14.10 同步器

Java.util.concurrent 包中有一些包含预支功能的线程集相关的类型

CyclicBarrier	公共栅栏，可以约定指定书目的线程到达指定起点后再执行
CountDownLatch	线程集等待知道计数器变0
Exchanger	交换两个线程对象
Semaphore	线程集等待直到允许继续运行
SynchronousQueue	线程把对象交给另一个线程

14.10.1 信号量

相当于有限的许可证，通过颁发指定书目的许可，限制可通过线程数量。任意线程可消耗许可，也可释放许可，无需保证获取者释放许可。

Semaphore semaphore = new Semaphore(10);//创建大小为10的信号量

semaphore.acquire();//获取许可，许可为0时阻塞

semaphore.release();//释放许可

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public class Main{
    public static void main(String[] args) throws InterruptedException {
        Main solution = new Main();
        Semaphore semaphore = new Semaphore(10);
        for(int i = 0; i < 50; i++){
            Thread t = new Thread(new A(semaphore,"number "+i));
            t.start();
        }
 
        Thread.sleep(8000);
    }
}
 
class A implements Runnable{
    private Semaphore semaphore;
    private String name;
    public A(Semaphore semaphore,String name){
        this.semaphore = semaphore;
        this.name = name;
    }
 
    @Override
    public void run() {
        try {
            semaphore.acquire();
            System.out.println(name+" is Running"+new Date().getTime()/1000);
            Thread.sleep(1000);
            semaphore.release();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

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运行结果：

每秒有10条同时运行的结果

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14.10.2 倒计时门栓

线程集等待计数，直到计数变为0，一旦计数为0，不可重用。

CountDownLatch c = new CountDownLatch(10);//创建大小为10的倒计时门栓

C.countDown();//倒计时计数减1

C.await();//等待，直到倒计时为0

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public class Main{
    public static void main(String[] args) throws InterruptedException {
        Main solution = new Main();
 
        CountDownLatch c = new CountDownLatch(10);
        for(int i = 0; i < 10; i++){
            Thread t = new Thread(new A(c,"number "+i));
            t.start();
            Thread.sleep(1000);
        }
 
 
    }
}
 
class A implements Runnable{
    private CountDownLatch c;
    private String name;
    public A(CountDownLatch c,String name){
        this.c = c;
        this.name = name;
    }
 
    @Override
    public void run() {
        try {
            c.countDown();
            c.await();
            System.out.println(name+" is Running"+System.currentTimeMillis());
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

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运行结果：

10个不同时启动的线程，可通过倒计时器同时开始执行

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