Java 并发编程中的 CountDownLatch 锁用于多个线程同时开始运行或主线程等待子线程结束

时间:2024-06-26 14:07:50

Java 5 开始引入的 Concurrent 并发软件包里面的 CountDownLatch 其实可以把它看作一个计数器,只不过这个计数器的操作是原子操作,同时只能有一个线程去操作这个计数器,也就是同时只能有一个线程去减这个计数器里面的值。CountDownLatch的一个非常典型的应用场景是:有一个任务想要往下执行,但必须要等到其他的任务执行完毕后才可以继续往下执行。假如我们这个想要继续往下执行的任务调用一个CountDownLatch对象的await()方法,其他的任务执行完自己的任务后调用同一个CountDownLatch对象上的countDown()方法,这个调用await()方法的任务将一直阻塞等待,直到这个CountDownLatch对象的计数值减到0为止。 所以,CountDownLatch类可以用于控制多个线程同时开始运行,或者用于主线程等待所有子线程都结束。
举个例子,有三个工人在为老板干活,这个老板有一个习惯,就是当三个工人把一天的活都干完了的时候,他就来检查所有工人所干的活。记住这个条件:三个工人先全部干完活,老板才检查。所以在这里用Java代码设计两个类,Worker代表工人,Boss代表老板,具体的代码实现如下:

 public class Worker implements Runnable{   

     private CountDownLatch downLatch;
private String name; public Worker(CountDownLatch downLatch, String name){
this.downLatch = downLatch;
this.name = name;
} public void run() {
this.doWork();
try
{
TimeUnit.SECONDS.sleep(new Random().nextInt(10));
}catch(InterruptedException ie){
}
System.out.println(this.name + "活干完了!");
this.downLatch.countDown(); } private void doWork()
{
System.out.println(this.name + "正在干活!");
} } ================== public class Boss implements Runnable { private CountDownLatch downLatch; public Boss(CountDownLatch downLatch){
this.downLatch = downLatch;
} public void run() {
System.out.println("老板正在等所有的工人干完活......");
try {
this.downLatch.await();
} catch (InterruptedException e) {
}
System.out.println("工人活都干完了,老板开始检查了!");
} } ============================ public class CountDownLatchDemo { public static void main(String[] args) {
ExecutorService executor = Executors.newCachedThreadPool(); CountDownLatch latch = new CountDownLatch(3); Worker w1 = new Worker(latch,"张三");
Worker w2 = new Worker(latch,"李四");
Worker w3 = new Worker(latch,"王二"); Boss boss = new Boss(latch); executor.execute(w3);
executor.execute(w2);
executor.execute(w1);
executor.execute(boss); executor.shutdown();
}
}