Semaphore
Semaphore
Semaphore翻译成字面意思为 信号量，Semaphore可以控同时访问的线程个数，通过 acquire() 获取一个许可，如果没有就等待，而 release() 释放一个许可。
Semaphore类位于java.util.concurrent包下，它提供了2个构造器:
//参数permits表示许可数目，即同时可以允许多少线程进行访问
public Semaphore(int permits) {}
//这个多了一个参数fair表示是否是公平的，即等待时间越久的越先获取许可
public Semaphore(int permits, boolean fair) {}下面说一下Semaphore类中比较重要的几个方法，首先是acquire()、release()方法：
public void acquire() throws InterruptedException { } //获取一个许可
public void acquire(int permits) throws InterruptedException { } //获取permits个许可
public void release() { } //释放一个许可
public void release(int permits) { } //释放permits个许可acquire()用来获取一个许可，若无许可能够获得，则会一直等待，直到获得许可。
release()用来释放许可。注意，在释放许可之前，必须先获获得许可。
这4个方法都会被阻塞，如果想立即得到执行结果，可以使用下面几个方法：
//尝试获取一个许可，若获取成功，则立即返回true，若获取失败，则立即返回false
public boolean tryAcquire() { };
//尝试获取一个许可，若在指定的时间内获取成功，则立即返回true，否则则立即返回false
public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException { };
//尝试获取permits个许可，若获取成功，则立即返回true，若获取失败，则立即返回false
public boolean tryAcquire(int permits) { };
//尝试获取permits个许可，若在指定的时间内获取成功，则立即返回true，否则则立即返回false
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException { };另外还可以通过availablePermits()方法得到可用的许可数目。
代码：
you can find the code on the github, get the result as you try:
public class SemaphoreLearn {
public static void main(String[] args) {
CountDownLatch countDownLatch = new CountDownLatch(10);
Semaphore semaphore = new Semaphore(3);
for(int i = 0;i<10;i++) {
new Thread(new SemaphoreThread(countDownLatch,semaphore)).start();
}
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("all worker has finish the work");
}
}
public class SemaphoreThread implements Runnable {
private CountDownLatch countDownLatch;
private Semaphore semaphore;
public SemaphoreThread(CountDownLatch countDownLatch, Semaphore semaphore) {
this.countDownLatch = countDownLatch;
this.semaphore = semaphore;
}
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see Thread#run()
*/
@Override
public void run() {
try {
semaphore.acquire();
System.out.println("thread: " +Thread.currentThread().getName()+"is asking a new acquirement.");
Thread.sleep(1000);
System.out.println("thread: " +Thread.currentThread().getName()+"release the acquirement.");
} catch (InterruptedException e) {
e.printStackTrace();
}finally {
countDownLatch.countDown();
semaphore.release();
}
}
}源码分析
构造方法：
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}这里和 ReentrantLock 类似，用了公平策略和非公平策略。
看 acquire 方法：
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
public void acquireUninterruptibly(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireShared(permits);
}这几个方法也是老套路了，大家基本都懂了吧，这边多了两个可以传参的 acquire 方法，不过大家也都懂的吧，如果我们需要一次获取超过一个的资源，会用得着这个的。
我们接下来看不抛出 InterruptedException 异常的 acquireUninterruptibly() 方法吧：
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}前面说了，Semaphore 分公平策略和非公平策略，我们对比一下两个 tryAcquireShared 方法：
// 公平策略：
protected int tryAcquireShared(int acquires) {
for (;;) {
// 区别就在于是不是会先判断是否有线程在排队，然后才进行 CAS 减操作
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
// 非公平策略：
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}也是老套路了，所以从源码分析角度的话，我们其实不太需要关心是不是公平策略还是非公平策略，它们的区别往往就那么一两行。
我们再回到 acquireShared 方法，
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}由于 tryAcquireShared(arg) 返回小于 0 的时候，说明 state 已经小于 0 了（没资源了），此时 acquire 不能立马拿到资源，需要进入到阻塞队列等待，虽然贴了很多代码，不在乎多这点了：
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}这个方法我就不介绍了，线程挂起后等待有资源被 release 出来。接下来，我们就要看 release 的方法了：
// 任务介绍，释放一个资源
public void release() {
sync.releaseShared(1);
}
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
// 溢出，当然，我们一般也不会用这么大的数
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}tryReleaseShared 方法总是会返回 true，然后是 doReleaseShared，这个也是我们熟悉的方法了，我就贴下代码，不分析了，这个方法用于唤醒所有的等待线程：
private void doReleaseShared() {
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}Semphore 的源码确实很简单，基本上都是分析过的老代码的组合使用了。