complicate decisions about necessary signals, each call to
* cleanQueue traverses the queue until a clean sweep. Nodes that
* become relinked as first are unconditionally unparked
* (sometimes unnecessarily, but those cases are not worth
* avoiding).
*
* A thread may try to acquire if it is first (frontmost) in the
* queue, and sometimes before. Being first does not guarantee
* success; it only gives the right to contend. We balance
* throughput, overhead, and fairness by allowing incoming threads
* to "barge" and acquire the synchronizer while in the process of
* enqueuing, in which case an awakened first thread may need to
* rewait. To counteract possible repeated unlucky rewaits, we
* exponentially increase retries (up to 256) to acquire each time
* a thread is unparked. Except in this case, AQS locks do not
* spin; they instead interleave attempts to acquire with
* bookkeeping steps. (Users who want spinlocks can use
* tryAcquire.)
*
* To improve garbage collectibility, fields of nodes not yet on
* list are null. (It is not rare to create and then throw away a
* node without using it.) Fields of nodes coming off the list are
* nulled out as soon as possible. This accentuates the challenge
* of externally determining the first waiting thread (as in
* method getFirstQueuedThread). This sometimes requires the
* fallback of traversing backwards from the atomically updated
* "tail" when fields appear null. (This is never needed in the
* process of signalling though.)
*
* CLH queues need a dummy header node to get started. But
* we don't create them on construction, because it would be wasted
* effort if there is never contention. Instead, the node
* is constructed and head and tail pointers are set upon first
* contention.
*
* Shared mode operations differ from Exclusive in that an acquire
* signals the next waiter to try to acquire if it is also
* Shared. The tryAcquireShared API allows users to indicate the
* degree of propagation, but in most applications, it is more
* efficient to ignore this, allowing the successor to try
* acquiring in any case.
*
* Threads waiting on Conditions use nodes with an additional
* link to maintain the (FIFO) list of conditions. Conditions only
* need to link nodes in simple (non-concurrent) linked queues
* because they are only accessed when exclusively held. Upon
* await, a node is inserted into a condition queue. Upon signal,
* the node is enqueued on the main queue. A special status field
* value is used to track and atomically trigger this.
*
* Accesses to fields head, tail, and state use full Volatile
* mode, along with CAS. Node fields status, prev and next also do
* so while threads may be signallable, but sometimes use weaker
* modes otherwise. Accesses to field "waiter" (the thread to be
* signalled) are always sandwiched between other atomic accesses
* so are used in Plain mode. We use jdk.internal Unsafe versions
* of atomic access methods rather than VarHandles to avoid
* potential VM bootstrap issues.
*
* Most of the above is performed by primary internal method
* acquire, that is invoked in some way by all exported acquire
* methods. (It is usually easy for compilers to optimize
* call-site specializations when heavily used.)
*
* Most AQS methods may be called by JDK components that cannot be
* allowed to fail when encountering OutOfMemoryErrors. The main
* acquire method resorts to spin-waits with backoff if nodes
* cannot be allocated. Condition waits release and reacquire
* locks upon OOME at a slow fixed rate (OOME_COND_WAIT_DELAY)
* designed with the hope that eventually enough memory will be
* recovered; if not performance can be very slow. Effectiveness
* is also limited by the possibility of class loading triggered
* by first-time usages, that may encounter unrecoverable
* OOMEs. Also, it is possible for OutOfMemoryErrors to be thrown
* when attempting to create and throw
* IllegalMonitorStateExceptions and InterruptedExceptions.
*
* There are several arbitrary decisions about when and how to
* check interrupts in both acquire and await before and/or after
* blocking. The decisions are less arbitrary in implementation
* updates because some users appear to rely on original behaviors
* in ways that are racy and so (rarely) wrong in general but hard
* to justify changing.
*
* Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
* Scherer and Michael Scott, along with members of JSR-166
* expert group, for helpful ideas, discussions, and critiques
* on the design of this class.
