描述

本文使用ReentrantLock和CountDownLatch演示獨占鎖和共享鎖的實現(xiàn)误债。

AQS屬性
Node head
Node tail
int status
Thread exclusiveOwnerThread

AQS內部Node屬性
int waitStatus
Node prev
Node next
Thread thread
Node nextWaiter

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獨占鎖
獲取鎖

• 1線程先獲取鎖，設置status和exclusiveOwnerThread辉哥。1線程獲取鎖
• 2線程去獲取鎖吞加，發(fā)現(xiàn)鎖已被使用灼芭，只能加入等待隊列。隊列先初始化一個head節(jié)點，然后將2線程封裝成Node碰酝，拼接到head的next節(jié)點上全庸，也就是tail節(jié)點纷宇。
• 3線程去獲取鎖，也加入等待隊列顷编。將3線程封裝成Node插入到隊列尾部暖释，也就是tail袭厂。

釋放鎖

• 1線程執(zhí)行完，設置status和exclusiveOwnerThread為初始值球匕，釋放鎖纹磺。并叫醒head的next節(jié)點。
• 2線程被喚醒亮曹，設置status和exclusiveOwnerThread橄杨。并將自己所在的Node的thread和prev設置為null，提升為head

共享鎖
通過status標識鎖

獨占鎖

ReentrantLock使用排他鎖照卦。AQS的status>0表示加鎖式矫，thread是當前獲取鎖的線程。該鎖時可重入鎖役耕，所以status>0采转。

public static void main(String[] args) {
    final ExecutorService pool = Executors.newFixedThreadPool(3);
    final X x = new X();
    for (int i = 0; i < 3; i++) {
        pool.execute(x::m);
    }
    pool.shutdown();

}
static class X {
    private final ReentrantLock lock = new ReentrantLock();
    private int i;
    void m() {
        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + i++);
        } finally {
            lock.unlock();
        }
    }
}

共享鎖

CountDownLatch使用共享鎖。AQS的status為共享鎖的標記位瞬痘，status>0就是加鎖故慈，等于0就是釋放鎖。每調用一次countDown()框全，status減1察绷。
線程會阻塞在await()，直到countDown()將status置為0

public static void main(String[] args) {
    final int down = 3;
    final CountDownLatch start = new CountDownLatch(1);
    final CountDownLatch count = new CountDownLatch(down);

    final ExecutorService pool = Executors.newFixedThreadPool(down);
    for (int i = 0; i < down; i++) {
        pool.execute(new WorkerRunnable(count, start, i));
    }
    count.countDown();
    pool.shutdown();
}

static class WorkerRunnable implements Runnable {
    private final CountDownLatch doneSignal;
    private final CountDownLatch startSignal;
    private final int i;

    WorkerRunnable(CountDownLatch doneSignal, CountDownLatch startSignal, int i) {
        this.startSignal = startSignal;
        this.doneSignal = doneSignal;
        this.i = i;
    }

    public void run() {
        try {
            startSignal.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(i);
        doneSignal.countDown();
    }
}