join、CountDownLatch、CyclicBarrier浅析
1、用法示例
（1）join是Thread提供的线程间协作的方法，通过查看代码可知是通过自旋wait实现的，使用方法比较简单，直接调用线程的join方法就会进入wait状态，直到该线程 !isAlive() 跳出循环，从而保证线程的执行顺序，适用于线程间执行的逻辑有依赖的情况。具体代码如下：
public final void join() throws InterruptedException {
join(0);
}
public final synchronized void join(long millis)
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (millis == 0) {
while (isAlive()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
public final synchronized void join(long millis, int nanos)
throws InterruptedException {
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
}
join(millis);
}
具体使用方法如下：
public static void main(String[] args) {
Random random = new Random();
Runnable runnable = () -> {
log.info("Runnable is starting ...");
int sleep = random.nextInt(1000);
try {
Thread.sleep(sleep);
} catch (InterruptedException e) {
e.printStackTrace();
}
log.info("Runnable{} is finished and work cost {}ms ...", Thread.currentThread().getName(), sleep);
};
Thread t1 = new Thread(runnable);
Thread t2 = new Thread(runnable);
Thread t3 = new Thread(runnable);
// 此处t1、t2调用join方法后其他线程会等待t1、t2执行结束后开始
t1.start();
t2.start();
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
t3.start();
}
运行结果如下：
2019-07-16 14:29:57.847 [Thread-0] INFO com.longc.demo.study.thread.TestJoin - Runnable is starting ... 2019-07-16 14:29:57.847 [Thread-1] INFO com.longc.demo.study.thread.TestJoin - Runnable is starting ... 2019-07-16 14:29:58.146 [Thread-0] INFO com.longc.demo.study.thread.TestJoin - RunnableThread-0 is finished and work cost 292ms ... 2019-07-16 14:29:58.304 [Thread-1] INFO com.longc.demo.study.thread.TestJoin - RunnableThread-1 is finished and work cost 450ms ... 2019-07-16 14:29:58.305 [Thread-2] INFO com.longc.demo.study.thread.TestJoin - Runnable is starting ... 2019-07-16 14:29:59.260 [Thread-2] INFO com.longc.demo.study.thread.TestJoin - RunnableThread-2 is finished and work cost 951ms ...
由运行结果可以看出线程2等待线程0、线程1执行结束后才开始执行。
（2）CountDownLatch是通过共享锁实现的一个线程间协作工具。利用的是AQS中的status（可以理解为锁的数量），逐渐减少锁直至全部锁释放掉，因此其核心是内部类Sync及AQS的原理，具体代码如下：
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
/*
*
*
*
*
*
* 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.AbstractQueuedSynchronizer;
/**
* A synchronization aid that allows one or more threads to wait until
* a set of operations being performed in other threads completes.
*
* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
* The {@link #await await} methods block until the current count reaches
* zero due to invocations of the {@link #countDown} method, after which
* all waiting threads are released and any subsequent invocations of
* {@link #await await} return immediately. This is a one-shot phenomenon
* -- the count cannot be reset. If you need a version that resets the
* count, consider using a {@link CyclicBarrier}.
*
* <p>A {@code CountDownLatch} is a versatile synchronization tool
* and can be used for a number of purposes. A
* {@code CountDownLatch} initialized with a count of one serves as a
* simple on/off latch, or gate: all threads invoking {@link #await await}
* wait at the gate until it is opened by a thread invoking {@link
* #countDown}. A {@code CountDownLatch} initialized to <em>N</em>
* can be used to make one thread wait until <em>N</em> threads have
* completed some action, or some action has been completed N times.
*
* <p>A useful property of a {@code CountDownLatch} is that it
* doesn't require that threads calling {@code countDown} wait for
* the count to reach zero before proceeding, it simply prevents any
* thread from proceeding past an {@link #await await} until all
* threads could pass.