*/
// Node status bits, also used as argument and return values
static final int WAITING = 1; // must be 1
static final int CANCELLED = 0x80000000; // must be negative
static final int COND = 2; // in a condition wait
/** CLH Nodes */
abstract static class Node {
volatile Node prev; // initially attached via casTail
volatile Node next; // visibly nonnull when signallable
Thread waiter; // visibly nonnull when enqueued
volatile int status; // written by owner, atomic bit ops by others
// methods for atomic operations
final boolean casPrev(Node c, Node v) { // for cleanQueue
return U.weakCompareAndSetReference(this, PREV, c, v);
}
final boolean casNext(Node c, Node v) { // for cleanQueue
return U.weakCompareAndSetReference(this, NEXT, c, v);
}
final int getAndUnsetStatus(int v) { // for signalling
return U.getAndBitwiseAndInt(this, STATUS, ~v);
}
final void setPrevRelaxed(Node p) { // for off-queue assignment
U.putReference(this, PREV, p);
}
final void setStatusRelaxed(int s) { // for off-queue assignment
U.putInt(this, STATUS, s);
}
final void clearStatus() { // for reducing unneeded signals
U.putIntOpaque(this, STATUS, 0);
}
private static final long STATUS
= U.objectFieldOffset(Node.class, "status");
private static final long NEXT
= U.objectFieldOffset(Node.class, "next");
private static final long PREV
= U.objectFieldOffset(Node.class, "prev");
}
// Concrete classes tagged by type
static final class ExclusiveNode extends Node { }
static final class SharedNode extends Node { }
static final class ConditionNode extends Node
implements ForkJoinPool.ManagedBlocker {
ConditionNode nextWaiter; // link to next waiting node
/**
* Allows Conditions to be used in ForkJoinPools without
* risking fixed pool exhaustion. This is usable only for
* untimed Condition waits, not timed versions.
*/
public final boolean isReleasable() {
return status <= 1 || Thread.currentThread().isInterrupted();
}
public final boolean block() {
while (!isReleasable()) LockSupport.park();
return true;
}
}
/**
* Head of the wait queue, lazily initialized.
*/
private transient volatile Node head;
/**
* Tail of the wait queue. After initialization, modified only via casTail.
*/
private transient volatile Node tail;
/**
* @serial The synchronization state.
*/
private volatile int state;
/**
* Returns the current value of synchronization state.
* This operation has memory semantics of a {@code volatile} read.
* @return current state value
*/
protected final int getState() {
return state;
}
/**
* Sets the value of synchronization state.
* This operation has memory semantics of a {@code volatile} write.
* @param newState the new state value
*/
protected final void setState(int newState) {
state = newState;
}
/**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a {@code volatile} read
* and write.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that the actual
* value was not equal to the expected value.
*/
protected final boolean compareAndSetState(int expect, int update) {
return U.compareAndSetInt(this, STATE, expect, update);
}
// Queuing utilities
private boolean casTail(Node c, Node v) {
return U.compareAndSetReference(this, TAIL, c, v);
}
/**
* Tries to CAS a new dummy node for head.
* Returns new tail, or null if OutOfMemory
*/
private Node tryInitializeHead() {
for (Node h = null, t;;) {
if ((t = tail) != null)
return t;
else if (head != null)
Thread.onSpinWait();
else {
if (h == null) {
try {
h = new ExclusiveNode();
} catch (OutOfMemoryError oome) {
return null;
}
}
if (U.compareAndSetReference(this, HEAD, null, h))
return tail = h;
}
}
}
/**
* Enqueues the node unless null. (Currently used only for
* ConditionNodes; other cases are interleaved with acquires.)
*/
final void enqueue(ConditionNode node) {
if (node != null) {
boolean unpark = false;
for (Node t;;) {
if ((t = tail) == null && (t = tryInitializeHead()) == null) {
unpark = true; // wake up to spin on OOME
break;
}
node.setPrevRelaxed(t); // avoid unnecessary fence
if (casTail(t, node)) {
t.next = node;
if (t.status < 0) // wake up to clean link
unpark = true;
break;
}
}
if (unpark)
LockSupport.unpark(node.waiter);
}
}
/** Returns true if node is found in traversal from tail */
final boolean isEnqueued(Node node) {
for (Node t = tail; t != null; t = t.prev)
if (t == node)
return true;
return false;
}
/**
* Wakes up the successor of given node, if one exists, and unsets its
* WAITING status to avoid park race. This may fail to wake up an
* eligible thread when one or more have been cancelled, but
* cancelAcquire ensures liveness.
*/
private static void signalNext(Node h) {
Node s;
if (h != null && (s = h.next) != null && s.status != 0) {
s.getAndUnsetStatus(WAITING);
LockSupport.unpark(s.waiter);
}
}
/** Wakes up the given node if in shared mode */
private static void signalNextIfShared(Node h) {
Node s;
if (h != null && (s = h.next) != null &&
(s instanceof SharedNode) && s.status != 0) {
s.getAndUnsetStatus(WAITING);
LockSupport.unpark(s.waiter);
}
}
/**
* Repeatedly invokes acquire, if its execution throws an Error or a Runtime Exception,
* using an Unsafe.park-based backoff
* @param node which to reacquire
* @param arg the acquire argument
*/
private final void reacquire(Node node, int arg) {
try {
acquire(node, arg, false, false, false, 0L);
} catch (Error | RuntimeException firstEx) {
// While we currently do not emit an JFR events in this situation, mainly
// because the conditions under which this happens are such that it
// cannot be presumed to be possible to actually allocate an event, and
// using a preconstructed one would have limited value in serviceability.
