java并发CountDownLatch

Posted 2022-12-03 Summer-Zheng

【应用场景】

       有这样一种业务需求：需要几个线程完成之后，主线程才能继续。举个例子：车辆出厂，一辆车出厂，需要生产各个配件，并且安装，都完成后还需要质检，最后才可以出厂。类似这样的需求，我们可以考虑使用CountDownLatch来控制并发。

【CountDownLatch】

       java.util.concurrent包有这样一个类，类似计数器，其内部维护着一个加锁的count计数，CountDownLatch通过构造函数传入一个初始计数值，调用cutDown()方法使count减1；调用await()方法，则线程就会一直阻塞，直到其他线程通过cutDown方法，将计数减到0，才可以继续执行。

public class CountDownLatch     
    private static final class Sync extends AbstractQueuedSynchronizer 
        private static final long serialVersionUID = 4982264981922014374L;
        Sync(int count) 
            setState(count);
        
        int getCount() 
            return getState();
        
        protected int tryAcquireShared(int acquires) 
            return (getState() == 0) ? 1 : -1;
        
        protected boolean tryReleaseShared(int releases) 
            for (;;) 
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            
        
    
    private final Sync sync;
    public CountDownLatch(int count) 
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
      
    public void await() throws InterruptedException 
        sync.acquireSharedInterruptibly(1);
      
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException 
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    
    public void countDown() 
        sync.releaseShared(1);
    
    public long getCount() 
        return sync.getCount();
    
    public String toString() 
        return super.toString() + "[Count = " + sync.getCount() + "]";
    

【应用实现】

package zheng;

import java.util.concurrent.CountDownLatch;

public class test 
	public enum EnumTest 
		发动机, 底盘, 车身, 其他;// 线程名称
	

	public static void main(String[] args) 
		final CountDownLatch countDownLatch = new CountDownLatch(3);
		new Thread(new Runnable() 
			@Override
			public void run() 
				System.out.println("主线程:正在等待车辆质检");
				try 
					countDownLatch.await();
					System.out.println("发动机、底盘、车身、其他,已配置好, 车辆出厂啦");
				 catch (InterruptedException e) 
					e.printStackTrace();
				
			
		).start();
		for (EnumTest threadName : EnumTest.values()) 
			final String tN = String.valueOf(threadName);
			new Thread(new Runnable() 
				@Override
				public void run() 
					System.out.println(tN + " 正在配置");
					try 
						Thread.sleep(100);
					 catch (Exception e) 
						e.printStackTrace();
					
					System.out.println(tN + " 安装完成");
					countDownLatch.countDown();
				
			).start();
		

	

运行结果：