概述
- 简介
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 cyclic because it can be re-used after the waiting threads are released.
CyclicBarrier是一个同步工具类，它允许一组线程互相等待，直到到达某个栅栏点（common barrier point）。与CountDownLatch不同的是该barrier在释放等待线程后可以重用，所以称它为循环（Cyclic）的屏障（Barrier）。
CyclicBarrier字面理解是循环的栅栏，之所以称之为循环的是因为在等待线程释放后，该栅栏还可以复用。
CyclicBarrier支持一个可选的Runnable命令，在一组线程中的最后一个线程到达之后（但在释放所有线程之前），该命令只在每个屏障点运行一次。若在继续所有参与线程之前更新共享状态，此屏障操作很有用。
- 使用场景
需要所有的子任务都完成时，才执行主任务，这个时候就可以选择使用CyclicBarrier
源码解析
成员变量
/** 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; //拦截的线程数量
/* The command to run when tripped */
private final Runnable barrierCommand; //当屏障撤销时，需要执行的屏障操作
/** The current generation */
//当前的Generation。每当屏障失效或者开闸之后都会自动替换掉。从而实现重置的功能。
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.
*/
//还能阻塞的线程数（即parties-当前阻塞的线程数），当新建generation或generation被破坏时，count会被重置。因为对Count的操作都是在获取锁之后，所以不需要其他同步措施
private int count;
构造方法
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
public CyclicBarrier(int parties) {
this(parties, null);
}成员方法
/**
* 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();
}
/**
* Returns 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.
*/
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.
*/
public int await(long timeout, TimeUnit unit)
throws InterruptedException,
BrokenBarrierException,
TimeoutException {
return dowait(true, unit.toNanos(timeout));
}
/**
* 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();
}
}
/**
* Queries if this barrier is in a broken state.
*/
public boolean isBroken() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return generation.broken;
} finally {
lock.unlock();
}
}
/**
* Resets the barrier to its initial state.
*/
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.
*/
public int getNumberWaiting() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return parties - count;
} finally {
lock.unlock();
}
}
内部类
/**
* Each use of the barrier is represented as a generation instance.
*/
private static class Generation {
boolean broken = false;
}循环栅栏CyclicBarrier应用实例
源码
import java.util.Random;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
//先执行Soldier的run方法，再执行BarrierRun的run方法
public class CyclicBarrierTest {
public static class Soldier implements Runnable {
private String soldierName;
private final CyclicBarrier cyclic;
public Soldier(CyclicBarrier cyclic, String soldierName) {
this.soldierName = soldierName;
this.cyclic = cyclic;
}
@Override
public void run() {
try {
// 栅栏点一，针对doTask方法
// 等待所有士兵到齐
cyclic.await();
doWork();
// 栅栏点二，针对doWork方法
// 等待所有士兵完成工作
cyclic.await();
} catch (InterruptedException e) {// 在等待过程中,线程被中断
e.printStackTrace();
} catch (BrokenBarrierException e) {// 表示当前CyclicBarrier已经损坏.系统无法等到所有线程到齐了.
e.printStackTrace();
}
}
void doWork() {
try {
Thread.sleep(Math.abs(new Random().nextInt() % 10000));
System.out.println(soldierName + ":任务完成");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static class BarrierRun implements Runnable {
boolean flag;
int num;
public BarrierRun(boolean flag, int num) {
this.flag = flag;
this.num = num;
}
@Override
public void run() {
if (flag) {
System.out.println("司令:[士兵" + num + "个,任务完成!]");
} else {
System.out.println("司令:[士兵" + num + "个,集合完毕!]");
flag = true;
}
}
}
public static void doTask(int i) {
System.out.println("士兵" + i + "报道! ");
}
public static void main(String[] args) {
final int soldierNum = 10;
Thread[] allSoldier = new Thread[soldierNum];
boolean flag = false;
CyclicBarrier cyclic = new CyclicBarrier(soldierNum, new BarrierRun(flag, soldierNum));
System.out.println("吹响号角，集合队伍! ");
for (int i = 0; i < soldierNum; i++) {
doTask(i);
allSoldier[i] = new Thread(new Soldier(cyclic, "士兵" + i));
allSoldier[i].start();
}
}
}