【背景】今天遇到一个并发问题，为了在开发环境复现这个bug，需要让多个线程同时执行到某条语句。

【解决方案】

java1.5的concurrent包下的CyclicBarrier 和 CountDownLatch都能解决这个问题。不得不佩服Doug Lea！

【方法1】使用CyclicBarrier

public class TestCyclicBarrier {

class Worker implements Runnable{

CyclicBarrier cyclicBarrier;

public Worker(CyclicBarrier cyclicBarrier){

this.cyclicBarrier = cyclicBarrier;

}

@Override

public void run() {

try {

cyclicBarrier.await(); // 等待其它线程

System.out.println(Thread.currentThread().getName() + "启动@" + System.currentTimeMillis());

} catch (InterruptedException e) {

e.printStackTrace();

} catch (BrokenBarrierException e) {

e.printStackTrace();

}

}

}

public void doTest() throws InterruptedException {

final int N = 5; // 线程数

CyclicBarrier cyclicBarrier = new CyclicBarrier(N);

for(int i=0;i

new Thread(new Worker(cyclicBarrier)).start();

}

}

public static void main(String[] args) throws InterruptedException {

TestCyclicBarrier testCyclicBarrier = new TestCyclicBarrier();

testCyclicBarrier.doTest();

}

}

【方法1结果】

【方法2】使用CountDownLatch

package thread;

import java.util.concurrent.CountDownLatch;

public class TestCountDownLatch {

class Worker implements Runnable{

CountDownLatch countDownLatch;

Worker(CountDownLatch countDownLatch){

this.countDownLatch = countDownLatch;

}

@Override

public void run() {

try {

countDownLatch.await(); // 等待其它线程

System.out.println(Thread.currentThread().getName() + "启动@" + System.currentTimeMillis());

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}

public void doTest() throws InterruptedException {

final int N = 5; // 线程数

CountDownLatch countDownLatch = new CountDownLatch(N);

for(int i=0;i

new Thread(new Worker(countDownLatch)).start();

countDownLatch.countDown();

}

}

public static void main(String[] args) throws InterruptedException {

TestCountDownLatch testCountDownLatch = new TestCountDownLatch();

testCountDownLatch.doTest();

}

}

【方法2结果】