// Having said that, the following place would be the more appropriate
// place to put such logic:
// emit JFR event
for (long nanos = 1L;;) {
U.park(false, nanos); // must use Unsafe park to sleep
if (nanos < 1L << 30) // max about 1 second
nanos <<= 1;
try {
acquire(node, arg, false, false, false, 0L);
} catch (Error | RuntimeException ignored) {
continue;
}
throw firstEx;
}
}
}
/**
* Main acquire method, invoked by all exported acquire methods.
*
* @param node null unless a reacquiring Condition
* @param arg the acquire argument
* @param shared true if shared mode else exclusive
* @param interruptible if abort and return negative on interrupt
* @param timed if true use timed waits
* @param time if timed, the System.nanoTime value to timeout
* @return positive if acquired, 0 if timed out, negative if interrupted
*/
final int acquire(Node node, int arg, boolean shared,
boolean interruptible, boolean timed, long time) {
Thread current = Thread.currentThread();
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
boolean interrupted = false, first = false;
Node pred = null; // predecessor of node when enqueued
/*
* Repeatedly:
* Check if node now first
* if so, ensure head stable, else ensure valid predecessor
* if node is first or not yet enqueued, try acquiring
* else if queue is not initialized, do so by attaching new header node
* resort to spinwait on OOME trying to create node
* else if node not yet created, create it
* resort to spinwait on OOME trying to create node
* else if not yet enqueued, try once to enqueue
* else if woken from park, retry (up to postSpins times)
* else if WAITING status not set, set and retry
* else park and clear WAITING status, and check cancellation
*/
for (;;) {
if (!first && (pred = (node == null) ? null : node.prev) != null &&
!(first = (head == pred))) {
if (pred.status < 0) {
cleanQueue(); // predecessor cancelled
continue;
} else if (pred.prev == null) {
Thread.onSpinWait(); // ensure serialization
continue;
}
}
if (first || pred == null) {
boolean acquired;
try {
if (shared)
acquired = (tryAcquireShared(arg) >= 0);
else
acquired = tryAcquire(arg);
} catch (Throwable ex) {
cancelAcquire(node, interrupted, false);
throw ex;
}
if (acquired) {
if (first) {
node.prev = null;
head = node;
pred.next = null;
node.waiter = null;
if (shared)
signalNextIfShared(node);
if (interrupted)
current.interrupt();
}
return 1;
}
}
Node t;
if ((t = tail) == null) { // initialize queue
if (tryInitializeHead() == null)
return acquireOnOOME(shared, arg);
} else if (node == null) { // allocate; retry before enqueue
try {
node = (shared) ? new SharedNode() : new ExclusiveNode();
} catch (OutOfMemoryError oome) {
return acquireOnOOME(shared, arg);
}
} else if (pred == null) { // try to enqueue
node.waiter = current;
node.setPrevRelaxed(t); // avoid unnecessary fence
if (!casTail(t, node))
node.setPrevRelaxed(null); // back out
else
t.next = node;
} else if (first && spins != 0) {
--spins; // reduce unfairness on rewaits
Thread.onSpinWait();
} else if (node.status == 0) {
node.status = WAITING; // enable signal and recheck
} else {
spins = postSpins = (byte)((postSpins << 1) | 1);
try {
long nanos;
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
else
break;
} catch (Error | RuntimeException ex) {
cancelAcquire(node, interrupted, interruptible); // cancel & rethrow
throw ex;
}
node.clearStatus();
if ((interrupted |= Thread.interrupted()) && interruptible)
break;
}
}
return cancelAcquire(node, interrupted, interruptible);
}
/**
* Spin-waits with backoff; used only upon OOME failures during acquire.
*/
private int acquireOnOOME(boolean shared, int arg) {
for (long nanos = 1L;;) {
if (shared ? (tryAcquireShared(arg) >= 0) : tryAcquire(arg))
return 1;
U.park(false, nanos); // must use Unsafe park to sleep
if (nanos < 1L << 30) // max about 1 second
nanos <<= 1;
}
}
/**
* Possibly repeatedly traverses from tail, unsplicing cancelled
* nodes until none are found. Unparks nodes that may have been
* relinked to be next eligible acquirer.
