CountDownLatch使用场景

Java 理论与实践: 正确使用 Volatile 变量：http://www.ibm.com/developerworks/cn/java/j-jtp06197.html
聊聊并发（一）——深入分析Volatile的实现原理：http://www.infoq.com/cn/articles/ftf-java-volatile
深入理解Java内存模型（四）——volatile：http://www.infoq.com/cn/articles/java-memory-model-4/
Java的多线程机制系列：(四)不得不提的volatile及指令重排序(happen-before)：
http://www.cnblogs.com/mengheng/p/3495379.html

CountDownLatch同步化的一个辅助工具，允许一个或多个线程等待，直到所有线程中执行完成
比如主线程计算一个复杂的计算表达式，将表达式分为多个子表达式在线程中去计算，
主线程要计算表达式的最后值，必须等所有的线程计算完子表达式计算，方可计算表达式的值；
再比如一个团队赛跑游戏，最后要计算团队赛跑的成绩，主线程计算最后成绩，要等到所有
团队成员跑完，方可计算总成绩。使用情况两种：第一种，所有线程等待一个开始信息号，当开始信息号启动时，所有线程执行，等待所有线程执行完；第二种，所有线程放在线程池中，执行，等待所有线程执行完，方可执行主线程任务方可执行主线程任务。

测试有统一开始信号的情况：

package juc;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class RunnerGames {
	public static void main(String[] args) {
		   CountDownLatch startSignal = new CountDownLatch(1);
		   CountDownLatch doneSignal = new CountDownLatch(3);
		   ExecutorService  exec = Executors.newCachedThreadPool();
		   RunnableMan rm1 = new RunnableMan(startSignal,doneSignal, 1000);
		   RunnableMan rm2 = new RunnableMan(startSignal,doneSignal, 2000);
		   RunnableMan rm3 = new RunnableMan(startSignal,doneSignal, 3000);
		   Future<Integer> score1 = exec.submit(rm1);
		   Future<Integer> score2 = exec.submit(rm2);
		   Future<Integer> score3 = exec.submit(rm3);
		   System.out.println("开始赛跑......");
		   startSignal.countDown();
		   try {
			   doneSignal.await();
			} catch (InterruptedException e1) {
				e1.printStackTrace();
			}  
		   int sumScores =0;
		   try {
			   try {
				sumScores  = score1.get()+score2.get()+score3.get();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			} catch (ExecutionException e) {
				e.printStackTrace();
			}
		   System.out.println("团队赛跑结束,最后成绩为："+sumScores);
		   exec.shutdown();
		 }
}

package juc;

import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;

class RunnableMan implements Callable<Integer> {  
	  private final CountDownLatch startSignal;
	  private final CountDownLatch doneSignal;             
	  private final int i;                                 
	  RunnableMan(CountDownLatch startSignal,CountDownLatch doneSignal, int i) {   
		 this.startSignal = startSignal;
	     this.doneSignal = doneSignal;                     
	     this.i = i;                                       
	  }                                                    
	  public Integer call() {        
	     try {
	    	startSignal.await();
			doRun(i);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}                                      
	     doneSignal.countDown();
	     return new Integer(i);
	  }                                                    
	  void doRun(int i) throws InterruptedException { 
		  System.out.println("选手"+i/1000+"正在赛跑中........");
		  Thread.sleep(i*2);
	  }                             
}

                                                     

测试结果：
开始赛跑......
选手3正在赛跑中........
选手2正在赛跑中........
选手1正在赛跑中........
团队赛跑结束,最后成绩为：6000


