CountDownLatch演示(陪老婆儿子买东西案例)

约定不等于承诺〃 2023-06-09 15:29 10阅读 0赞

# CountDownLatch是什么？ #

CountDownLatch是一个非常实用的多线程控制工具类，一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待

# CountDownLatch如何工作？ #

CountDownLatch有一个正数计数器，countDown()方法对计数器做减操作，await()方法等待计数器达到0。所有await的线程都会阻塞直到计数器为0或者等待线程中断或者超时

# CountDownLatch适用场景 #

*  当需要将任务分解的时候，可以用CountDownLatch来实现，每个任务完成，CountDownLatch计数减一，直到减为0,表示所有任务完成

# 示例演示 #

## 示例背景说明 ##

背景描述：我陪儿子和老婆一起逛街，着急想回家，老婆买衣服需要60秒，儿子要买玩具需要20秒，儿子买衣服要30秒

## 演示代码 ##

*  创建动作接口：买东西

public interface Buy { 
        void buy();
    }

*  创建任务类：儿子买东西

public class SonBuy implements Buy, Runnable { 
        private CountDownLatch countDownLatch;
    
        public SonBuy(CountDownLatch countDownLatch) { 
            this.countDownLatch = countDownLatch;
        }
    
        @Override
        public void buy() { 
            System.out.println("儿子买玩具，需要花费20秒");
            try { 
                TimeUnit.SECONDS.sleep(20);
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }finally { 
                System.out.println("儿子已经买好了玩具");
            }
    
            System.out.println("儿子买了玩具，还要买衣服，需要花费30秒");
            try { 
                TimeUnit.SECONDS.sleep(30);
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }finally { 
                System.out.println("儿子已经买好了衣服");
            }
            if (countDownLatch != null) { 
                countDownLatch.countDown();
            }
        }
    
        @Override
        public void run() { 
            this.buy();
        }
    }

*  创建任务类：老婆买东西

public class WifeBuy implements Buy, Runnable { 
        private CountDownLatch countDownLatch;
    
        public WifeBuy(CountDownLatch countDownLatch) { 
            this.countDownLatch = countDownLatch;
        }
    
        @Override
        public void buy() { 
            System.out.println("老婆买衣服，需要花60秒");
            try { 
                TimeUnit.SECONDS.sleep(60);
            } catch (InterruptedException e) { 
                e.printStackTrace();
            } finally { 
                System.out.println("老婆已经买好了衣服");
            }
            if (countDownLatch != null) { 
                countDownLatch.countDown();
            }
        }
    
        @Override
        public void run() { 
            this.buy();
        }
    }

*  测试1：演示不使用CountDownLatch的效果，看需要多久才能回家

public class Test1 { 
        public static void main(String[] args) { 
    
            // Test1用来演示不使用CountDownLatch的效果
            SonBuy sonBuy = new SonBuy(null);
            WifeBuy wifeBuy = new WifeBuy(null);
            long start = System.currentTimeMillis();
            sonBuy.buy();
            wifeBuy.buy();
    
            System.out.println("结束回家,耗费时间(秒):" + (System.currentTimeMillis() - start) / 1000);
    
    
            // 结论：
            // 不引入CountDownLatch的话，三个人一起去买东西，买了再去买另外一个人的，总共耗费60+20+30=110秒
        }
    }

测试结果：打印输出

儿子买玩具，需要花费20秒
    儿子已经买好了玩具
    儿子买了玩具，还要买衣服，需要花费30秒
    儿子已经买好了衣服
    老婆买衣服，需要花60秒
    老婆已经买好了衣服
    结束回家,耗费时间(秒):110

*  测试2：演示使用CountDownLatch的效果，看需要多久才能回家

public class Test2 { 
        public static void main(String[] args) { 
    
            CountDownLatch countDownLatch = new CountDownLatch(2);
            // Test2用来演示使用CountDownLatch的效果
            SonBuy sonBuy = new SonBuy(countDownLatch);
            WifeBuy wifeBuy = new WifeBuy(countDownLatch);
            // 规范要求不建议通过这种方式来创建线程池，要通过new的方式，这样可以明确的知道线程池的参数,这里为了演示方便，就这样使用了
            long start = System.currentTimeMillis();
            ExecutorService executorService = Executors.newFixedThreadPool(2);
            executorService.execute(sonBuy);
            executorService.execute(wifeBuy);
    
            // 他们分别去买东西了，我(主线程)要在这里等
            try { 
                countDownLatch.await();
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }
            System.out.println("结束回家,耗费时间(秒):" + (System.currentTimeMillis() - start) / 1000);
    
            // 结论：
            // 引入CountDownLatch之后，老婆儿子各自去买东西，谁买好了就到我（主线程）这里等着，
            // 总共耗费时间为最费时间的那个人消耗的时间，也就是老婆买衣服60秒
    
        }
    }

测试结果：打印输出

老婆买衣服，需要花60秒
    儿子买玩具，需要花费20秒
    儿子已经买好了玩具
    儿子买了玩具，还要买衣服，需要花费30秒
    儿子已经买好了衣服
    老婆已经买好了衣服
    结束回家,耗费时间(秒):60

`源码地址：`[https://gitee.com/mengbaoyuxuan/xbs-demo.git][https_gitee.com_mengbaoyuxuan_xbs-demo.git]

*  最后附上CountDownLatch的源码

/* * 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.AbstractQueuedSynchronizer;
 
 /** * A synchronization aid that allows one or more threads to wait until * a set of operations being performed in other threads completes. * * A {@code CountDownLatch} is initialized with a given count. * 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}. * * 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 N * can be used to make one thread wait until N threads have * completed some action, or some action has been completed N times. * * 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. * * Sample usage: Here is a pair of classes in which a group * of worker threads use two countdown latches: * <ul> * <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. * </ul> * * <pre> {@code * 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 * startSignal.countDown(); // let all threads proceed * doSomethingElse(); * 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 { * startSignal.await(); * doWork(); * doneSignal.countDown(); * } catch (InterruptedException ex) {} // return; * } * * void doWork() { ... } * }}</pre> * * 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}.) * * <pre> {@code * 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> * * Memory consistency effects: Until the count reaches * zero, actions in a thread prior to calling * {@code countDown()} * <a href="package-summary.html#MemoryVisibility">happen-before</a> * 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. */
 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) { 
 // 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}. * * If the current count is zero then this method returns immediately. * * 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: * <ul> * <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. * </ul> * * If the current thread: * <ul> * <li>has its interrupted status set on entry to this method; or * <li>is {@linkplain Thread#interrupt interrupted} while waiting, * </ul> * 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. * * If the current count is zero then this method returns immediately * with the value {@code true}. * * 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: * <ul> * <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. * </ul> * * If the count reaches zero then the method returns with the * value {@code true}. * * If the current thread: * <ul> * <li>has its interrupted status set on entry to this method; or * <li>is {@linkplain Thread#interrupt interrupted} while waiting, * </ul> * then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * * 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. * * 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. * * If the current count equals zero then nothing happens. */
 public void countDown() { 
 sync.releaseShared(1);
 }
 
 /** * Returns the current count. * * 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() + "]";
 }
 }

[https_gitee.com_mengbaoyuxuan_xbs-demo.git]: https://gitee.com/mengbaoyuxuan/xbs-demo.git