*/
private void cleanQueue() {
for (;;) { // restart point
for (Node q = tail, s = null, p, n;;) { // (p, q, s) triples
if (q == null || (p = q.prev) == null)
return; // end of list
if (s == null ? tail != q : (s.prev != q || s.status < 0))
break; // inconsistent
if (q.status < 0) { // cancelled
if ((s == null ? casTail(q, p) : s.casPrev(q, p)) &&
q.prev == p) {
p.casNext(q, s); // OK if fails
if (p.prev == null)
signalNext(p);
}
break;
}
if ((n = p.next) != q) { // help finish
if (n != null && q.prev == p) {
p.casNext(n, q);
if (p.prev == null)
signalNext(p);
}
break;
}
s = q;
q = q.prev;
}
}
}
/**
* Cancels an ongoing attempt to acquire.
*
* @param node the node (may be null if cancelled before enqueuing)
* @param interrupted true if thread interrupted
* @param interruptible if should report interruption vs reset
*/
private int cancelAcquire(Node node, boolean interrupted,
boolean interruptible) {
if (node != null) {
node.waiter = null;
node.status = CANCELLED;
if (node.prev != null)
cleanQueue();
}
if (interrupted) {
if (interruptible)
return CANCELLED;
else
Thread.currentThread().interrupt();
}
return 0;
}
// Main exported methods
/**
* Attempts to acquire in exclusive mode. This method should query
* if the state of the object permits it to be acquired in the
* exclusive mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread. This can be used
* to implement method {@link Lock#tryLock()}.
*
* <p>The default
* implementation throws {@link UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return {@code true} if successful. Upon success, this object has
* been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in exclusive
* mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this object is now in a fully released
* state, so that any waiting threads may attempt to acquire;
* and {@code false} otherwise.
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to acquire in shared mode. This method should query if
* the state of the object permits it to be acquired in the shared
* mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread.
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return a negative value on failure; zero if acquisition in shared
* mode succeeded but no subsequent shared-mode acquire can
* succeed; and a positive value if acquisition in shared
* mode succeeded and subsequent shared-mode acquires might
* also succeed, in which case a subsequent waiting thread
* must check availability. (Support for three different
* return values enables this method to be used in contexts
* where acquires only sometimes act exclusively.) Upon
* success, this object has been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in shared mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this release of shared mode may permit a
* waiting acquire (shared or exclusive) to succeed; and
* {@code false} otherwise
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Returns {@code true} if synchronization is held exclusively with
* respect to the current (calling) thread. This method is invoked
* upon each call to a {@link ConditionObject} method.
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}. This method is invoked
* internally only within {@link ConditionObject} methods, so need
* not be defined if conditions are not used.
*
* @return {@code true} if synchronization is held exclusively;
* {@code false} otherwise
* @throws UnsupportedOperationException if conditions are not supported
*/
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
}
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg))
acquire(null, arg, false, false, false, 0L);
}
/**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted() ||
(!tryAcquire(arg) && acquire(null, arg, false, true, false, 0L) < 0))
throw new InterruptedException();
}
/**
* Attempts to acquire in exclusive mode, aborting if interrupted,
* and failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquire}, returning on success. Otherwise, the thread is
* queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquire} until success or the thread is interrupted
* or the timeout elapses. This method can be used to implement
* method {@link Lock#tryLock(long, TimeUnit)}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (!Thread.interrupted()) {
if (tryAcquire(arg))
return true;
if (nanosTimeout <= 0L)
return false;
int stat = acquire(null, arg, false, true, true,
System.nanoTime() + nanosTimeout);
if (stat > 0)
return true;
if (stat == 0)
return false;
}
throw new InterruptedException();
}
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
signalNext(head);
return true;
}
return false;
}
/**
* Acquires in shared mode, ignoring interrupts. Implemented by
* first invoking at least once {@link #tryAcquireShared},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquireShared} until success.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
*/
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
acquire(null, arg, true, false, false, 0L);
}
/**
* Acquires in shared mode, aborting if interrupted. Implemented
* by first checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted.
* @param arg the acquire argument.
* This value is conveyed to {@link #tryAcquireShared} but is
* otherwise uninterpreted and can represent anything
* you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted() ||
(tryAcquireShared(arg) < 0 &&
acquire(null, arg, true, true, false, 0L) < 0))
throw new InterruptedException();
}
/**
* Attempts to acquire in shared mode, aborting if interrupted, and
* failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquireShared}, returning on success. Otherwise, the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted or the timeout elapses.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (!Thread.interrupted()) {
if (tryAcquireShared(arg) >= 0)
return true;
if (nanosTimeout <= 0L)
return false;
int stat = acquire(null, arg, true, true, true,
System.nanoTime() + nanosTimeout);
if (stat > 0)
return true;
if (stat == 0)
return false;
}
throw new InterruptedException();
}
/**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
signalNext(head);
return true;
}
return false;
}
// Queue inspection methods
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations due to interrupts and timeouts may occur
* at any time, a {@code true} return does not guarantee that any
* other thread will ever acquire.