测试第二种情况，没有开始信号

package juc;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;
/**AtomicInteger，一个提供原子操作的Integer的类。
在Java语言中，++i和i++操作并不是线程安全的，在使用的时候，
不可避免的会用到synchronized关键字。而AtomicInteger则通过一种线程安全的加减操作接口。*/
public class ShopTickets {
	private static volatile AtomicInteger tickets = new AtomicInteger(10);
	public static void main(String[] args) throws InterruptedException {
		  CountDownLatch doneSignal = new CountDownLatch(3);
		   Executor exec = Executors.newCachedThreadPool();
		   exec.execute(new TicketSales(doneSignal,"售票员1",tickets));
		   exec.execute(new TicketSales(doneSignal,"售票员2",tickets));
		   exec.execute(new TicketSales(doneSignal,"售票员3",tickets));
		   doneSignal.await();  
		   System.out.println("票已售完,所有售票员，停止售票");
		 }
}

package juc;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;

class TicketSales implements Runnable {  
	  private final CountDownLatch doneSignal;   
	  private final AtomicInteger tickets;
	  private String saleName;
	  
	  TicketSales(CountDownLatch doneSignal, String saleName,AtomicInteger tickets) { 
	     this.doneSignal = doneSignal;                   
	     this.saleName = saleName; 
	     this.tickets = tickets;
	  }         
	  public  void run() {                                
		  doSales(tickets);                                    
	  }                                                  
	  public  void  doSales(AtomicInteger tickets) { 
		   try {
				   while(tickets.get()>0){
					System.out.println(saleName+"卖完一张票，还有："+tickets.decrementAndGet()+"张");
					Thread.sleep(1000);
			   }
				doneSignal.countDown();
		  }catch (InterruptedException e) {
			e.printStackTrace();
		}
	  }                              
}                             
 

                     
测试结果:                                                                                                                                      
售票员1卖完一张票，还有：8张
售票员3卖完一张票，还有：7张
售票员2卖完一张票，还有：9张
售票员3卖完一张票，还有：5张
售票员1卖完一张票，还有：6张
售票员3卖完一张票，还有：3张
售票员2卖完一张票，还有：4张
售票员3卖完一张票，还有：1张
售票员1卖完一张票，还有：2张
售票员2卖完一张票，还有：0张
票已售完,所有售票员，停止售票
有时测试结果为：

售票员1卖完一张票，还有：9张
售票员2卖完一张票，还有：8张
售票员3卖完一张票，还有：7张
售票员2卖完一张票，还有：6张
售票员1卖完一张票，还有：5张
售票员3卖完一张票，还有：4张
售票员3卖完一张票，还有：2张
售票员2卖完一张票，还有：1张
售票员1卖完一张票，还有：3张
售票员1卖完一张票，还有：0张
售票员2卖完一张票，还有：-1张
票已售完,所有售票员，停止售票

把Thread.sleep(1000)这句去掉，票数为负数的概率较小，但还是会出现，
我测试了3分钟，出现2次。
关注这一句：
while(tickets.get()>0){
问题可能在tickets.get()的时候，票数不为零，在卖票的时候，其他线程已经在其基础上，修改
tickets，而当前线程tickets.get()已经做过判断，不为零，导致的错误。
再次修改：

package juc;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class ShopTickets2 {
	/**
	 * AtomicInteger，一个提供原子操作的Integer的类。 在Java语言中，++i和i++操作并不是线程安全的，在使用的时候，
	 * 不可避免的会用到synchronized关键字。而AtomicInteger则通过一种线程安全的加减操作接口。
	 */
	private static volatile AtomicInteger tickets = new AtomicInteger(10);
	public static void main(String[] args) throws InterruptedException {
		CountDownLatch doneSignal = new CountDownLatch(3);
		Executor exec = Executors.newCachedThreadPool();
		exec.execute(new TicketSales2(doneSignal, "售票员1", tickets));
		exec.execute(new TicketSales2(doneSignal, "售票员2", tickets));
		exec.execute(new TicketSales2(doneSignal, "售票员3", tickets));
		doneSignal.await();
		System.out.println("票已售完,所有售票员，停止售票");
	}
}

package juc;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;

class TicketSales2 implements Runnable { 
	  private static volatile AtomicBoolean isDoned = new AtomicBoolean(Boolean.TRUE);
	  private final CountDownLatch doneSignal;   
	  private final AtomicInteger tickets;
	  private String saleName;
	  
