java并发之同步辅助类CountDownLatch

CountDownLatch

含义：

CountDownLatch可以理解为一个计数器在初始化时设置初始值，当一个线程需要等待某些操作先完成时，需要调用await()方法。这个方法让线程进入休眠状态直到等待的所有线程都执行完成。每调用一次countDown()方法内部计数器减1,直到计数器为0时唤醒。这个可以理解为特殊的CyclicBarrier。线程同步点比较特殊，为内部计数器值为0时开始。

方法：

核心方法两个：countDown()和await()

countDown():使CountDownLatch维护的内部计数器减1,每个被等待的线程完成的时候调用

await():线程在执行到CountDownLatch的时候会将此线程置于休眠

例子

开会的例子：会议室里等与会人员到齐了会议才能开始。

import java.util.concurrent.CountDownLatch;

public class VideoConference implements Runnable {

private final CountDownLatch controller;

public VideoConference(int number) {

controller = new CountDownLatch(number);

}

public void arrive(String name) {

System.out.printf("%s has arrived.\n", name);

controller.countDown();// 调用countDown()方法，使内部计数器减1

System.out.printf("VideoConference: Waiting for %d participants.\n", controller.getCount());

}

@Override

public void run() {

System.out.printf("VideoConference: Initialization: %d participants.\n", controller.getCount());

try {

controller.await();// 等待，直到CoutDownLatch计数器为0

System.out.printf("VideoConference: All the participants have come\n");

System.out.printf("VideoConference: Let's start...\n");

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}

参加会议人员类

import java.util.concurrent.Semaphore;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.locks.Lock;

import java.util.concurrent.locks.ReentrantLock;

public class PrintQueue {

//信号量

private Semaphore semaphore;

//是否空闲打印机

private boolean freePrinters[];

private Lock lockPrinters;

public PrintQueue(){

//初始化三个信号

semaphore=new Semaphore(3);

//三台空闲打印机

freePrinters=new boolean[3];

for (int i=0; i<3; i++){

freePrinters[i]=true;

}

lockPrinters=new ReentrantLock();

}

public void printJob (Object document){

try {

//获取信号量

semaphore.acquire();

int assignedPrinter=getPrinter();

Long duration=(long)(Math.random()*10);

System.out.printf("%s: PrintQueue: Printing a Job in Printer %d during %d seconds\n",Thread.currentThread().getName(),assignedPrinter,duration);

TimeUnit.SECONDS.sleep(duration);

freePrinters[assignedPrinter]=true;

} catch (InterruptedException e) {

e.printStackTrace();

} finally {

// Free the semaphore

semaphore.release();

}

}

private int getPrinter() {

int ret=-1;

try {

lockPrinters.lock();

for (int i=0; i<freePrinters.length; i++) {

if (freePrinters[i]){

ret=i;

freePrinters[i]=false;

break;

}

}

} catch (Exception e) {

e.printStackTrace();

} finally {

lockPrinters.unlock();

}

return ret;

}

}

测试类：

public class CountDownLatchMain {

public static void main(String[] args) {

VideoConference conference = new VideoConference(10);

Thread threadConference = new Thread(conference);

threadConference.start();// 开启await()方法，在内部计数器为0之前线程处于等待状态

for (int i = 0; i < 10; i++) {

Participant p = new Participant(conference, "Participant " + i);

Thread t = new Thread(p);

t.start();

}

}

}