java编程思想笔记-并发之CountDownLatch

CountDownLatch使用简介

1.CountDownLatch用来同步一个或者多个任务，强制它们等待由其他任务执行的一组操作
2.CountDownLatch设定的初始值只能设置一次，不能重置，使用countDown()来减小这个值，如果希望这个值可以被重置可以使用CycliBarrier

CountDownLatch使用示例

class TaskPortion implements Runnable{
private static int counter=0;
private final int id=counter++;
private static Random rand=new Random(47);
private final CountDownLatch latch;
    TaskPortion(CountDownLatch latch){
this.latch=latch;
    }
    @Override
public void run() {
try {
            doWork();
//每调用一次就减小1
            latch.countDown();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
public void doWork() throws InterruptedException{
        TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000));
        System.out.println(this+"-completed");
    }
    @Override
public String toString() {
return String.format("%d$ ", id);
    }
}

class WaitingTask implements Runnable{
private static int counter=0;
private final int id=counter++;
private final CountDownLatch latch;
    WaitingTask(CountDownLatch latch){
this.latch=latch;
    }
    @Override
public void run() {
try {
//所有的WaitingTask都在这里阻塞，直到CountDownLatch中的值降为0
            latch.await();
            System.out.println("Latch barrier passed for "+this);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    @Override
public String toString() {
return String.format("waitingTask %d ", id);
    }
}

public class CountDownLatchDemo {
static final int SIZE=100;
public static void main(String[] args) {
        ExecutorService exec=Executors.newCachedThreadPool();
        CountDownLatch countDownLatch=new CountDownLatch(SIZE);
for (int i = 0; i < 10; i++) {
            exec.execute(new WaitingTask(countDownLatch));
        }
for (int i = 0; i < SIZE; i++) {
            exec.execute(new TaskPortion(countDownLatch));
        }
        System.out.println("Latched all tasks");
        exec.shutdown();
    }
}