JAVA并发CountDownLatch、CyclicBarrier、Semaphore
2017-04-24 15:16
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CountDownLatch
一个同步辅助类,在完成一组正在其他线程中执行的操作之前,它允许一个或多个线程一直等待。主要方法
public CountDownLatch(int count);
public void countDown();
public void await() throws InterruptedException
构造方法参数指定了计数的次数
countDown方法,当前线程调用此方法,则计数减一
awaint方法,调用此方法会一直阻塞当前线程,直到计时器的值为0
例子
public class CountDownLatchDemo { final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); public static void main(String[] args) throws InterruptedException { CountDownLatch latch=new CountDownLatch(2);//两个工人的协作 Worker worker1=new Worker("zhang san", 5000, latch); Worker worker2=new Worker("li si", 8000, latch); worker1.start();// worker2.start();// latch.await();//等待所有工人完成工作 System.out.println("all work done at "+sdf.format(new Date())); } static class Worker extends Thread{ String workerName; int workTime; CountDownLatch latch; public Worker(String workerName ,int workTime ,CountDownLatch latch){ this.workerName=workerName; this.workTime=workTime; this.latch=latch; } public void run(){ System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date())); doWork();//工作了 System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date())); latch.countDown();//工人完成工作,计数器减一 } private void doWork(){ try { Thread.sleep(workTime); } catch (InterruptedException e) { e.printStackTrace(); } } } }
CyclicBarrier
CyclicBarrier初始化时规定一个数目,然后计算调用了CyclicBarrier.await()进入等待的线程数。当线程数达到了这个数目时,所有进入等待状态的线程被唤醒并继续。CyclicBarrier就象它名字的意思一样,可看成是个障碍, 所有的线程必须到齐后才能一起通过这个障碍。
CyclicBarrier初始时还可带一个Runnable的参数, 此Runnable任务在CyclicBarrier的数目达到后,所有其它线程被唤醒前被执行。
public class TestCyclicBarrier { private static final int THREAD_NUM = 5; public static class WorkerThread implements Runnable{ CyclicBarrier barrier; public WorkerThread(CyclicBarrier b){ this.barrier = b; } @Override public void run() { // TODO Auto-generated method stub try{ System.out.println("Worker's waiting"); //线程在这里等待,直到所有线程都到达barrier。 barrier.await(); System.out.println("ID:"+Thread.currentThread().getId()+" Working"); }catch(Exception e){ e.printStackTrace(); } } } /** * @param args */ public static void main(String[] args) { // TODO Auto-generated method stub CyclicBarrier cb = new CyclicBarrier(THREAD_NUM, new Runnable() { //当所有线程到达barrier时执行 @Override public void run() { // TODO Auto-generated method stub System.out.println("Inside Barrier"); } }); for(int i=0;i<THREAD_NUM;i++){ new Thread(new WorkerThread(cb)).start(); } } } /* 以下是输出: Worker's waiting Worker's waiting Worker's waiting Worker's waiting Worker's waiting Inside Barrier ID:12 Working ID:8 Working ID:11 Working ID:9 Working ID:10 Working */
Semaphore
Semaphore 直译是信号量,可能称它是许可量更容易理解。当然,因为在计算机科学中这个名字由来已久,所以不能乱改。它的功能比较好理解,就是通过构造函数设定一个数量的许可,然后通过 acquire 方法获得许可,release 方法释放许可。它还有 tryAcquire 和 acquireUnintb64a
erruptibly 方法,可以根据自己的需要选择
class Pool { private static final int MAX_AVAILABLE = 100; private final Semaphore available = new Semaphore(MAX_AVAILABLE, true); public Object getItem() throws InterruptedException { available.acquire(); return getNextAvailableItem(); } public void putItem(Object x) { if (markAsUnused(x)) available.release(); } // Not a particularly efficient data structure; just for demo protected Object[] items = ... whatever kinds of items being managed protected boolean[] used = new boolean[MAX_AVAILABLE]; protected synchronized Object getNextAvailableItem() { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (!used[i]) { used[i] = true; return items[i]; } } return null; // not reached } protected synchronized boolean markAsUnused(Object item) { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (item == items[i]) { if (used[i]) { used[i] = false; return true; } else return false; } } return false; } }
总结下
CountDownLatch 是能使一组线程等另一组线程都跑完了再继续跑;CyclicBarrier 能够使一组线程在一个时间点上达到同步,可以是一起开始执行全部任务或者一部分任务。同时,它是可以循环使用的;Semaphore 是只允许一定数量的线程同时执行一段任务。相关文章推荐
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