*
* <p><b>Sample usage:</b> Here is a pair of classes in which a group
* of worker threads use two countdown latches:
* <ul>
* <li>The first is a start signal that prevents any worker from proceeding
* until the driver is ready for them to proceed;
* <li>The second is a completion signal that allows the driver to wait
* until all workers have completed.
* </ul>
*
* <pre> {@code
* class Driver { // ...
* void main() throws InterruptedException {
* CountDownLatch startSignal = new CountDownLatch(1);
* CountDownLatch doneSignal = new CountDownLatch(N);
*
* for (int i = 0; i < N; ++i) // create and start threads
* new Thread(new Worker(startSignal, doneSignal)).start();
*
* doSomethingElse(); // don't let run yet
* startSignal.countDown(); // let all threads proceed
* doSomethingElse();
* doneSignal.await(); // wait for all to finish
* }
* }
*
* class Worker implements Runnable {
* private final CountDownLatch startSignal;
* private final CountDownLatch doneSignal;
* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
* this.startSignal = startSignal;
* this.doneSignal = doneSignal;
* }
* public void run() {
* try {
* startSignal.await();
* doWork();
* doneSignal.countDown();
* } catch (InterruptedException ex) {} // return;
* }
*
* void doWork() { ... }
* }}</pre>
*
* <p>Another typical usage would be to divide a problem into N parts,
* describe each part with a Runnable that executes that portion and
* counts down on the latch, and queue all the Runnables to an
* Executor. When all sub-parts are complete, the coordinating thread
* will be able to pass through await. (When threads must repeatedly
* count down in this way, instead use a {@link CyclicBarrier}.)
*
* <pre> {@code
* class Driver2 { // ...
* void main() throws InterruptedException {
* CountDownLatch doneSignal = new CountDownLatch(N);
* Executor e = ...
*
* for (int i = 0; i < N; ++i) // create and start threads
* e.execute(new WorkerRunnable(doneSignal, i));
*
* doneSignal.await(); // wait for all to finish
* }
* }
*
* class WorkerRunnable implements Runnable {
* private final CountDownLatch doneSignal;
* private final int i;
* WorkerRunnable(CountDownLatch doneSignal, int i) {
* this.doneSignal = doneSignal;
* this.i = i;
* }
* public void run() {
* try {
* doWork(i);
* doneSignal.countDown();
* } catch (InterruptedException ex) {} // return;
* }
*
* void doWork() { ... }
* }}</pre>
*
* <p>Memory consistency effects: Until the count reaches
* zero, actions in a thread prior to calling
* {@code countDown()}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions following a successful return from a corresponding
* {@code await()} in another thread.
*
* @since 1.5
* @author Doug Lea
*/
public class CountDownLatch {
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c-1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
private final Sync sync;
/**
* Constructs a {@code CountDownLatch} initialized with the given count.
*
* @param count the number of times {@link #countDown} must be invoked
* before threads can pass through {@link #await}
* @throws IllegalArgumentException if {@code count} is negative
*/
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>If the current count is zero then this method returns immediately.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of two things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
* or the specified waiting time elapses.
*
* <p>If the current count is zero then this method returns immediately
* with the value {@code true}.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of three things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the count reaches zero then the method returns with the
* value {@code true}.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <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 the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if the count reached zero and {@code false}
* if the waiting time elapsed before the count reached zero
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*
* <p>If the current count is greater than zero then it is decremented.
* If the new count is zero then all waiting threads are re-enabled for
* thread scheduling purposes.
*
* <p>If the current count equals zero then nothing happens.
*/
public void countDown() {
sync.releaseShared(1);
}
/**
* Returns the current count.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the current count
*/
public long getCount() {
return sync.getCount();
}
/**
* Returns a string identifying this latch, as well as its state.
* The state, in brackets, includes the String {@code "Count ="}
* followed by the current count.