	  TicketSales2(CountDownLatch doneSignal, String saleName,AtomicInteger tickets) { 
	     this.doneSignal = doneSignal;                   
	     this.saleName = saleName; 
	     this.tickets = tickets;
	  }         
	  public  void run() {                                
		  doSales(tickets);                                    
	  }                                                  
	  public  void  doSales(AtomicInteger tickets) { 
		 while(isDoned.get()){
				System.out.println(saleName+"卖完一张票，还有："+tickets.decrementAndGet()+"张");
				if(tickets.get()==0){
					isDoned.compareAndSet(true, false);
				}
            }
		doneSignal.countDown();
	  }                              
}     

                                              
                                                    
测试：
售票员1卖完一张票，还有：9张
售票员3卖完一张票，还有：7张
售票员2卖完一张票，还有：8张
售票员3卖完一张票，还有：5张
售票员1卖完一张票，还有：6张
售票员3卖完一张票，还有：3张
售票员2卖完一张票，还有：4张
售票员2卖完一张票，还有：0张
售票员3卖完一张票，还有：1张
售票员1卖完一张票，还有：2张
票已售完,所有售票员，停止售票
不在出现上述票数为负的情况，关键是添加了所有线程都可见，且线程安全的余票状态
private static volatile AtomicBoolean isDoned = new AtomicBoolean(Boolean.TRUE);


下面来看使用同步锁的场景：

package juc;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class ShopTickets3 {
	/**
	 * AtomicInteger，一个提供原子操作的Integer的类。 在Java语言中，++i和i++操作并不是线程安全的，在使用的时候，
	 * 不可避免的会用到synchronized关键字。而AtomicInteger则通过一种线程安全的加减操作接口。
	 */
	private static volatile AtomicInteger tickets = new AtomicInteger(10);
	private static volatile Object saleLock = new Object();
	public static void main(String[] args) throws InterruptedException {
		CountDownLatch doneSignal = new CountDownLatch(3);
		Executor exec = Executors.newCachedThreadPool();
		exec.execute(new TicketSales3(doneSignal, "售票员1", tickets,saleLock));
		exec.execute(new TicketSales3(doneSignal, "售票员2", tickets,saleLock));
		exec.execute(new TicketSales3(doneSignal, "售票员3", tickets,saleLock));
		doneSignal.await();
		System.out.println("票已售完,所有售票员，停止售票");
	}
}

package juc;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;

class TicketSales3 implements Runnable {  
	  private final CountDownLatch doneSignal;   
	  private final AtomicInteger tickets;
	  private final Object saleLock;
	  private String saleName;
	  
	  TicketSales3(CountDownLatch doneSignal, String saleName,AtomicInteger tickets, Object saleLock) { 
	     this.doneSignal = doneSignal;                   
	     this.saleName = saleName; 
	     this.tickets = tickets;
	     this.saleLock = saleLock;
	  }         
	  public  void run() {                                
		  doSales(tickets);                                    
	  }                                                  
	  public  void  doSales(AtomicInteger tickets) { 
		 boolean flag = true;
		 while(flag){
			 synchronized(saleLock){
				   if(tickets.get()>0){
					   System.out.println(saleName+"卖完一张票，还有："+tickets.decrementAndGet()+"张");
					   saleLock.notifyAll();
				   }
				   else{
					   flag= false;
				   }
			   }
            }
		doneSignal.countDown();
	  }                              
}                                                    

