深浅模式
CyclicBarrier
更新: 3/4/2026 字数: 0 字
Java 并发编程中，CyclicBarrier（循环屏障）是一个非常实用的同步辅助类。它允许一组线程互相等待，直到所有线程都到达某个公共屏障点 (Common Barrier Point)。
它的核心特性是可重用性：当所有等待线程被释放后，屏障可以被重置并再次使用，这也是它被称为“循环”的原因
CyclicBarrier 的实现原理
原理
- 核心实现原理 (JDK 8 源码分析) CyclicBarrier 的底层并没有直接使用 AQS (AbstractQueuedSynchronizer) 的共享模式，而是通过 ReentrantLock 和 Condition 组合实现的。
核心内部成员
java
/** 屏障入口的锁 */
private final ReentrantLock lock = new ReentrantLock();
/** 线程等待条件 */
private final Condition trip = lock.newCondition();
/** 屏障需要拦截的线程总数 */
private final int parties;
/** 当前代（Generation）执行完后的回调任务 */
private final Runnable barrierCommand;
/** 当前代 */
private Generation generation = new Generation();
/** 剩余还未到达屏障的线程数 */
private int count;关键概念：Generation (代)
- CyclicBarrier 使用一个内部类 Generation 来表示屏障的“当前周期”
- 每当所有线程到达屏障，或者屏障被重置/损坏时，generation 就会更新
- 这解决了并发中的“过早释放”问题，确保线程能识别自己是在哪一轮等待
核心方法：dowait 无论是 await() 还是带超时的 await(long, TimeUnit)，最终都调用了私有的 dowait 方法。其逻辑如下：
获取锁：所有操作都在 lock.lock() 保护下进行
计数减少：count--
判断是否为最后一个到达者：
- 如果是最后一个：执行 barrierCommand（如果有），然后调用 nextGeneration()。这个方法会唤醒所有在 trip 条件上等待的线程，并重置 count
- 如果不是最后一个：进入
for(;;)循环，调用 trip.await() 挂起线程，等待被唤醒或中断
核心源码片段解析
java
private int dowait(boolean timed, long nanos) throws ... {
final ReentrantLock lock = this.lock;
lock.lock();
try {
final Generation g = generation;
// 如果屏障已损坏，抛出异常
if (g.broken) throw new BrokenBarrierException();
int index = --count; // 剩余计数减一
if (index == 0) { // 最后一个线程到达
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null) command.run(); // 执行预定任务
ranAction = true;
nextGeneration(); // 唤醒所有人并开启下一代
return 0;
} finally {
if (!ranAction) breakBarrier(); // 任务执行失败则损坏屏障
}
}
// 非最后一个线程，进入自旋/等待
for (;;) {
try {
if (!timed) trip.await();
else if (nanos > 0L) nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
// 如果等待期间被中断，标记屏障损坏并唤醒其它人
if (g == generation && ! g.broken) {
breakBarrier();
throw ie;
} else {
Thread.currentThread().interrupt();
}
}
// ... 检查异常状态 ...
if (g != generation) return index; // 正常唤醒，返回索引
}
} finally {
lock.unlock();
}
}- 与 CountDownLatch 的区别
| 特性 | CountDownLatch | CyclicBarrier |
|---|---|---|
| 可复用性 | 一次性，计数归零后失效 | 可循环使用 (reset()) |
| 实现机制 | 基于 AQS 共享锁 | 基于 ReentrantLock + Condition |
| 等待角色 | 主线程等待多个子线程 | 多个子线程互相等待 |
| 额外功能 | 无 | 支持 barrierAction (合并任务) |
常用方案演示
模拟 3 个运动员参加田径赛。要求所有人都在起跑线准备好后，裁判才鸣枪出发
java
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
public class CyclicBarrierDemo {
public static void main(String[] args) {
int runnerCount = 3;
// 定义屏障：3个线程，外加一个所有人到达后的回调任务
CyclicBarrier barrier = new CyclicBarrier(runnerCount, () -> {
System.out.println("\n--- 所有人已就位，裁判鸣枪：跑！ ---");
});
for (int i = 1; i <= runnerCount; i++) {
final int id = i;
new Thread(() -> {
try {
System.out.println("运动员 " + id + " 正在穿鞋...");
Thread.sleep((long) (Math.random() * 2000));
System.out.println("运动员 " + id + " 已到达起跑线，等待他人...");
// 线程在此处阻塞，直到第3个线程调用 await()
barrier.await();
System.out.println("运动员 " + id + " 冲出了起跑线！");
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
}).start();
}
}
}运行结果预期： 三个运动员随机完成准备。
前两个到达的运动员会显示“等待他人”。
第三个到达后，触发 barrierCommand 输出“鸣枪”。
随后三个运动员几乎同时输出“冲出了起跑线”。
源码
CyclicBarrier
java
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
/**
* A synchronization aid that allows a set of threads to all wait for
* each other to reach a common barrier point. CyclicBarriers are
* useful in programs involving a fixed sized party of threads that
* must occasionally wait for each other. The barrier is called
* <em>cyclic</em> because it can be re-used after the waiting threads
* are released.
