Semaphore和Exchanger的使用

Semaphore

功能介绍

Semaphore 主要作用就是限制线程并发数量。
Semaphore 的构造函数中permits 可以控制最大并发数。每个线程可以acquire指定数量的 permit 。但是acquire n个 则需要释放n个。防止被阻塞

public class MySemaphore {
    // permits 控制最大许可数
    private Semaphore mySemaphore = new Semaphore(2);

    public void test(int permits){
        try {
            //获取锁 permits 表示该线程需要的许可数量 如果 当前许可数 > semaphore 的许可数量将会被阻塞
            mySemaphore.acquire(permits);
            System.out.println("acquire " + Thread.currentThread().getName() + " " + System.currentTimeMillis());
            Thread.sleep(3000);
            System.out.println("release " + Thread.currentThread().getName() + " " + System.currentTimeMillis());
            //释放锁 获取了n个许可 则需要释放n个许可
            mySemaphore.release(permits);
        }catch (Exception e){
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        MySemaphore mySemaphore = new MySemaphore();

        MyThread myThread1 = new MyThread(mySemaphore);
        MyThread myThread2 = new MyThread(mySemaphore);
        MyThread myThread3 = new MyThread(mySemaphore);
        myThread1.start();
        myThread2.start();
        myThread3.start();
    }

    @AllArgsConstructor
    @NoArgsConstructor
    public static class MyThread extends Thread{

        private MySemaphore mySemaphore;

        @Override
        public void  run(){
            mySemaphore.test(2);
        }
    }
}

使用acquireUninterruptibly() 可以防止 thread被 println

public class MySemaphore {
    // permits 控制最大许可数
    private Semaphore mySemaphore = new Semaphore(1);

    public void test(){
        try {
            //获取锁 permits 表示该线程需要的许可数量 如果 当前许可数 > semaphore 的许可数量将会被阻塞
            mySemaphore.acquireUninterruptibly();
            System.out.println("acquire " + Thread.currentThread().getName() + " " + System.currentTimeMillis());
//            Thread.sleep(3000);
            for (int i = 0; i < Integer.MAX_VALUE/50; i++){
                String str = new String();
                Math.random();
            }
            System.out.println("release " + Thread.currentThread().getName() + " " + System.currentTimeMillis());
            //释放锁 获取了n个许可 则需要释放n个许可
            mySemaphore.release();
        }catch (Exception e){
            System.out.println(Thread.currentThread().getName() + " interrupt");
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        MySemaphore mySemaphore = new MySemaphore();

        MyThread myThread1 = new MyThread(mySemaphore);
        MyThread myThread2 = new MyThread(mySemaphore);
        MyThread myThread3 = new MyThread(mySemaphore);
        myThread1.setName("myThread1");
        myThread1.start();
        myThread2.setName("myThread2");
        myThread2.start();
        myThread3.setName("myThread3");
        myThread3.start();
        Thread.sleep(1000);
        System.out.println("start interrupt a");
        myThread2.interrupt();
    }

    @AllArgsConstructor
    @NoArgsConstructor
    public static class MyThread extends Thread{

        private MySemaphore mySemaphore;

        @Override
        public void  run(){
            mySemaphore.test();
        }
    }
}

availablePermits() 获取当前可用的许可数
drainPermits() 立即返回可用的所有许可数，并且可讲许可数设置为0
hasQueuedThreads() 获取等待许可的线程数
hasQueuedThreads() 判断是都有线程在等待许可

可以 使用 tryAcquire 尝试获取许可 该方法可以保证线程不被阻塞。

应用场景

• 多进路-多处理-多出路（多线程并发）
• 多进路-单处理-多出路（多线程同时触发，串行运行）
• pool池 （限制线程数量）
• 多生产者/多消费者

Exchanger

功能介绍

Exchanger 可以使2个线程之间传输数据，比生产者/消费者更加方便。

public class MyExchanger {

    @AllArgsConstructor
    @NoArgsConstructor
    public static class MyThreadA extends Thread{

        private Exchanger<String> exchanger;

        @Override
        public void run(){
            try {
                System.out.println("A: " + exchanger.exchange("aaa"));
                System.out.println(Thread.currentThread().getName() + " end!");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    @AllArgsConstructor
    @NoArgsConstructor
    public static class MyThreadB extends Thread{

        private Exchanger<String> exchanger;

        @Override
        public void run(){
            try {
                System.out.println("B: " + exchanger.exchange("bbb"));
                System.out.println(Thread.currentThread().getName() + " end!");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        Exchanger<String> exchanger = new Exchanger<>();
        MyThreadA myThread = new MyThreadA(exchanger);
        myThread.start();
        MyThreadB myThreadB = new MyThreadB(exchanger);
        myThreadB.start();
        System.out.println("main end!");
    }
}