                                  
控制台输出：
售票员1卖完一张票，还有：9张
售票员1卖完一张票，还有：8张
售票员1卖完一张票，还有：7张
售票员1卖完一张票，还有：6张
售票员1卖完一张票，还有：5张
售票员1卖完一张票，还有：4张
售票员1卖完一张票，还有：3张
售票员1卖完一张票，还有：2张
售票员3卖完一张票，还有：1张
售票员3卖完一张票，还有：0张
票已售完,所有售票员，停止售票
虽然也可以控制卖票，但只有两个售票员在售票，不能有效的利用资源，达到最大性能。
从上面可以看出，最有效方式为第二种。

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;

/**
 * A synchronization aid that allows one or more threads to wait until
 * a set of operations being performed in other threads completes.
 * 同步化的一个辅助工具，允许一个或多个线程等待，直到所有线程中执行完成
 *比如主线程计算一个复杂的计算表达式，将表达式分为多个子表达式在线程中去计算，
 *主线程要计算表达式的最后值，必须等所有的线程计算完子表达式计算，方可计算表达式的值；
 *再比如一个团队赛跑游戏，最后要计算团队赛跑的成绩，主线程计算最后成绩，要等到所有
 *团队成员跑完，方可计算总成绩。
 * <p>A {@code CountDownLatch} is initialized with a given [i]count[/i].
 * The {@link #await await} methods block until the current count reaches
 * zero due to invocations of the {@link #countDown} method, after which
 * all waiting threads are released and any subsequent invocations of
 * {@link #await await} return immediately.  This is a one-shot phenomenon
 * -- the count cannot be reset.  If you need a version that resets the
 * count, consider using a {@link CyclicBarrier}.
 *CountDownLatch有一个初始值，调用await的线程，要等待CountDownLatch值为0时，方可
 *往下执行
 * <p>A {@code CountDownLatch} is a versatile synchronization tool
 * and can be used for a number of purposes.  A
 * {@code CountDownLatch} initialized with a count of one serves as a
 * simple on/off latch, or gate: all threads invoking {@link #await await}
 * wait at the gate until it is opened by a thread invoking {@link
 * #countDown}.  A {@code CountDownLatch} initialized to [i]N[/i]
 * can be used to make one thread wait until [i]N[/i] threads have
 * completed some action, or some action has been completed N times.
 *
 * <p>A useful property of a {@code CountDownLatch} is that it
 * doesn't require that threads calling {@code countDown} wait for
 * the count to reach zero before proceeding, it simply prevents any
 * thread from proceeding past an {@link #await await} until all
 * threads could pass.
 *
 * <p><b>Sample usage:</b> Here is a pair of classes in which a group
 * of worker threads use two countdown latches:
 * [list]
 * <li>The first is a start signal that prevents any worker from proceeding
 * until the driver is ready for them to proceed;
 * <li>The second is a completion signal that allows the driver to wait
 * until all workers have completed.
 * [/list]
 *第一种，所有线程等待一个开始信息号，当开始信息号启动时，所有线程执行，等待所有线程执行完
 *方可执行主线程任务
 * <pre>
 * class Driver { // ...
 *   void main() throws InterruptedException {
 *     CountDownLatch startSignal = new CountDownLatch(1);
 *     CountDownLatch doneSignal = new CountDownLatch(N);
 *
 *     for (int i = 0; i < N; ++i) // create and start threads
 *       new Thread(new Worker(startSignal, doneSignal)).start();
 *
 *     doSomethingElse();  // don't let run yet
       //给所有线程一个开始信号，比如上面的团队赛发令枪
 *     startSignal.countDown();      // let all threads proceed
 *     doSomethingElse();
       //等待所有团队赛跑选手，释放跑完信号
 *     doneSignal.await();           // wait for all to finish
 *   }
 * }
 *
 * class Worker implements Runnable {
 *   private final CountDownLatch startSignal;
 *   private final CountDownLatch doneSignal;
 *   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
 *      this.startSignal = startSignal;
 *      this.doneSignal = doneSignal;
 *   }
 *   public void run() {
 *      try {
         //等待开始信息号
 *        startSignal.await();
         //赛跑
 *        doWork();
          //跑完，计时员收到跑完信号通知
 *        doneSignal.countDown();
 *      } catch (InterruptedException ex) {} // return;
 *   }
 *
 *   void doWork() { ... }
 * }
 *
 * </pre>
 *
 * <p>Another typical usage would be to divide a problem into N parts,
 * describe each part with a Runnable that executes that portion and
 * counts down on the latch, and queue all the Runnables to an
 * Executor.  When all sub-parts are complete, the coordinating thread
 * will be able to pass through await. (When threads must repeatedly
 * count down in this way, instead use a {@link CyclicBarrier}.)
 *所有线程放在线程池中，执行，等待所有线程执行完，方可执行主线程任务
 * <pre>
 * class Driver2 { // ...
 *   void main() throws InterruptedException {
 *     CountDownLatch doneSignal = new CountDownLatch(N);
 *     Executor e = ...
 *
 *     for (int i = 0; i < N; ++i) // create and start threads
 *       e.execute(new WorkerRunnable(doneSignal, i));
 *
 *     doneSignal.await();           // wait for all to finish
 *   }
 * }
 *
 * class WorkerRunnable implements Runnable {
 *   private final CountDownLatch doneSignal;
 *   private final int i;
 *   WorkerRunnable(CountDownLatch doneSignal, int i) {
 *      this.doneSignal = doneSignal;
 *      this.i = i;
 *   }
 *   public void run() {
 *      try {
 *        doWork(i);
 *        doneSignal.countDown();
 *      } catch (InterruptedException ex) {} // return;
 *   }
 *
 *   void doWork() { ... }
 * }
 *
 * </pre>
 *
 * <p>Memory consistency effects: Until the count reaches
 * zero, actions in a thread prior to calling
 * {@code countDown()}
 * [url=package-summary.html#MemoryVisibility]<i>happen-before</i>[/url]
 * actions following a successful return from a corresponding
 * {@code await()} in another thread.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CountDownLatch {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     *用一个 AQS状态代表信号量数
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }
        //获取信号量
        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }
       //释放信号量
        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