*
* <p>A {@code CyclicBarrier} supports an optional {@link Runnable}
* command that is run once per barrier point, after the last thread
* in the party arrives, but before any threads are released. This
* <em>barrier action</em> is useful for updating shared-state before
* any of the parties continue.
*
* <p><b>Sample usage:</b> Here is an example of using a barrier in a
* parallel decomposition design:
*
* <pre> {@code
* class Solver {
* final int N;
* final float[][] data;
* final CyclicBarrier barrier;
*
* class Worker implements Runnable {
* int myRow;
* Worker(int row) { myRow = row; }
* public void run() {
* while (!done()) {
* processRow(myRow);
*
* try {
* barrier.await();
* } catch (InterruptedException ex) {
* return;
* } catch (BrokenBarrierException ex) {
* return;
* }
* }
* }
* }
*
* public Solver(float[][] matrix) {
* data = matrix;
* N = matrix.length;
* Runnable barrierAction =
* new Runnable() { public void run() { mergeRows(N); }};
* barrier = new CyclicBarrier(N, barrierAction);
*
* List<Thread> threads = new ArrayList<Thread>(N);
* for (int i = 0; i < N; i++) {
* Thread thread = new Thread(new Worker(i));
* threads.add(thread);
* thread.start();
* }
*
* // wait until done
* for (Thread thread : threads)
* try { thread.join(); } catch (InterruptedException ex) {}
* }
* }}</pre>
*
* Here, each worker thread processes a row of the matrix then waits at the
* barrier until all rows have been processed. When all threads have
* reached the barrier, the barrier action executes, which merges the
* rows. If the merger determines that a solution has been found then
* {@code done()} will return {@code true} and each worker will
* terminate.
*
* <p>If the barrier action does not rely on the parties being suspended when
* it is executed, then any of the threads in the party could execute that
* action when it is released. To facilitate this, each invocation of
* {@link #await()} returns the arrival index of that thread at the barrier.
* You can then choose which thread should execute the barrier action, for
* example:
*
* <pre> {@code
* if (barrier.await() == 0) {
* // log the completion of this iteration
* }}</pre>
*
* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
* for failed synchronization attempts: If a thread leaves a barrier
* point prematurely because of interruption, failure, or timeout, all
* other threads waiting at that barrier point will also leave
* abnormally via {@link BrokenBarrierException} (or
* {@link InterruptedException} if they too were interrupted at about
* the same time).
*
* <p>Memory consistency effects: Actions in a thread prior to calling
* {@code await()} <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions that are part of the barrier action, which in turn
* <i>happen-before</i> actions following a successful return from the
* corresponding {@code await()} in other threads.
*
* @since 1.5
* @see CountDownLatch
* @author Doug Lea
*/
public class CyclicBarrier {
/**
* Each use of the barrier is represented as a generation instance.
* The generation changes whenever the barrier is tripped, or
* is reset. There can be many generations associated with threads
* using the barrier - due to the reset() method.
*/
private static class Generation {
boolean broken = false;
}
/** The lock for guarding barrier entry */
private final ReentrantLock lock = new ReentrantLock();
/** Condition to wait on until tripped */
private final Condition trip = lock.newCondition();
/** The number of parties */
private final int parties;
/* The command to run when tripped */
private final Runnable barrierCommand;
/** The current generation */
private Generation generation = new Generation();
/**
* Number of parties still waiting. Counts down from parties to 0
* on each generation. It is reset to parties on each new
* generation or when broken.
*/
private int count;
/**
* Updates state on barrier trip and wakes up everyone.
* Called only while holding lock.
*/
private void nextGeneration() {
// signal completion of last generation
trip.signalAll();
// set up next generation
count = parties;
generation = new Generation();
}
/**
* Sets current barrier generation as broken and wakes up everyone.
* Called only while holding lock.
*/
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
/**
* Main barrier code, covering the various policies.
*/
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
final Generation g = generation;
if (g.broken)
throw new BrokenBarrierException();
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
int index = --count;
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
// loop until tripped, broken, interrupted, or timed out
for (;;) {
try {
if (!timed)
trip.await();
else if (nanos > 0L)
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
if (g == generation && ! g.broken) {
breakBarrier();
throw ie;
} else {
// We're about to finish waiting even if we had been
// interrupted, so this interrupt is deemed to "belong"
// to subsequent execution.
Thread.currentThread().interrupt();
}
}
if (g.broken)
throw new BrokenBarrierException();
if (g != generation)
return index;
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}
/**
* Creates a new {@code CyclicBarrier} that will trip when the
* given number of parties (threads) are waiting upon it, and which
* will execute the given barrier action when the barrier is tripped,
* performed by the last thread entering the barrier.
