CountDownLatch介紹

CountDownLatch是一個同步輔助類，在完成一組正在其他線程中執行的操作之前，它允許一個或多個線程一直等待。

uml類圖

CountDownLatch的數據結構很簡單，它是通過”共享鎖”實現的。它包含了sync對象，sync是Sync類型。Sync是實例類，它繼承于AQS。

CountDownLatch源碼分析

1. CountDownLatch(int count)

public CountDownLatch(int count) {if (count < 0) throw new IllegalArgumentException("count < 0");this.sync = new Sync(count); }

該函數是創建一個Sync對象，而Sync是繼承于AQS類。Sync構造函數如下：

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

setState()在AQS中實現，源碼如下：

protected final void setState(long newState) {state = newState; }

說明：在AQS中，state是一個private volatile long類型的對象。對于CountDownLatch而言，state表示的”鎖計數器“。CountDownLatch中的getCount()最終是調用AQS中的getState()，返回的state對象，即”鎖計數器“。

2.await()

public void await() throws InterruptedException {sync.acquireSharedInterruptibly(1); }

說明：該函數實際上是調用的AQS的acquireSharedInterruptibly(1);

AQS中的acquireSharedInterruptibly()的源碼如下：

public final void acquireSharedInterruptibly(long arg)throws InterruptedException {//如果當前線程是中斷狀態，則拋出異常InterruptedExceptionif (Thread.interrupted())throw new InterruptedException();//調用tryAcquireShared(arg)嘗試獲取共享鎖,嘗試成功則返回//否則就調用doAcquireSharedInterruptibly()if (tryAcquireShared(arg) < 0)doAcquireSharedInterruptibly(arg); }

tryAcquireShared()在CountDownLatch.java中被重寫，它的源碼如下：

protected int tryAcquireShared(int acquires) {return (getState() == 0) ? 1 : -1; }

如果”鎖計數器=0”，即鎖是可獲取狀態，則返回1；否則，鎖是不可獲取狀態，則返回-1。

doAcquireSharedInterruptibly()方法

private void doAcquireSharedInterruptibly(long arg)throws InterruptedException {// 創建"當前線程"的Node節點，且Node中記錄的鎖是"共享鎖"類型；并將該節點添加到CLH隊列末尾。final Node node = addWaiter(Node.SHARED);boolean failed = true;try {for (;;) {// 獲取上一個節點。// 如果上一節點是CLH隊列的表頭，則"嘗試獲取共享鎖"。final Node p = node.predecessor();if (p == head) {long r = tryAcquireShared(arg);if (r >= 0) {setHeadAndPropagate(node, r);p.next = null; // help GCfailed = false;return;}}// (上一節點不是CLH隊列的表頭) 當前線程一直等待，直到獲取到共享鎖。// 如果線程在等待過程中被中斷過，則再次中斷該線程(還原之前的中斷狀態)。if (shouldParkAfterFailedAcquire(p, node) &&parkAndCheckInterrupt())throw new InterruptedException();}} finally {if (failed)cancelAcquire(node);} }

doAcquireSharedInterruptibly()會使當前線程一直等待，直到當前線程獲取到共享鎖(或被中斷)才返回。跟共享鎖ReadLock的doAcquireShare()方法非常類似，這里就不進行贅述了。

3. countDown()

public void countDown() {sync.releaseShared(1); }

說明：該函數實際上調用releaseShared(1)釋放共享鎖。
releaseShared()在AQS中實現，源碼如下：

public final boolean releaseShared(int arg) {if (tryReleaseShared(arg)) {doReleaseShared();return true;}return false; }

說明：releaseShared()的目的是讓當前線程釋放它所持有的共享鎖。
它首先會通過tryReleaseShared()去嘗試釋放共享鎖。嘗試成功，則直接返回；嘗試失敗，則通過doReleaseShared()去釋放共享鎖。

tryReleaseShared()在CountDownLatch.java中被重寫，源碼如下：

protected boolean tryReleaseShared(int releases) {// Decrement count; signal when transition to zerofor (;;) {// 獲取“鎖計數器”的狀態int c = getState();if (c == 0)return false;// “鎖計數器”-1int nextc = c-1;// 通過CAS函數進行賦值。if (compareAndSetState(c, nextc))return nextc == 0;} }

說明：tryReleaseShared()的作用是釋放共享鎖，將“鎖計數器”的值-1。

總結：CountDownLatch是通過“共享鎖”實現的。在創建CountDownLatch中時，會傳遞一個int類型參數count，該參數是“鎖計數器”的初始狀態，表示該“共享鎖”最多能被count給線程同時獲取。當某線程調用該CountDownLatch對象的await()方法時，該線程會等待“共享鎖”可用時，才能獲取“共享鎖”進而繼續運行。而“共享鎖”可用的條件，就是“鎖計數器”的值為0！而“鎖計數器”的初始值為count，每當一個線程調用該CountDownLatch對象的countDown()方法時，才將“鎖計數器”-1；通過這種方式，必須有count個線程調用countDown()之后，“鎖計數器”才為0，而前面提到的等待線程才能繼續運行！

CountDownLatch的使用示例

import java.util.concurrent.CountDownLatch; import java.util.concurrent.CyclicBarrier;public class CountDownLatchTest1 {private static int LATCH_SIZE = 5;private static CountDownLatch doneSignal;public static void main(String[] args) {try {doneSignal = new CountDownLatch(LATCH_SIZE);// 新建5個任務for(int i=0; i<LATCH_SIZE; i++)new InnerThread().start();System.out.println("main await begin.");// "主線程"等待線程池中5個任務的完成doneSignal.await();System.out.println("main await finished.");} catch (InterruptedException e) {e.printStackTrace();}}static class InnerThread extends Thread{public void run() {try {Thread.sleep(1000);System.out.println(Thread.currentThread().getName() + " sleep 1000ms.");// 將CountDownLatch的數值減1doneSignal.countDown();} catch (InterruptedException e) {e.printStackTrace();}}} }

運行結果：

main await begin. Thread-0 sleep 1000ms. Thread-2 sleep 1000ms. Thread-1 sleep 1000ms. Thread-4 sleep 1000ms. Thread-3 sleep 1000ms. main await finished.

總結

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