*
* @return a string identifying this latch, as well as its state
*/
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}
核心方法：
CountDownLatch(int count) int值参数的构造函数，count代表倒数计数。
countDown() 执行计数减操作（每次调用释放一个锁）。
await() 等待指令，调用CountDownLatch.await()之后当前线程会进入等待状态，直至预设的锁全部释放为止。
下面我们以一个经典的场景做示例，假设一项工作需要三个工作做好准备工作之后才能开始，示例如下：
@Test
public void work() {
int needCount = 3;
CountDownLatch ready = new CountDownLatch(needCount);
for (int i = 0; i < needCount; i++) {
Thread thread = new Thread(new Worker(i, ready));
thread.start();
}
try {
ready.await();
log.info("Workers is ready begin next Job ...");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
@Slf4j
static class Worker implements Runnable {
int id;
CountDownLatch ready;
Worker(int id, CountDownLatch ready) {
this.id = id;
this.ready = ready;
}
@Override
public void run() {
try {
Random random = new Random();
int cost = random.nextInt(10000);
log.info("Worker{} begin prepare job ...", id);
Thread.sleep(cost);
log.info("Worker{} is ready and cost {}ms ...", id, cost);
ready.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
运行结果如下：
2019-07-16 14:52:27.376 [Thread-1] INFO com.longc.demo.study.thread.TestCountDownLatch$Worker - Worker1 begin prepare job ... 2019-07-16 14:52:27.376 [Thread-2] INFO com.longc.demo.study.thread.TestCountDownLatch$Worker - Worker2 begin prepare job ... 2019-07-16 14:52:27.376 [Thread-0] INFO com.longc.demo.study.thread.TestCountDownLatch$Worker - Worker0 begin prepare job ... 2019-07-16 14:52:30.609 [Thread-0] INFO com.longc.demo.study.thread.TestCountDownLatch$Worker - Worker0 is ready and cost 3225ms ... 2019-07-16 14:52:33.750 [Thread-1] INFO com.longc.demo.study.thread.TestCountDownLatch$Worker - Worker1 is ready and cost 6362ms ... 2019-07-16 14:52:34.724 [Thread-2] INFO com.longc.demo.study.thread.TestCountDownLatch$Worker - Worker2 is ready and cost 7337ms ... 2019-07-16 14:52:34.724 [main] INFO com.longc.demo.study.thread.TestCountDownLatch - Workers is ready begin next Job ...
根据运行结果可知主线程调用CountDownLatch.await()之后就会进入等待状态，直至线程0、线程1、线程2分别完成自己的工作后计数器减（释放一个锁）后，所有锁全部释放才开始执行后续的操作。此种方式相对灵活，不需要获取线程对象即可操作，对于使用线程池管理线程的我们来说是很方便的。这里有什么不懂的地方可以仔细学习一下AQS相关知识。
（3）CyclicBarrier字面意义是循环屏障，其原理大致为让一组线程到达同步点后被阻塞，先到先阻塞，直到指定数量的线程到达同步点后解除屏障。此时所有被阻塞的线程才会继续工作。先把源码贴出来：
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
/*
*
*
*
*
*
* 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(...); }};
* 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)
* thread.join();
* }
* }}</pre>
*
* Here, each worker thread processes a row of the matrix then waits at the
* barrier until all rows have been processed. When all rows are processed
* the supplied {@link Runnable} barrier action is executed and 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 non-deterministic way the lock
* may be allocated to waiting threads - but only one of these
* can be active at a time (the one to which {@code count} applies)
* and all the rest are either broken or tripped.
* There need not be an active generation if there has been a break
* but no subsequent reset.
*/
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 not
// 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; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* one of the other waiting threads; or
* <li>Some other thread times out while waiting for barrier; or
* <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; or
* <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,
* or if the barrier {@linkplain #isBroken is broken} when
* {@code await} is invoked, or while any thread is waiting, then
* {@link BrokenBarrierException} is thrown.
