Semaphore不明白为什么直接放在juc包下，不是应该放locks下面嘛，这里还是当初locks学习吧。
Semaphore英文是信号量的意思，在这里我喜欢叫资源或者许可，实现的功能就是获取资源，获取到就干活，获取不到就排队，等别人释放了资源，然后所有排队的再去获取。实现AQS的共享api，看个入门demo：
public class SemaphoreTest {
//3个钥匙
private static Semaphore semaphore = new Semaphore(3);
public static void main(String[] args) throws InterruptedException {
for(int i =0; i<10; i++){
new Thread(new Runnable() {
@Override
public void run() {
try {
semaphore.acquire();
System.out.println(Thread.currentThread() + "得到一把钥匙");
//模拟干些事情，要不然控制台看不出效果
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}finally{
semaphore.release();
System.out.println(Thread.currentThread() + "丢掉一把钥匙");
}
}
}).start();
}
//不是占用锁的线程也可以release
// semaphore.release(4);
// Thread.sleep(4000);
// System.out.println(semaphore.availablePermits());
}
}另外Semaphore也支持公平和非公平，区别跟ReetrantLock的公平非公平差不多，非公平就是获取的时候有可用的就插队，公平的就老老实实排队。不过lock的release必须是持有锁的线程去release，而Semaphore不是。
看下内部对AQS的实现：
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
//AQS的state保存许可的数量
Sync(int permits) {
setState(permits);
}
final int getPermits() {
return getState();
}
//非公平的Acquire
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
//许可量小于0就排队，要不然就cas设置返回可用的数量，肯定是大于0，不用排队
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
//对AQS共享模式tryReleaseShard的release
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
//这是减少许可量，
//举例说我项目要求有5个干活小弟，但是老大说人太多，只能给你3个人，好吧，那就减少2个吧
final void reducePermits(int reductions) {
for (;;) {
int current = getState();
int next = current - reductions;
if (next > current) // underflow
throw new Error("Permit count underflow");
if (compareAndSetState(current, next))
return;
}
}
//这里是获取所有可用的许可量
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}看下sync的公平和非公平的子类实现：
tatic final class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
//父类设置state许可量
NonfairSync(int permits) {
super(permits);
}
//非公平的共享Acquire，调用分类实现
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
}
static final class FairSync extends Sync {
private static final long serialVersionUID = 2014338818796000944L;
//父类设置state许可量
FairSync(int permits) {
super(permits);
}
//公平的共享Acquire
protected int tryAcquireShared(int acquires) {
for (;;) {
//这里是区分，看下pre是否有非自己线程排队的，非公平的没有这一步
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}看下Semaphore的实现：
//传入许可量，调用sync设置AQS的state的值，默认非公平
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
/**
许可量跟是否公平标识
*/
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
/**
调用AQS响应中断的Acquire
*/
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
这个不响应中断的Acquire
*/
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
/**
直接调用sync的非公平Acquire，如果你构造的时候使用的是公平模式，肯定会打破公平
*/
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
/**
响应中断跟超时的Acquire
*/
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
直接release一个许可
*/
public void release() {
sync.releaseShared(1);
}
/**
响应Acquire指定数量的许可
*/
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
/**
不响应中断的Acquire指定数量的许可
*/
public void acquireUninterruptibly(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireShared(permits);
}
/**
非公平的Acquire指定数量的许可
*/
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return sync.nonfairTryAcquireShared(permits) >= 0;
}
/**
响应中断和超时的Acquire指定数量的许可
*/
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
}
/**
release指定数量的许可
*/
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}
/**
查询许可量
*/
public int availablePermits() {
return sync.getPermits();
}
/**
Acquire所有可用的许可并返回许可量
*/
public int drainPermits() {
return sync.drainPermits();
}
/**
扣减指定数量的许可，会导致许可量为负数，使用的时候注意，自己可以定义个子类看看
*/
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
sync.reducePermits(reduction);
}
/**
验证是否是公平
*/
public boolean isFair() {
return sync instanceof FairSync;
}
/**
调用AQS检查队列是否还有等待节点
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
返回AQS中节点数量
*/
public final int getQueueLength() {
return sync.getQueueLength();
}
/**
返回AQS同步等待队列所有等待Acquire的数量
*/
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}Semaphore还是比较简单的，因为实现的是共享模式API，所以不管lock还是lock的线程都可以release，另外感觉收缩许可量的时候可能会导致许可量为负，使用的时候需要注意。