*
* @return {@code true} if there may be other threads waiting to acquire
*/
public final boolean hasQueuedThreads() {
for (Node p = tail, h = head; p != h && p != null; p = p.prev)
if (p.status >= 0)
return true;
return false;
}
/**
* Queries whether any threads have ever contended to acquire this
* synchronizer; that is, if an acquire method has ever blocked.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there has ever been contention
*/
public final boolean hasContended() {
return head != null;
}
/**
* Returns the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued.
*
* <p>In this implementation, this operation normally returns in
* constant time, but may iterate upon contention if other threads are
* concurrently modifying the queue.
*
* @return the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued
*/
public final Thread getFirstQueuedThread() {
Thread first = null, w; Node h, s;
if ((h = head) != null && ((s = h.next) == null ||
(first = s.waiter) == null ||
s.prev == null)) {
// traverse from tail on stale reads
for (Node p = tail, q; p != null && (q = p.prev) != null; p = q)
if ((w = p.waiter) != null)
first = w;
}
return first;
}
/**
* Returns true if the given thread is currently queued.
*
* <p>This implementation traverses the queue to determine
* presence of the given thread.
*
* @param thread the thread
* @return {@code true} if the given thread is on the queue
* @throws NullPointerException if the thread is null
*/
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
for (Node p = tail; p != null; p = p.prev)
if (p.waiter == thread)
return true;
return false;
}
/**
* Returns {@code true} if the apparent first queued thread, if one
* exists, is waiting in exclusive mode. If this method returns
* {@code true}, and the current thread is attempting to acquire in
* shared mode (that is, this method is invoked from {@link
* #tryAcquireShared}) then it is guaranteed that the current thread
* is not the first queued thread. Used only as a heuristic in
* ReentrantReadWriteLock.
*/
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
return (h = head) != null && (s = h.next) != null &&
!(s instanceof SharedNode) && s.waiter != null;
}
/**
* Queries whether any threads have been waiting to acquire longer
* than the current thread.
*
* <p>An invocation of this method is equivalent to (but may be
* more efficient than):
* <pre> {@code
* getFirstQueuedThread() != Thread.currentThread()
* && hasQueuedThreads()}</pre>
*
* <p>Note that because cancellations due to interrupts and
* timeouts may occur at any time, a {@code true} return does not
* guarantee that some other thread will acquire before the current
* thread. Likewise, it is possible for another thread to win a
* race to enqueue after this method has returned {@code false},
* due to the queue being empty.
*
* <p>This method is designed to be used by a fair synchronizer to
* avoid <a href="AbstractQueuedSynchronizer.html#barging">barging</a>.
* Such a synchronizer's {@link #tryAcquire} method should return
* {@code false}, and its {@link #tryAcquireShared} method should
* return a negative value, if this method returns {@code true}
* (unless this is a reentrant acquire). For example, the {@code
* tryAcquire} method for a fair, reentrant, exclusive mode
* synchronizer might look like this:
*
* <pre> {@code
* protected boolean tryAcquire(int arg) {
* if (isHeldExclusively()) {
* // A reentrant acquire; increment hold count
* return true;
* } else if (hasQueuedPredecessors()) {
* return false;
* } else {
* // try to acquire normally
* }
* }}</pre>
*
* @return {@code true} if there is a queued thread preceding the
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty
* @since 1.7
*/
public final boolean hasQueuedPredecessors() {
Thread first = null; Node h, s;
if ((h = head) != null && ((s = h.next) == null ||
(first = s.waiter) == null ||
s.prev == null))
first = getFirstQueuedThread(); // retry via getFirstQueuedThread
return first != null && first != Thread.currentThread();
}
// Instrumentation and monitoring methods
/**
* Returns an estimate of the number of threads waiting to
* acquire. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring system state, not for synchronization control.
*
* @return the estimated number of threads waiting to acquire
*/
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.waiter != null)
++n;
}
return n;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
public final Collection<Thread> getQueuedThreads() {
ArrayList<Thread> list = new ArrayList<>();
for (Node p = tail; p != null; p = p.prev) {
Thread t = p.waiter;
if (t != null)
list.add(t);
}
return list;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in exclusive mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to an exclusive acquire.
