java多線程編程(4)CountDownLatch,CyclicBarrier,Semaphore的舉例使用

前言

這3個組件具有相似的用途

CountDownLatch倒數鎖

await()方法的線程會被掛起，它會等待直到count值爲0才繼續執行，也提供有超時時間的await()方法，當超過這個時間還在等待會拋出異常
countDown()將count值減1

import java.util.concurrent.CountDownLatch;

public class Test16 extends Thread{
	//如果把3改爲4，可以看到3個線程都等待在那裏了
    public static CountDownLatch countDownLatch = new CountDownLatch(3);
    public void run(){
        System.out.println("thread"+Thread.currentThread().getName()+"wait");	
        //countDownLatch-1，減少一個倒數
        countDownLatch.countDown();
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("thread"+Thread.currentThread().getName()+"again");
    }
    public static void main(String[] args) {
        Test16 test1 = new Test16();
        Test16 test2 = new Test16();
        Test16 test3 = new Test16();
        test1.setName("test1");
        test2.setName("test2");
        test3.setName("test3");
        test1.start();
        test2.start();
        test3.start();
    }
}

CyclicBarrier循環屏障

類似於CountDownLatch的複用版本，可以重置屏障進行再次多個線程的等待
await()方法的線程告訴CyclicBarrier自己已經到達同步點，然後當前線程被阻塞。直到n（設定好）個參與線程調用了await方法，同樣提供帶超時時間的await和不帶超時時間的await方法

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class Test17 extends Thread{
    public static CyclicBarrier cyclicBarrier = new CyclicBarrier (3);
    public void run(){
        System.out.println("thread"+Thread.currentThread().getName()+"start");
            try {
                cyclicBarrier.await();
            } catch (BrokenBarrierException | InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("thread"+Thread.currentThread().getName()+"again");
    }
    public static void main(String[] args) {
        Test17 test1 = new Test17();
        Test17 test2 = new Test17();
        Test17 test3 = new Test17();
        test1.setName("test1");
        test2.setName("test2");
        test3.setName("test3");
        test1.start();
        test2.start();
        test3.start();

        try {

            Thread.sleep(2000);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }
        //重置了屏障，又需要3次等待
        System.out.println("準備開始下一輪");
        cyclicBarrier.reset();

        Test17 test4 = new Test17();
        Test17 test5 = new Test17();
        Test17 test6 = new Test17();
        test4.setName("test4");
        test5.setName("test5");
        test6.setName("test6");
        test4.start();
        test5.start();
        test6.start();
    }
}

Semaphore信號量

void acquire():從此信號量獲取一個許可，在提供一個許可前一直將線程阻塞，否則線程被中斷。
void release():釋放一個許可，將其返回給信號量。
int availablePermits():返回此信號量中當前可用的許可數。
boolean hasQueuedThreads():查詢是否有線程正在等待獲取。

import java.util.concurrent.Semaphore;

public class Test18 extends Thread{
    public static Semaphore semaphore = new Semaphore(3);
    public void run(){
        while (true){
            try {
                semaphore.acquire();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("thread"+Thread.currentThread().getName()+"acquire begin");
            System.out.println("thread"+Thread.currentThread().getName()+"release");
            semaphore.release();

            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }
    public static void main(String[] args) {
        Test18 test1 = new Test18();
        Test18 test2 = new Test18();
        Test18 test3 = new Test18();
        Test18 test4 = new Test18();
        Test18 test5 = new Test18();
        Test18 test6 = new Test18();
        test4.setName("test4");
        test5.setName("test5");
        test6.setName("test6");
        test1.setName("test1");
        test2.setName("test2");
        test3.setName("test3");
        test1.start();
        test2.start();
        test3.start();
        test4.start();
        test5.start();
        test6.start();
    }
}