测试代码
/**
* CountDownLatch
* CyclicBarrier 区别
*
* 网上说 CyclicBarrier 与 CountDownLatch 区别只是一个能重置一个不行
* 其实看过源码不止区别在此
*
* 调用
* countDownLatch.countDown();
* 线程会直接执行完，计数器减1
*
* countDownLatch.await(); 只会阻塞当前线程
*
* 而
* 调用
* cyclicBarrier.await();
* 线程会阻塞，直到最后一个线程也调用 await(); 时，才能全部释放
* 这时候会有个 惊群 的问题出现
*
* 所以
* 使用 countDownLatch 会比较好
*
* Created by zhuangjiesen on 2018/1/24.
*/
public class CountDownLatchAndBarrierTest {
public static void main(String[] args) {
//保活
Thread t4 = new Thread(new Runnable() {
@Override
public void run() {
while(true) {
ThreadHelper.sleep(300);
}
}
});
t4.start();
// testCountDownLatch();
testCyclicBarrier();
}
public static void testCountDownLatch(){
final CountDownLatch countDownLatch = new CountDownLatch(4);
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(" t1 start ....");
countDownLatch.countDown();
System.out.println(" t1 await over ....");
ThreadHelper.sleep(300);
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(" t2 start ....");
countDownLatch.countDown();
System.out.println(" t2 await over ....");
ThreadHelper.sleep(300);
}
});
Thread t3 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(" t3 start ....");
countDownLatch.countDown();
System.out.println(" t3 await over ....");
ThreadHelper.sleep(300);
}
});
t1.start();
t2.start();
t3.start();
}
public static void testCyclicBarrier(){
final CyclicBarrier cyclicBarrier = new CyclicBarrier(4);
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(" t1 start ....");
try {
cyclicBarrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println(" t1 await over ....");
ThreadHelper.sleep(300);
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(" t2 start ....");
try {
cyclicBarrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println(" t2 await over ....");
ThreadHelper.sleep(300);
}
});
Thread t3 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(" t3 start ....");
try {
cyclicBarrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println(" t3 await over ....");
ThreadHelper.sleep(300);
}
});
t1.start();
t2.start();
t3.start();
}
}
运行结果：
testCyclicBarrier();
t1 start ....
t3 start ....
t2 start ....
testCountDownLatch();
t1 start ....
t3 start ....
t2 start ....
t3 await over ....
t1 await over ....
t2 await over ....
结论：
CyclicBarrier 源码是通过 lock / condition
signal 和 await 的， 所以它实现的效果是阻塞当前线程
CountDownLatch 源码是设计一个 继承 aqs 的内部类，
设置 state 的值
当 countDown(); 时，是去 releaseShared(); 释放锁，
然后当state 等于 0 时，就唤醒await(); 等待的线程；
所以 CountDownLatch.countDown(); 时，线程是直接继续执行的，所以也不用进行异常处理
