CountdownLatch的實現原理如下
設定建構函式的
【1】建構函式

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

【2】await 函式

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

public
 
 final void acquireSharedInterruptible(int arg)throw InterruptedException{
    if(Thread.interrupted()){
        throw new InterruptedException();
    }
    if(tryAcquired(arg)<0){//嘗試獲取共享鎖
         doAcquireSharedInterruptibly(arg);//實現執行緒的入列與阻塞  獲取共享鎖失敗
    }
}

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

private void doAcquireSharedInterruptibly(int arg)throws InterruptedException{
    final Node node = addWariter(Node.Shared);//新增至等待佇列
    boolean failed = true;
    try{
        for(;;){
            final Node p = node.predecessor();
            if(p==head){//前驅節點為頭節點
                int
 
 r = tryAcquireShared(arg);
                if(r>0){//獲取成功
                    //設定頭節點並進行反戰
                    setHeadAndPropagate(node,r);
                    //設定節點next 域
                    p.next = null;
                    failed = false;
                    return;
                }
            }
        }
    }

}

【3】countdown()方法
此函式將遞減鎖存器的計數，如果計數達到0，則釋放所有等待的執行緒。

public void countDown(){
    sync.releaseShared(1);//釋放共享鎖
}

releaseShared方法呼叫如下

//以共享模式釋放物件，並且函式種會呼叫到CountdownLatch的tryReleaseeShared函式
//並且可能會呼叫AQS的doreleaseShared函式，其中tryReleaseShared函式
public final boolean releaseShared(int arg){
    if(tryReleaseShared(arg)){//返回state減1以後,state==0的結果。如果是0，執行下面的程式碼
        doReleaseShared();//實現會喚醒第一個節點，第一個節點會喚醒第二個節點。
        return true;
    }
    return false;
}


//tryReleaseShared()的程式碼如下所示
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;//返回當前state與0相等的結果
            }
        }