java.util.concurrent.CountDownLatch是一个并发构造，它允许多个线程等候特定的操作完成。
CountDownLatch用一个数字初始化，通过调用countDown()方法来减低这个值。当一个线程调用await()方法后，它必须等候数值变成0以后才能往后继续执行。
调用await()方法会阻塞线程，直到这个数值变为0.
代码例子：
package com.dufeng.testjava.countdownlatch;
import java.util.concurrent.CountDownLatch;
public class TestCountDownLatch {
public static void main(String[] args) {
CountDownLatch latch = new CountDownLatch(3);
Waiter waiter = new Waiter(latch);
Decrementer decrementer = new Decrementer(latch);
new Thread(waiter).start();
new Thread(decrementer).start();
System.out.println("end");
}
}
package com.dufeng.testjava.countdownlatch;
import java.util.concurrent.CountDownLatch;
public class Waiter implements Runnable {
private CountDownLatch latch;
public Waiter(CountDownLatch latch) {
this.latch = latch;
}
@Override
public void run() {
try {
latch.await();
} catch (InterruptedException e) {
System.out.println(e.getMessage());
}
System.out.println("Waiter Released");
}
}
package com.dufeng.testjava.countdownlatch;
import java.util.concurrent.CountDownLatch;
public class Decrementer implements Runnable {
private CountDownLatch latch;
public Decrementer(CountDownLatch latch) {
this.latch = latch;
}
@Override
public void run() {
try {
Thread.sleep(1000);
latch.countDown();
System.out.println(latch.getCount());
Thread.sleep(1000);
latch.countDown();
System.out.println(latch.getCount());
Thread.sleep(1000);
latch.countDown();
System.out.println(latch.getCount());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
执行结果：
end
2
1
0
Waiter Released