*
* @return the collection of threads
*/
public final Collection<Thread> getExclusiveQueuedThreads() {
ArrayList<Thread> list = new ArrayList<>();
for (Node p = tail; p != null; p = p.prev) {
if (!(p instanceof SharedNode)) {
Thread t = p.waiter;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in shared mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to a shared acquire.
*
* @return the collection of threads
*/
public final Collection<Thread> getSharedQueuedThreads() {
ArrayList<Thread> list = new ArrayList<>();
for (Node p = tail; p != null; p = p.prev) {
if (p instanceof SharedNode) {
Thread t = p.waiter;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* Returns a string identifying this synchronizer, as well as its state.
* The state, in brackets, includes the String {@code "State ="}
* followed by the current value of {@link #getState}, and either
* {@code "nonempty"} or {@code "empty"} depending on whether the
* queue is empty.
*
* @return a string identifying this synchronizer, as well as its state
*/
public String toString() {
return super.toString()
+ "[State = " + getState() + ", "
+ (hasQueuedThreads() ? "non" : "") + "empty queue]";
}
// Instrumentation methods for conditions
/**
* Queries whether the given ConditionObject
* uses this synchronizer as its lock.
*
* @param condition the condition
* @return {@code true} if owned
* @throws NullPointerException if the condition is null
*/
public final boolean owns(ConditionObject condition) {
return condition.isOwnedBy(this);
}
/**
* Queries whether any threads are waiting on the given condition
* associated with this synchronizer. Note that because timeouts
* and interrupts may occur at any time, a {@code true} return
* does not guarantee that a future {@code signal} will awaken
* any threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final boolean hasWaiters(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.hasWaiters();
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this synchronizer. Note that
* because timeouts and interrupts may occur at any time, the
* estimate serves only as an upper bound on the actual number of
* waiters. This method is designed for use in monitoring system
* state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final int getWaitQueueLength(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitQueueLength();
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this
* synchronizer. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitingThreads();
}
/**
* Condition implementation for a {@link AbstractQueuedSynchronizer}
* serving as the basis of a {@link Lock} implementation.
*
* <p>Method documentation for this class describes mechanics,
* not behavioral specifications from the point of view of Lock
* and Condition users. Exported versions of this class will in
* general need to be accompanied by documentation describing
* condition semantics that rely on those of the associated
* {@code AbstractQueuedSynchronizer}.
*
* <p>This class is Serializable, but all fields are transient,
* so deserialized conditions have no waiters.
*/
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
/** First node of condition queue. */
private transient ConditionNode firstWaiter;
/** Last node of condition queue. */
private transient ConditionNode lastWaiter;
/**
* Fixed delay in nanoseconds between releasing and reacquiring
* lock during Condition waits that encounter OutOfMemoryErrors
*/
static final long OOME_COND_WAIT_DELAY = 10L * 1000L * 1000L; // 10 ms
/**
* Creates a new {@code ConditionObject} instance.
*/
public ConditionObject() { }
// Signalling methods
/**
* Removes and transfers one or all waiters to sync queue.
*/
private void doSignal(ConditionNode first, boolean all) {
while (first != null) {
ConditionNode next = first.nextWaiter;
if ((firstWaiter = next) == null)
lastWaiter = null;
else
first.nextWaiter = null; // GC assistance
if ((first.getAndUnsetStatus(COND) & COND) != 0) {
enqueue(first);
if (!all)
break;
}
first = next;
}
}
/**
* Moves the longest-waiting thread, if one exists, from the
* wait queue for this condition to the wait queue for the
* owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
public final void signal() {
ConditionNode first = firstWaiter;
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
else if (first != null)
doSignal(first, false);
}
/**
* Moves all threads from the wait queue for this condition to
* the wait queue for the owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
public final void signalAll() {
ConditionNode first = firstWaiter;
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
else if (first != null)
doSignal(first, true);
}
// Waiting methods
/**
* Adds node to condition list and releases lock.
*
* @param node the node
* @return savedState to reacquire after wait
*/
private int enableWait(ConditionNode node) {
if (isHeldExclusively()) {
node.waiter = Thread.currentThread();
node.setStatusRelaxed(COND | WAITING);
ConditionNode last = lastWaiter;
if (last == null)
firstWaiter = node;
else
last.nextWaiter = node;
lastWaiter = node;
int savedState = getState();
if (release(savedState))
return savedState;
}
node.status = CANCELLED; // lock not held or inconsistent
throw new IllegalMonitorStateException();
}
/**
* Returns true if a node that was initially placed on a condition
* queue is now ready to reacquire on sync queue.