*
* @param parties the number of threads that must invoke {@link #await()}
* before the barrier is tripped
* @param barrierAction the command to execute when the barrier is
* tripped, or {@code null} if there is no action
* @throws IllegalArgumentException if {@code parties} is less than 1
*/
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0)
throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
/**
* Creates a new {@code CyclicBarrier} that will trip when the
* given number of parties (threads) are waiting upon it, and
* does not perform a predefined action when the barrier is tripped.
*
* @param parties the number of threads that must invoke {@link #await()}
* before the barrier is tripped
* @throws IllegalArgumentException if {@code parties} is less than 1
*/
public CyclicBarrier(int parties) {
this(parties, null);
}
/**
* Returns the number of parties required to trip this barrier.
*
* @return the number of parties required to trip this barrier
*/
public int getParties() {
return parties;
}
/**
* Waits until all {@linkplain #getParties parties} have invoked
* {@code await} on this barrier.
*
* <p>If the current thread is not the last to arrive then it is
* disabled for thread scheduling purposes and lies dormant until
* one of the following things happens:
* <ul>
* <li>The last thread arrives;
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread;
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* one of the other waiting threads;
* <li>Some other thread times out while waiting for the barrier;
* <li>Some other thread invokes {@link #reset} on this barrier.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method;
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the barrier is {@link #reset} while any thread is waiting, then
* {@link BrokenBarrierException} is thrown for those threads.
*
* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
* then all other waiting threads will throw
* {@link BrokenBarrierException} and the barrier is placed in the broken
* state.
*
* <p>If the current thread is the last thread to arrive, and a
* non-null barrier action was supplied in the constructor, then
* the current thread runs the action before allowing the other
* threads to continue. If an exception occurs during the barrier
* action then that exception will be propagated in the current
* thread and the barrier is placed in the broken state.
*
* @return the arrival index of the current thread, where index
* {@code getParties() - 1} indicates the first to arrive
* and zero indicates the last to arrive
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws BrokenBarrierException if <em>another</em> thread was
* interrupted or timed out while the current thread was
* waiting, or the barrier was reset, or the barrier was
* broken when {@code await} was called, or the barrier
* action (if present) failed due to an exception
*/
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
/**
* Waits until all {@linkplain #getParties parties} have invoked
* {@code await} on this barrier, or the specified waiting time elapses.
*
* <p>If the current thread is not the last to arrive then it is
* disabled for thread scheduling purposes and lies dormant until
* one of the following things happens:
* <ul>
* <li>The last thread arrives;
* <li>The specified timeout elapses;
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread;
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* one of the other waiting threads;
* <li>Some other thread times out while waiting for the barrier;
* <li>Some other thread invokes {@link #reset} on this barrier.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method;
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then {@link TimeoutException}
* is thrown. If the time is less than or equal to zero, the method
* will not wait at all.
*
* <p>If the barrier is {@link #reset} while any thread is waiting, then
* {@link BrokenBarrierException} is thrown for those threads.
*
* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
* then all other waiting threads will throw
* {@link BrokenBarrierException} and the barrier is placed in the broken
* state.
*
* <p>If the current thread is the last thread to arrive, and a
* non-null barrier action was supplied in the constructor, then
* the current thread runs the action before allowing the other
* threads to continue. If an exception occurs during the barrier
* action then that exception will be propagated in the current
* thread and the barrier is placed in the broken state.
*
* @param timeout the time to wait for the barrier
* @param unit the time unit of the timeout parameter
* @return the arrival index of the current thread, where index
* {@code getParties() - 1} indicates the first to arrive
* and zero indicates the last to arrive
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the specified timeout elapses
* @throws BrokenBarrierException if <em>another</em> thread was
* interrupted or timed out while the current thread was
* waiting, or the barrier was reset, or the barrier was
* broken when {@code await} was called, or the barrier
* action (if present) failed due to an exception
*/
public int await(long timeout, TimeUnit unit)
throws InterruptedException, BrokenBarrierException, TimeoutException {
return dowait(true, unit.toNanos(timeout));
}
/**
* Queries if this barrier is in a broken state.
*
* @return {@code true} if one or more parties broke out of this
* barrier due to interruption or timeout since
* construction or the last reset, or a barrier action
* failed due to an exception; {@code false} otherwise.
*/
public boolean isBroken() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return generation.broken;
} finally {
lock.unlock();
}
}
/**
* Resets the barrier to its initial state.
*
* <p>If any parties are currently waiting at the barrier, they will
* return with a {@link BrokenBarrierException}. Note that resetting
* a barrier that is already in a broken state has no effect.
*/
public void reset() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
breakBarrier(); // break the current generation
nextGeneration(); // start a new generation
} finally {
lock.unlock();
}
}
/**
* Returns the number of parties currently waiting at the barrier.
* This method is primarily useful for debugging and assertions.
*
* @return the number of parties currently blocked in {@link #await()}
*/
public int getNumberWaiting() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return parties - count;
} finally {
lock.unlock();
}
}
}