并发编程之基础( 四)

前面已经把并发编程的基础知识讲的差不多了，这章主要介绍一下JAVA中其它一些关于并发编程的类库,主要有一下几个类库。

• CountDownLatch
• CyclicBarrier
• BlockingQueue
• ScheduleExecutor
• Semaphore
• Exchanger

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1. CountDownLatch

　　该类主要是同步一个或多个任务，强制一个或多个任务等待其它任务执行的一组操作完成。可以给该对象设置一个初始计数值，当计数值不为0时，调用该对象的await()方法就会阻塞，调用counDown()方法会让计数值减1，当计数值为0时阻塞任务会被唤醒。其典型用法就是将一个程序分成多个独立的任务，并给CountDownLatch设定一个初始值，该初始值应该为首先需要执行的线程的个数(比如赛跑，5个运动员都做好准备之后，裁判才能打枪，这时初始值应该设置为5)。一些任务需要等待其它任务先完成或者其它任务的一部分完成，那么可以待用await()将自己挂起。而另一些任务的某些操作完成时调用countDown()方法来减小计数值，等待计数值为0时，挂起的任务则则认为当前所有的条件以满足继续执行的需要了，则可以继续运行。注意：计数值只能被设置一次且在new的时候就要指定初值，而且该对象只能使用一次，如果想重复使用，请考虑CyclicBarrier

 1 package com.dy.xidian;
 2 
 3 import java.util.Random;
 4 import java.util.concurrent.CountDownLatch;
 5 import java.util.concurrent.ExecutorService;
 6 import java.util.concurrent.Executors;
 7 import java.util.concurrent.TimeUnit;
 8 
 9 class TaskPortion implements Runnable {
10     private static int counter = 0;
11     private final int id = counter++;
12     private static Random rand = new Random(47);
13     private final CountDownLatch latch;
14 
15     public TaskPortion(CountDownLatch latch) {
16         super();
17         this.latch = latch;
18     }
19 
20     @Override
21     public void run() {
22         try {
23             doWork();
24             latch.countDown();
25         } catch (InterruptedException e) {
26         }
27 
28     }
29 
30     public void doWork() throws InterruptedException {
31         TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000));
32         System.out.println(this + "completed");
33     }
34 
35     public String toString() {
36         return String.format("%1$-3d", id);
37     }
38 }
39 
40 class WaitingTask implements Runnable {
41     private static int counter = 0;
42     private final int id = counter++;
43     private final CountDownLatch latch;
44 
45     public WaitingTask(CountDownLatch latch) {
46         super();
47         this.latch = latch;
48     }
49 
50     @Override
51     public void run() {
52         try {
53             latch.await();
54             System.out.println("Latch barrier passed for " + this);
55         } catch (InterruptedException e) {
56             System.out.println(this + " interrupted");
57         }
58     }
59 
60     public String toString() {
61         return String.format("WaitingTask %1$-3d ", id);
62     }
63 }
64 public class CountDownLatchDemo {
65     static final int SIZE = 100;
66     public static void main(String[] args) {
67         ExecutorService exec = Executors.newCachedThreadPool();
68         CountDownLatch latch = new CountDownLatch(SIZE);
69         for (int i = 0; i < 10; i++)
70             exec.execute(new WaitingTask(latch));
71         for (int i = 0; i < SIZE; i++)
72             exec.execute(new TaskPortion(latch));
73         System.out.println("Launched all tasks");
74         exec.shutdownNow();
75     }
76 }

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