一、SemaphoreDemo

Semaphore的应用场景就是加锁、异步、控制并发数

下面demo可以理解为抢夺共享资源

import java.util.concurrent.Semaphore;public class SemaphoreDemo {public static void main(String[] args) {String[] name = {"小胖", "大花", "大肥", "小桌子", "小椅子"};Semaphore semaphore = new Semaphore(3);for (int i = 0; i < 5; i++) {//五个人去争夺三个共享资源//五个人一放学就去网吧抢机子，网吧只有三台电脑Thread thread = new Thread(new Runnable() {@Overridepublic void run() {try {System.out.println(Thread.currentThread().getName() + "进入网吧");semaphore.acquire();System.out.println(Thread.currentThread().getName() + "抢到了电脑");Thread.sleep(5000);System.out.println(Thread.currentThread().getName() + "下机了");} catch (InterruptedException e) {e.printStackTrace();} finally {semaphore.release();}}}, name[i]);thread.start();}}
}

运行结果

抢不到电脑的要等别人下机之后才能上网

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二、加锁

在调用semaphore.acquire()方法后，进入InterruptedException()，然后点击进入tryAcquireShared()

public final void acquireSharedInterruptibly(int arg)throws InterruptedException {if (Thread.interrupted())throw new InterruptedException();if (tryAcquireShared(arg) < 0)doAcquireSharedInterruptibly(arg);}

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从tryAcquireShared()进入nonfairTryAcquireShared()

拿到state值后进行减一操作，然后CAS修改state值后返回

final int nonfairTryAcquireShared(int acquires) {for (;;) {int available = getState();int remaining = available - acquires;if (remaining < 0 ||compareAndSetState(available, remaining))return remaining;}}

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如果state值小于0进入doAcquireSharedInterruptibly()

1）addWaiter(Node.SHARED)方法将当前线程加入同步队列，是一个双向链表，如果初始化为空，则进入enq()；

2）enq()方法中：new Node()作为head，指向当前线程结点作为tail；

3）如果prev为head则进入tryAcquireShared()，然后尝试CAS修改state；

4）将waitStatus设置为-1并且LockSupport.unpark()阻塞线程

private void doAcquireSharedInterruptibly(int arg)throws InterruptedException {final Node node = addWaiter(Node.SHARED);boolean failed = true;try {for (;;) {final Node p = node.predecessor(); // 1）如果prev为head则进入tryAcquireShared()if (p == head) {int r = tryAcquireShared(arg); // 3）尝试CAS修改stateif (r >= 0) {setHeadAndPropagate(node, r);  p.next = null; // help GCfailed = false;return;}}if (shouldParkAfterFailedAcquire(p, node) &&  //4）将waitStatus设置为-1并且LockSupport.unpark()阻塞线程parkAndCheckInterrupt())throw new InterruptedException();}} finally {if (failed)cancelAcquire(node);}}

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for(;)循环尝试修改state，成功后进入setHeadAndPropagate()

设置当前node为head，如果当前waitStauts为-1，则判断下一结点是否为null或者new Node()，如果是则直接唤醒

private void setHeadAndPropagate(Node node, int propagate) {Node h = head; // Record old head for check belowsetHead(node);/** Try to signal next queued node if:*   Propagation was indicated by caller,*     or was recorded (as h.waitStatus either before*     or after setHead) by a previous operation*     (note: this uses sign-check of waitStatus because*      PROPAGATE status may transition to SIGNAL.)* and*   The next node is waiting in shared mode,*     or we don't know, because it appears null** The conservatism in both of these checks may cause* unnecessary wake-ups, but only when there are multiple* racing acquires/releases, so most need signals now or soon* anyway.*/if (propagate > 0 || h == null || h.waitStatus < 0 ||(h = head) == null || h.waitStatus < 0) {Node s = node.next;if (s == null || s.isShared())doReleaseShared();}}

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三、释放锁

进入releaseShared()

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

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tryReleaseShared()

释放锁，将state值加一，CAS修改成功后返回true

protected final boolean tryReleaseShared(int releases) {for (;;) {int current = getState();int next = current + releases;if (next < current) // overflowthrow new Error("Maximum permit count exceeded");if (compareAndSetState(current, next))return true;}}

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tryReleaseShared()返回true后进入doReleaseShared()

for(;;)循环判断head头结点不会null也不会尾结点，如果waitStatus为-1，则先CAS修改waitStatus为0，然后唤醒下一结点

private void doReleaseShared() {/** Ensure that a release propagates, even if there are other* in-progress acquires/releases.  This proceeds in the usual* way of trying to unparkSuccessor of head if it needs* signal. But if it does not, status is set to PROPAGATE to* ensure that upon release, propagation continues.* Additionally, we must loop in case a new node is added* while we are doing this. Also, unlike other uses of* unparkSuccessor, we need to know if CAS to reset status* fails, if so rechecking.*/for (;;) {Node h = head;if (h != null && h != tail) {int ws = h.waitStatus;if (ws == Node.SIGNAL) {if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) // CAS自旋修改waitStatus为0continue;            // loop to recheck casesunparkSuccessor(h);}else if (ws == 0 &&!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))continue;                // loop on failed CAS}if (h == head)                   // loop if head changedbreak;}}

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如果CAS自旋修改waitStatus=0成功，进入unparkSuccessor()

保障判断waitStatus是否为0，判断下一结点是否为空，不为空则唤醒下一结点。

private void unparkSuccessor(Node node) {/** If status is negative (i.e., possibly needing signal) try* to clear in anticipation of signalling.  It is OK if this* fails or if status is changed by waiting thread.*/int ws = node.waitStatus;if (ws < 0)compareAndSetWaitStatus(node, ws, 0);/** Thread to unpark is held in successor, which is normally* just the next node.  But if cancelled or apparently null,* traverse backwards from tail to find the actual* non-cancelled successor.*/Node s = node.next;if (s == null || s.waitStatus > 0) {s = null;for (Node t = tail; t != null && t != node; t = t.prev)if (t.waitStatus <= 0)s = t;}if (s != null)LockSupport.unpark(s.thread); // 唤醒线程}