* @param node the node
* @return true if is reacquiring
*/
private boolean canReacquire(ConditionNode node) {
// check links, not status to avoid enqueue race
Node p; // traverse unless known to be bidirectionally linked
return node != null && (p = node.prev) != null &&
(p.next == node || isEnqueued(node));
}
/**
* Unlinks the given node and other non-waiting nodes from
* condition queue unless already unlinked.
*/
private void unlinkCancelledWaiters(ConditionNode node) {
if (node == null || node.nextWaiter != null || node == lastWaiter) {
ConditionNode w = firstWaiter, trail = null;
while (w != null) {
ConditionNode next = w.nextWaiter;
if ((w.status & COND) == 0) {
w.nextWaiter = null;
if (trail == null)
firstWaiter = next;
else
trail.nextWaiter = next;
if (next == null)
lastWaiter = trail;
} else
trail = w;
w = next;
}
}
}
/**
* Constructs objects needed for condition wait. On OOME,
* releases lock, sleeps, reacquires, and returns null.
*/
private ConditionNode newConditionNode() {
int savedState;
if (tryInitializeHead() != null) {
try {
return new ConditionNode();
} catch (OutOfMemoryError oome) {
}
}
// fall through if encountered OutOfMemoryError
if (!isHeldExclusively() || !release(savedState = getState()))
throw new IllegalMonitorStateException();
U.park(false, OOME_COND_WAIT_DELAY);
acquireOnOOME(false, savedState);
return null;
}
/**
* Implements uninterruptible condition wait.
* <ol>
* <li>Save lock state returned by {@link #getState}.
* <li>Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li>Block until signalled.
* <li>Reacquire by invoking underlying version of
* {@link #acquire(int)} with saved state as argument.
* </ol>
*/
public final void awaitUninterruptibly() {
ConditionNode node = newConditionNode();
if (node == null)
return;
int savedState = enableWait(node);
LockSupport.setCurrentBlocker(this); // for back-compatibility
boolean interrupted = false, rejected = false;
while (!canReacquire(node)) {
if (Thread.interrupted())
interrupted = true;
else if ((node.status & COND) != 0) {
try {
if (rejected)
node.block();
else
ForkJoinPool.managedBlock(node);
} catch (RejectedExecutionException ex) {
rejected = true;
} catch (InterruptedException ie) {
interrupted = true;
}
} else
Thread.onSpinWait(); // awoke while enqueuing
}
LockSupport.setCurrentBlocker(null);
node.clearStatus();
reacquire(node, savedState);
if (interrupted)
Thread.currentThread().interrupt();
}
/**
* Implements interruptible condition wait.
* <ol>
* <li>If current thread is interrupted, throw InterruptedException.
* <li>Save lock state returned by {@link #getState}.
* <li>Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li>Block until signalled or interrupted.
* <li>Reacquire by invoking underlying version of
* {@link #acquire(int)} with saved state as argument.
* <li>If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
ConditionNode node = newConditionNode();
if (node == null)
return;
int savedState = enableWait(node);
LockSupport.setCurrentBlocker(this); // for back-compatibility
boolean interrupted = false, cancelled = false, rejected = false;
while (!canReacquire(node)) {
if (interrupted |= Thread.interrupted()) {
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
break; // else interrupted after signal
} else if ((node.status & COND) != 0) {
try {
if (rejected)
node.block();
else
ForkJoinPool.managedBlock(node);
} catch (RejectedExecutionException ex) {
rejected = true;
} catch (InterruptedException ie) {
interrupted = true;
}
} else
Thread.onSpinWait(); // awoke while enqueuing
}
LockSupport.setCurrentBlocker(null);
node.clearStatus();
reacquire(node, savedState);
if (interrupted) {
if (cancelled) {
unlinkCancelledWaiters(node);
throw new InterruptedException();
}
Thread.currentThread().interrupt();
}
}
/**
* Implements timed condition wait.
* <ol>
* <li>If current thread is interrupted, throw InterruptedException.
* <li>Save lock state returned by {@link #getState}.
* <li>Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li>Block until signalled, interrupted, or timed out.
* <li>Reacquire by invoking underlying version of
* {@link #acquire(int)} with saved state as argument.
* <li>If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final long awaitNanos(long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
ConditionNode node = newConditionNode();
if (node == null)
return nanosTimeout - OOME_COND_WAIT_DELAY;
int savedState = enableWait(node);
long nanos = (nanosTimeout < 0L) ? 0L : nanosTimeout;
long deadline = System.nanoTime() + nanos;
boolean cancelled = false, interrupted = false;
while (!canReacquire(node)) {
if ((interrupted |= Thread.interrupted()) ||
(nanos = deadline - System.nanoTime()) <= 0L) {
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
break;
} else
LockSupport.parkNanos(this, nanos);
}
node.clearStatus();
reacquire(node, savedState);
if (cancelled) {
unlinkCancelledWaiters(node);
if (interrupted)
throw new InterruptedException();
} else if (interrupted)
Thread.currentThread().interrupt();
long remaining = deadline - System.nanoTime(); // avoid overflow
return (remaining <= nanosTimeout) ? remaining : Long.MIN_VALUE;
}
/**
* Implements absolute timed condition wait.