*
* <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; or
* <li>The specified timeout elapses; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* one of the other waiting threads; or
* <li>Some other thread times out while waiting for barrier; or
* <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; or
* <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,
* or if the barrier {@linkplain #isBroken is broken} when
* {@code await} is invoked, or while any thread is waiting, then
* {@link BrokenBarrierException} is thrown.
*
* <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.
* In this case the barrier will be broken.
* @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. If any parties are
* currently waiting at the barrier, they will return with a
* {@link BrokenBarrierException}. Note that resets <em>after</em>
* a breakage has occurred for other reasons can be complicated to
* carry out; threads need to re-synchronize in some other way,
* and choose one to perform the reset. It may be preferable to
* instead create a new barrier for subsequent use.
*/
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();
}
}
}
实现原理：在CyclicBarrier的内部定义了一个Lock对象，每当一个线程调用CyclicBarrier的await方法时，将剩余拦截的线程数减1，然后判断剩余拦截数是否为0，如果不是，进入Lock对象的条件队列等待。如果是，执行barrierAction对象的Runnable方法，然后将锁的条件队列中的所有线程放入锁等待队列中，这些线程会依次的获取锁、释放锁，接着先从await方法返回，再从CyclicBarrier的await方法中返回。
下面我们再以一个经典的场景作为示例，运动员赛跑，需要所有运动员均准备好后裁判发令出发，运动员接收到指令后出发，代码如下：
public static void main(String[] args) {
int runnerCount = 4;
CyclicBarrier barrier = new CyclicBarrier(runnerCount);
for (int i = 0; i < runnerCount; i++)
new Runner(barrier).start();
}
static class Runner extends Thread {
private CyclicBarrier cyclicBarrier;
Runner(CyclicBarrier cyclicBarrier) {
this.cyclicBarrier = cyclicBarrier;
log.info("运动员{} 进入场地 ...", getName());
}
@Override
public void run() {
try {
Random random = new Random();
int sleep = random.nextInt(10000);
Thread.sleep(sleep);
log.info("运动员{}准备完毕，耗时{}ms，等待起跑指令...", getName(), sleep);
cyclicBarrier.await();
log.info("运动员{}开始出发...", getName());
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
}
}
运行结果如下：
2019-07-16 15:34:39.643 [main] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-0 进入场地 ... 2019-07-16 15:34:39.649 [main] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-1 进入场地 ... 2019-07-16 15:34:39.649 [main] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-2 进入场地 ... 2019-07-16 15:34:39.649 [main] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-3 进入场地 ... 2019-07-16 15:34:41.519 [Thread-2] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-2准备完毕，耗时1866ms，等待起跑指令... 2019-07-16 15:34:44.264 [Thread-3] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-3准备完毕，耗时4611ms，等待起跑指令... 2019-07-16 15:34:44.816 [Thread-1] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-1准备完毕，耗时5162ms，等待起跑指令... 2019-07-16 15:34:49.454 [Thread-0] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-0准备完毕，耗时9801ms，等待起跑指令... 2019-07-16 15:34:49.454 [Thread-0] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-0开始出发... 2019-07-16 15:34:49.454 [Thread-2] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-2开始出发... 2019-07-16 15:34:49.454 [Thread-3] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-3开始出发... 2019-07-16 15:34:49.454 [Thread-1] INFO com.longc.demo.study.thread.TestCyclicBarrier - 运动员Thread-1开始出发...
简单做个总结：
（1）使用场景不同
CyclicBarrier适用于一组线程之间的相互等待，而CountDownLatch、join适用于某线程或某组线程等待另一组线程的场景。
CountDownLatch两个方法配合使用也可以实现CyclicBarrier的功能的，即在线程内调用countDown() 和 await()
（2）实现方式不同
CountDownLatch是通过AQS共享锁实现的
CyclicBarrier核心是通过ReentranLock非公平锁（独占锁）实现的
join则是利用自旋Object.wait()实现。
（3）使用规则不同
CountDownLatch的计数器无法被重置；CyclicBarrier的计数器可以被重置后使用，因此它被称为是循环的barrier。
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