java併發之同步輔助類CountDownLatch

CountDownLatch

含義：

CountDownLatch能夠理解爲一個計數器在初始化時設置初始值，當一個線程須要等待某些操做先完成時，須要調用await()方法。這個方法讓線程進入休眠狀態直到等待的全部線程都執行完成。每調用一次countDown()方法內部計數器減1,直到計數器爲0時喚醒。這個能夠理解爲特殊的CyclicBarrier。線程同步點比較特殊，爲內部計數器值爲0時開始。

 

方法：

核心方法兩個：countDown()和await()

countDown():使CountDownLatch維護的內部計數器減1,每一個被等待的線程完成的時候調用

await():線程在執行到CountDownLatch的時候會將此線程置於休眠

 

例子

開會的例子：會議室裏等與會人員到齊了會議才能開始。

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();// 調用countDown()方法，使內部計數器減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計數器爲0

 

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

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

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}

 

參加會議人員類

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 {

//信號量

    private Semaphore semaphore;

    

    //是否空閒打印機

    private boolean freePrinters[];

    

    private Lock lockPrinters;

    

    public PrintQueue(){

     //初始化三個信號

        semaphore=new Semaphore(3);

        //三臺空閒打印機

        freePrinters=new boolean[3];

        for (int i=0; i<3; i++){

            freePrinters[i]=true;

        }

        lockPrinters=new ReentrantLock();

    }

    

    public void printJob (Object document){

        try {

         //獲取信號量

            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;

    }

}

 

 

測試類：

 

public class CountDownLatchMain {

 

public static void main(String[] args) {

VideoConference conference = new VideoConference(10);

Thread threadConference = new Thread(conference);

threadConference.start();// 開啓await()方法，在內部計數器爲0以前線程處於等待狀態

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

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

Thread t = new Thread(p);

t.start();

}

}

 

}

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