Semaphore 是一个计数信号量，它的本质是一个共享锁。信号量维护了一个信号量许可集。线程可以通过调用acquire()来获取信号量的许可；当信号量中有可用的许可时，线程能获取该许可；否则线程必须等待，直到有可用的许可为止。 线程可以通过release()来释放它所持有的信号量许可（用完信号量之后必须释放，不然其他线程可能会无法获取信号量）。
简单示例：
package me.socketthread;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class SemaphoreLearn {
//信号量总数
private static final int SEM_MAX = 12;
public static void main(String[] args) {
Semaphore sem = new Semaphore(SEM_MAX);
//创建线程池
ExecutorService threadPool = Executors.newFixedThreadPool(3);
//在线程池中执行任务
threadPool.execute(new MyThread(sem, 7));
threadPool.execute(new MyThread(sem, 4));
threadPool.execute(new MyThread(sem, 2));
//关闭池
threadPool.shutdown();
}
}
class MyThread extends Thread {
private volatile Semaphore sem; // 信号量
private int count; // 申请信号量的大小
MyThread(Semaphore sem, int count) {
this.sem = sem;
this.count = count;
}
public void run() {
try {
// 从信号量中获取count个许可
sem.acquire(count);
Thread.sleep(2000);
System.out.println(Thread.currentThread().getName() + " acquire count="+count);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// 释放给定数目的许可，将其返回到信号量。
sem.release(count);
System.out.println(Thread.currentThread().getName() + " release " + count + "");
}
}
}
执行结果：
pool-1-thread-2 acquire count=4 pool-1-thread-1 acquire count=7 pool-1-thread-1 release 7 pool-1-thread-2 release 4 pool-1-thread-3 acquire count=2 pool-1-thread-3 release 2
线程1和线程2会并发执行，因为两者的信号量和没有超过总信号量，当前两个线程释放掉信号量之后线程3才能继续执行。
源码分析：
1、构造函数
在构造函数中会初始化信号量值，这值最终是作为锁标志位state的值
Semaphore sem = new Semaphore(12);//简单来说就是给锁标识位state赋值为12
2、Semaphore.acquire(n);简单理解为获取锁资源，如果获取不到线程阻塞
Semaphore.acquire(n);//从锁标识位state中获取n个信号量，简单来说是state = state-n 此时state大于0表示可以获取信号量，如果小于0则将线程阻塞
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
//获取锁
sync.acquireSharedInterruptibly(permits);
}
acquireSharedInterruptibly中的操作是获取锁资源，如果可以获取则将state= state-permits，否则将线程阻塞
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)//tryAcquireShared中尝试获取锁资源
doAcquireSharedInterruptibly(arg); //将线程阻塞
}
tryAcquireShared中的操作是尝试获取信号量值，简单来说就是state=state-acquires ，如果此时小于0则返回负值，否则返回大于新值，再判断是否将当线程线程阻塞
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
//获取state值
int available = getState();
//从state中获取信号量
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
//如果信号量小于0则直接返回，表示无法获取信号量，否则将state值修改为新值
return remaining;
}
}
doAcquireSharedInterruptibly中的操作简单来说是将当前线程添加到FIFO队列中并将当前线程阻塞。
/会将线程添加到FIFO队列中，并阻塞
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
//将线程添加到FIFO队列中
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
//parkAndCheckInterrupt完成线程的阻塞操作
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
3、Semaphore.release(int permits)，这个函数的实现操作是将state = state+permits并唤起处于FIFO队列中的阻塞线程。
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
//state = state+permits，并将FIFO队列中的阻塞线程唤起
sync.releaseShared(permits);
}
releaseShared中的操作是将state = state+permits，并将FIFO队列中的阻塞线程唤起。
public final boolean releaseShared(int arg) {
//tryReleaseShared将state设置为state = state+arg
if (tryReleaseShared(arg)) {
//唤起FIFO队列中的阻塞线程
doReleaseShared();
return true;
}
return false;
}
tryReleaseShared将state设置为state = state+arg
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");
//将state值设置为state=state+releases
if (compareAndSetState(current, next))
return true;
}
}
doReleaseShared()唤起FIFO队列中的阻塞线程
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
//完成阻塞线程的唤起操作
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
总结：Semaphore简单来说设置了一个信号量池state，当线程执行时会从state中获取值，如果可以获取则线程执行，并且在执行后将获取的资源返回到信号量池中，并唤起其他阻塞线程；如果信号量池中的资源无法满足某个线程的需求则将此线程阻塞。
Semaphore源码：
public class Semaphore implements java.io.Serializable {
private static final long serialVersionUID = -3222578661600680210L;
private final Sync sync;
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
//设置锁标识位state的初始值
Sync(int permits) {
setState(permits);
}
//获取锁标识位state的值，如果state值大于其需要的值则表示锁可以获取
final int getPermits() {
return getState();
}
//获取state值减去acquires后的值，如果大于等于0则表示锁可以获取
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
//释放锁
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
//将state值加上release值
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
//将state的值减去reductions
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;
}
}
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}
//非公平锁
static final class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
NonfairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
}
//公平锁
static final class FairSync extends Sync {
private static final long serialVersionUID = 2014338818796000944L;
FairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}
//设置信号量
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
//获取锁
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
public void release() {
sync.releaseShared(1);
}
//获取permits值锁
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);
}
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return sync.nonfairTryAcquireShared(permits) >= 0;
}
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
}
//释放
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}
public int availablePermits() {
return sync.getPermits();
}
public int drainPermits() {
return sync.drainPermits();
}
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
sync.reducePermits(reduction);
}
public boolean isFair() {
return sync instanceof FairSync;
}
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
public final int getQueueLength() {
return sync.getQueueLength();
}
protected Collection getQueuedThreads() {
return sync.getQueuedThreads();
}
public String toString() {
return super.toString() + "[Permits = " + sync.getPermits() + "]";
}
}
总结
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