    /**
     * Constructs a {@code CountDownLatch} initialized with the given count.
     *
     * @param count the number of times {@link #countDown} must be invoked
     *        before threads can pass through {@link #await}
     * @throws IllegalArgumentException if {@code count} is negative
     */
    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
     *
     * <p>If the current count is zero then this method returns immediately.
     *
     * <p>If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of two things happen:
     * [list]
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * [/list]
     *
     * <p>If the current thread:
     * [list]
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
     * [/list]
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     * <p>If the current count is zero then this method returns immediately
     * with the value {@code true}.
     *
     * <p>If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of three things happen:
     * [list]
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>The specified waiting time elapses.
     * [/list]
     *
     * <p>If the count reaches zero then the method returns with the
     * value {@code true}.
     *
     * <p>If the current thread:
     * [list]
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
     * [/list]
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then the value {@code false}
     * is returned.  If the time is less than or equal to zero, the method
     * will not wait at all.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the {@code timeout} argument
     * @return {@code true} if the count reached zero and {@code false}
     *         if the waiting time elapsed before the count reached zero
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     *
     * <p>If the current count is greater than zero then it is decremented.
     * If the new count is zero then all waiting threads are re-enabled for
     * thread scheduling purposes.
     *
     * <p>If the current count equals zero then nothing happens.
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     * Returns the current count.
     *
     * <p>This method is typically used for debugging and testing purposes.
     *
     * @return the current count
     */
    public long getCount() {
        return sync.getCount();
    }

    /**
     * Returns a string identifying this latch, as well as its state.
     * The state, in brackets, includes the String {@code "Count ="}
     * followed by the current count.
     *
     * @return a string identifying this latch, as well as its state
     */
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }
}

评论

1 楼 di1984HIT 2017-07-31  

xuexile~~~