【1】CountDownLatch是什麼

CountDownLatch，英文翻譯為倒計時鎖存器。是一個同步輔助類，在完成一組正在其他執行緒中執行的操作之前，它允許一個或多個執行緒一直等待。

閉鎖可以延遲執行緒的進度直到其到達終止狀態，閉鎖可以用來確保某些活動直到其他活動都完成才繼續執行：

• 確保某個計算在其需要的所有資源都被初始化之後才繼續執行;
• 確保某個服務在其依賴的所有其他服務都已經啟動之後才啟動;
• 等待直到某個操作所有參與者都準備就緒再繼續執行。

CountDownLatch有一個正數計數器，countDown()方法對計數器做減操作，await()方法等待計數器達到0。所有await的執行緒都會阻塞直到計數器為0或者等待執行緒中斷或者超時。

如下示例，在多執行緒執行情況下，計算多執行緒耗費時間：

public class TestCountDownLatch {

    public static void main(String[] args){

        LatchDemo latchDemo = new LatchDemo();

        long begin = System.currentTimeMillis();
		//多執行緒
        for (int i = 0; i <5 ; i++) {
            new Thread(latchDemo).start();
        }
        //主執行緒
        long end = System.currentTimeMillis();
        
        System.out.println("耗費時間："+(end-begin));

    }
}

class LatchDemo implements  Runnable{

    @Override
    public void run() {

        for (int i = 0; i < 50000; i++) {
            if (i%2==0){
                System.out.println(i);
            }
        }

    }
}
 

如上示例，很顯然讓不能計算出多執行緒執行的時間！！這時，就可以使用 閉鎖解決這個問題。

【2】CountDownLatch原始碼分析與使用

CountDownLatch原始碼如下：

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.
 *//一種同步輔助工具，允許一個或多個執行緒等待，直到在其它執行緒中執行的一組操作完成。
 
 * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
 * // 一個CountDownLatch物件通常使用一個計數器(count)進行初始化。
 * 
 * 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. 
 * //await方法將會阻塞直到由於countDown方法執行將count (計數器)歸零，
 * //隨後將釋放所有等待的執行緒，await後面的方法將會立即返回。
 * 
 * This is a one-shot phenomenon-- the count cannot be reset.  
 * /*這是一次性的現象，不能重置計數。*/
 * If you need a version that resets the count, consider using a {@link CyclicBarrier}.
 */*如果你需要重置計數器，考慮使用CyclicBarrier--柵欄*/
 
 * <p>A {@code CountDownLatch} is a versatile synchronization tool
 * and can be used for a number of purposes.  
 * /*這個多功能的同步工具有許多作用*/
 * 
 * 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}. 
 * //以計數1初始化的{@code CountDownLatch}用作簡單的開/關鎖存器或閘門：
 * //呼叫{@linka wait a wait}的所有執行緒都在閘門等待，
 * //直到它被呼叫{@linkcountDown}的執行緒開啟為止。
 * 
 *  A {@code CountDownLatch} initialized to <em>N</em>
 * can be used to make one thread wait until <em>N</em> threads have
 * completed some action, or some action has been completed N times.
 *  /*初始化為N的{@code CountDownLatch}可用於使一個執行緒等待，*/
	/* 直到<em>N</em>執行緒已經完成某個操作，或者某個操作已經完成N次。*/
	
 * <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.
 */*一個有用特性是，它不要求呼叫{@code countDown}的執行緒在繼續之前等待計數達到零，*/
 /*它只是防止任何執行緒通過{@linka wait a wait}直到所有執行緒都可以通過。*/

//下面是使用開關鎖的一個簡單例子
 * <p><b>Sample usage:</b> 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>
 *

//另外一個典型應用
 * <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}.)
 *
 *  <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>
 *
 * <p>Memory consistency effects: Until the count reaches
 * zero, actions in a thread prior to calling
 * {@code countDown()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a successful return from a corresponding
 * {@code await()} in another thread.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CountDownLatch {
    /**
     * CountDownLatch的同步控制，使用AQS狀態呈現Count
     * 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}.
     *
     * <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:
     * <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>
     *
     * <p>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.
     *
     * <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:
     * <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>
     *
     * <p>If the count reaches zero then the method returns with the
     * value {@code true}.
     *
     * <p>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.
     *
     * <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));
    }

    /**
     * 將count-1 ，如果count減一後為0 ，則釋放所有等待的執行緒
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     返回當前的count，該方法通常用在debug或者測試中
     */
    public long getCount() {
        return sync.getCount();
    }

  
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }
}
 

簡單總結如下：

• 初始化使用計數器count；
• count代表多個執行緒執行或者某個操作執行次數；
• countDown()方法將會將count-1；
• count為0將會釋放所有等待執行緒；
• await方法將會阻塞直到count為0；
• 開關鎖應用；
• 問題分解應用。

修改【1】中程式碼如下：

public class TestCountDownLatch {

    public static void main(String[] args){
		//CountDownLatch 為唯一的、共享的資源
        final CountDownLatch latch = new CountDownLatch(5);
		
        LatchDemo latchDemo = new LatchDemo(latch);

        long begin = System.currentTimeMillis();

        for (int i = 0; i <5 ; i++) {
            new Thread(latchDemo).start();
        }
        try {
            //多執行緒執行結束前一直等待
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        long end = System.currentTimeMillis();
        
        System.out.println("耗費時間："+(end-begin));

    }
}

class LatchDemo implements  Runnable{

    private CountDownLatch latch;

    public LatchDemo(CountDownLatch latch){
        this.latch=latch;
    }
    public LatchDemo(){
        super();
    }

    @Override
    public void run() {
        //當前物件唯一，使用當前物件加鎖，避免多執行緒問題
        synchronized (this){
            try {
                for (int i = 0; i < 50000; i++) {
                    if (i%2==0){
                        System.out.println(i);
                    }
                }
            }finally {
                //保證肯定執行
                latch.countDown();
            }
        }
    }
}

測試結果如下圖：