* <ol>
* <li>If current thread is interrupted, throw InterruptedException.
* <li>Save lock state returned by {@link #getState}.
* <li>Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li>Block until signalled, interrupted, or timed out.
* <li>Reacquire by invoking underlying version of
* {@link #acquire(int)} with saved state as argument.
* <li>If interrupted while blocked in step 4, throw InterruptedException.
* <li>If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean awaitUntil(Date deadline)
throws InterruptedException {
long abstime = deadline.getTime();
if (Thread.interrupted())
throw new InterruptedException();
ConditionNode node = newConditionNode();
if (node == null)
return false;
int savedState = enableWait(node);
boolean cancelled = false, interrupted = false;
while (!canReacquire(node)) {
if ((interrupted |= Thread.interrupted()) ||
System.currentTimeMillis() >= abstime) {
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
break;
} else
LockSupport.parkUntil(this, abstime);
}
node.clearStatus();
reacquire(node, savedState);
if (cancelled) {
unlinkCancelledWaiters(node);
if (interrupted)
throw new InterruptedException();
} else if (interrupted)
Thread.currentThread().interrupt();
return !cancelled;
}
/**
* Implements timed condition wait.
* <ol>
* <li>If current thread is interrupted, throw InterruptedException.
* <li>Save lock state returned by {@link #getState}.
* <li>Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li>Block until signalled, interrupted, or timed out.
* <li>Reacquire by invoking underlying version of
* {@link #acquire(int)} with saved state as argument.
* <li>If interrupted while blocked in step 4, throw InterruptedException.
* <li>If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean await(long time, TimeUnit unit)
throws InterruptedException {
long nanosTimeout = unit.toNanos(time);
if (Thread.interrupted())
throw new InterruptedException();
ConditionNode node = newConditionNode();
if (node == null)
return false;
int savedState = enableWait(node);
long nanos = (nanosTimeout < 0L) ? 0L : nanosTimeout;
long deadline = System.nanoTime() + nanos;
boolean cancelled = false, interrupted = false;
while (!canReacquire(node)) {
if ((interrupted |= Thread.interrupted()) ||
(nanos = deadline - System.nanoTime()) <= 0L) {
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
break;
} else
LockSupport.parkNanos(this, nanos);
}
node.clearStatus();
reacquire(node, savedState);
if (cancelled) {
unlinkCancelledWaiters(node);
if (interrupted)
throw new InterruptedException();
} else if (interrupted)
Thread.currentThread().interrupt();
return !cancelled;
}
// support for instrumentation
/**
* Returns true if this condition was created by the given
* synchronization object.
*
* @return {@code true} if owned
*/
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
return sync == AbstractQueuedSynchronizer.this;
}
/**
* Queries whether any threads are waiting on this condition.
* Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
*
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final boolean hasWaiters() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
for (ConditionNode w = firstWaiter; w != null; w = w.nextWaiter) {
if ((w.status & COND) != 0)
return true;
}
return false;
}
/**
* Returns an estimate of the number of threads waiting on
* this condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
*
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final int getWaitQueueLength() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int n = 0;
for (ConditionNode w = firstWaiter; w != null; w = w.nextWaiter) {
if ((w.status & COND) != 0)
++n;
}
return n;
}
/**
* Returns a collection containing those threads that may be
* waiting on this Condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
*
* @return the collection of threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final Collection<Thread> getWaitingThreads() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
ArrayList<Thread> list = new ArrayList<>();
for (ConditionNode w = firstWaiter; w != null; w = w.nextWaiter) {
if ((w.status & COND) != 0) {
Thread t = w.waiter;
if (t != null)
list.add(t);
}
}
return list;
}
}
// Unsafe
private static final Unsafe U = Unsafe.getUnsafe();
private static final long STATE
= U.objectFieldOffset(AbstractQueuedSynchronizer.class, "state");
private static final long HEAD
= U.objectFieldOffset(AbstractQueuedSynchronizer.class, "head");
private static final long TAIL
= U.objectFieldOffset(AbstractQueuedSynchronizer.class, "tail");
static {
Class<?> ensureLoaded = LockSupport.class;
}
}:::