多线程（八同步计数器-CyclicBarrier）

Posted 2021-12-19

CyclicBarrier-循环栅栏

线程到达栅栏时调用await方法被阻塞，只有线程数据达到栅栏设置的阈值时，栅栏放行，所有线程继续执行，此轮结束，栅栏进入下一轮。

案例：3个线程等待栅栏放行

Task

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

public class Task implements Runnable

    private CyclicBarrier cyclicBarrier;

    public Task(CyclicBarrier cyclicBarrier) 
        this.cyclicBarrier = cyclicBarrier;
    

    @Override
    public void run() 
        try 
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName() + "准备就绪");
            cyclicBarrier.await();
         catch (InterruptedException e) 
            System.out.println(Thread.currentThread().getName() + "中断了......");
         catch (BrokenBarrierException e) 
            System.out.println(Thread.currentThread().getName() + "抛出损坏异常....");
        
    

启动文件

import java.util.concurrent.CyclicBarrier;

public class Main 

    public static void main(String[] args) throws InterruptedException 

        System.out.println("主线程开始......");

        CyclicBarrier cb = new CyclicBarrier(3, new Runnable()

            @Override
            public void run() 
                System.out.println("子线程全部就位，开始执行制定任务......");
            
        );

        Thread t1 = new Thread(new Task(cb), "Thread-1");
        Thread t2 = new Thread(new Task(cb), "Thread-2");
        Thread t3 = new Thread(new Task(cb), "Thread-3");

        t1.start();
        t2.start();
        t3.start();

        Thread.sleep(2000);
        System.out.println("主线程执行完毕");
    

结果：

CyclicBarrier的异常

CyclicBarrier如果不能满足阈值，超时或者等待的线程被中断了，则所有等待线程都会抛出损坏异常，然后进入下一轮。

案例：损坏异常，假设阈值为4，已经有3个线程等待，其中一个中断。

Task代码不变

启动文件修改

import java.util.concurrent.CyclicBarrier;

public class Main 

    public static void main(String[] args) throws InterruptedException 

        System.out.println("主线程开始......");

        CyclicBarrier cb = new CyclicBarrier(4, new Runnable()

            @Override
            public void run() 
                System.out.println("子线程全部就位，开始执行制定任务......");
            
        );

        Thread t1 = new Thread(new Task(cb), "Thread-1");
        Thread t2 = new Thread(new Task(cb), "Thread-2");
        Thread t3 = new Thread(new Task(cb), "Thread-3");

        t1.start();
        t2.start();
        t3.start();
        Thread.sleep(1000);
        t1.interrupt();

        Thread.sleep(2000);
        System.out.println("主线程执行完毕");
    

结果：

源码分析:

CyclicBarrier基于ReentrantLock和Condition实现锁机制。

成员变量

await方法：

breakBarrier损坏方法：