CountDownLatch的使用
concurrent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。
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();
}
}
为什么不行呢？
为什么不行呢？
使用System.nanoTime()试试吧...
为什么不行呢？
使用System.nanoTime()试试吧...
System.out.println()是main的线程 不受ExecutorService 管理,他们互补干扰,package methodtest;import java.util.Random;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.TimeUnit;public class CountDonwLatchTest {public static void main(String[] a) {long startTime = System.currentTimeMillis();ExecutorService executor = Executors.newCachedThreadPool();CountDownLatch latch = new CountDownLatch(3);CountDownLatch doneLatch = new CountDownLatch(1);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, doneLatch);executor.execute(w3);executor.execute(w2);executor.execute(w1);executor.execute(boss);executor.shutdown();try {doneLatch.await();} catch (InterruptedException e) {e.printStackTrace();}System.out.println(System.currentTimeMillis() - startTime);}}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!");}}class Boss implements Runnable {private CountDownLatch downLatch;private CountDownLatch doneLatch;public Boss(CountDownLatch downLatch,CountDownLatch doneLatch) {this.downLatch = downLatch;this.doneLatch = doneLatch;}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！");doneLatch.countDown();}}
