基于AQS的前世今生，来学习并发工具类Semaphore。本文将从Semaphore的应用场景、源码原理解析来学习这个并发工具类。
1、 应用场景
Semaphore用来控制同时访问某个特定资源的操作数量，或者同时执行某个指定操作的数量。还可以用来实现某种资源池限制，或者对容器施加边界。
1.1 当成锁使用
控制同时访问某个特定资源的操作数量，代码如下：
public class SemaphoreLock {
public static void main(String[] args) {
//1、信号量为1时 相当于普通的锁 信号量大于1时 共享锁
Output o = new Output();
for (int i = 0; i < 5; i++) {
new Thread(() -> o.output()).start();
}
}
}
class Output {
Semaphore semaphore = new Semaphore(1);
public void output() {
try {
semaphore.acquire();
System.out.println(Thread.currentThread().getName() + " start at " + System.currentTimeMillis());
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + " stop at " + System.currentTimeMillis());
}catch(Exception e) {
e.printStackTrace();
}finally {
semaphore.release();
}
}
}
1.2 线程通信信号
线程间通信，代码如下：
public class SemaphoreCommunication {
public static void main(String[] args) {
//2、线程间进行通信
Semaphore semaphore = new Semaphore(1);
new SendingThread(semaphore,"SendingThread");
new ReceivingThread(semaphore,"ReceivingThread");
}
}
class SendingThread extends Thread {
Semaphore semaphore;
String name;
public SendingThread(Semaphore semaphore,String name) {
this.semaphore = semaphore;
this.name = name;
new Thread(this).start();
}
public void run() {
try {
semaphore.acquire();
for (int i = 0; i < 5; i++) {
System.out.println(name + ":" + i);
Thread.sleep(1000);
}
} catch (Exception e) {
e.printStackTrace();
}
semaphore.release();
}
}
class ReceivingThread extends Thread {
Semaphore semaphore;
String name;
public ReceivingThread(Semaphore semaphore,String name) {
this.semaphore = semaphore;
this.name = name;
new Thread(this).start();
}
public void run() {
try {
semaphore.acquire();
for (int i = 0; i < 5; i++) {
System.out.println(name + ":" + i);
Thread.sleep(1000);
}
} catch (Exception e) {
e.printStackTrace();
}
semaphore.release();
}
}
1.3 资源池限制
对资源池进行资源限制，代码如下：
public class SemaphoreConnect {
public static void main(String[] args) throws Exception {
//3、模拟连接池数量限制
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < 200; i++) {
executorService.submit(new Runnable() {
@Override
public void run() {
Connection.getInstance().connect();
}
});
}
executorService.shutdown();
executorService.awaitTermination(1, TimeUnit.DAYS);
}
}
class Connection {
private static Connection instance = new Connection();
private Semaphore semaphores = new Semaphore(10,true);
private int connections = 0;
private Connection() {
}
public static Connection getInstance() {
return instance;
}
public void connect() {
try {
semaphores.acquire();
doConnect();
} catch (InterruptedException e) {
e.printStackTrace();
}finally {
semaphores.release();
}
}
private void doConnect() {
synchronized (this) {
connections ++;
System.out.println("current get connections is : " + connections);
}
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (this) {
connections --;
System.out.println("after release current connections is : " + connections);
}
}
}
1.4 容器边界限制
对容器进行边界限制，代码如下：
public class SemaphoreBoundedList {
public static void main(String[] args) {
//4、容器边界限制
final BoundedList ba = new BoundedList(5);
Runnable runnable1 = new Runnable() {
public void run() {
try {
ba.add("John");
ba.add("Martin");
ba.add("Adam");
ba.add("Prince");
ba.add("Tod");
System.out.println("Available Permits : " + ba.getSemaphore().availablePermits());
ba.add("Tony");
System.out.println("Final list: " + ba.getArrayList());
}catch (InterruptedException ie) {
Thread.interrupted();
}
}
};
Runnable runnable2 = new Runnable() {
public void run() {
try {
System.out.println("Before removing elements: "+ ba.getArrayList());
Thread.sleep(5000);
ba.remove("Martin");
ba.remove("Adam");
}catch (InterruptedException ie) {
Thread.interrupted();
}
}
};
Thread thread1 = new Thread(runnable1);
Thread thread2 = new Thread(runnable2);
thread1.start();
thread2.start();
}
}
class BoundedList<T> {
private final Semaphore semaphore;
private List arrayList;
BoundedList(int limit) {
this.arrayList = Collections.synchronizedList(new ArrayList());
this.semaphore = new Semaphore(limit);
}
public boolean add(T t) throws InterruptedException {
boolean added = false;
semaphore.acquire();
try {
added = arrayList.add(t);
return added;
} finally {
if (!added)
semaphore.release();
}
}
public boolean remove(T t) {
boolean wasRemoved = arrayList.remove(t);
if (wasRemoved)
semaphore.release();
return wasRemoved;
}
public void remove(int index) {
arrayList.remove(index);
semaphore.release();
}
public List getArrayList() {
return arrayList;
}
public Semaphore getSemaphore() {
return semaphore;
}
}
2、 源码原理解析
2.1 获取信号
获取信号的方法如下:
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);//共享式获取AQS的同步状态
}
调用的是AQS的acquireSharedInterruptibly方法：
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())//线程中断 说明信号量对线程中断敏感
throw new InterruptedException();
if (tryAcquireShared(arg) < 0) //获取信号量失败 线程进入同步队列自旋等待
doAcquireSharedInterruptibly(arg);
}
其中tryAcquireShared依赖的是Sync的实现，Sync提供了公平和非公平式的方式，先看非公平式。
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();//同步状态 当前的信号量许可数
int remaining = available - acquires;//减去释放的信号量 剩余信号量许可数
if (remaining < 0 ||//剩余信号量小于0 直接返回remaining 不做CAS
compareAndSetState(available, remaining))//CAS更新
return remaining;
}
}
再看下公平式的。
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())//判断同步队列如果存在前置节点 获取信号量失败 其他和非公平式是一致的
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
最后来看下，如果未获取到信号量的处理方法doAcquireSharedInterruptibly。
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED);//线程进入同步队列
boolean failed = true;
try {
for (;;) {//自旋
final Node p = node.predecessor();
if (p == head) {//当前节点的前置节点是AQS的头节点 即自己是AQS同步队列的第一个节点
int r = tryAcquireShared(arg); //再去获取信号量
if (r >= 0) {//获取成功
setHeadAndPropagate(node, r);//退出自旋
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node); //获取失败 就取消获取
}
}
2.2 释放信号
释放信号的方法如下：
public void release() {
sync.releaseShared(1);
}
调用的是AQS的releaseShared方法：
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {//释放信号量
doReleaseShared();//唤醒后续的线程节点
return true;
}
return false;
}
tryReleaseShared交由子类Sync实现，代码如下：
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();//当前信号量许可数
int next = current + releases; //当前信号量许可数+释放的信号量许可数
if (next < current) // overflow 这个分支我看着永远走不进来呢
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))//CAS更新当前信号量许可数
return true;
}
}
释放成功后，则继续调用doReleaseShared，唤醒后续线程节点可以来争取信号量了。
private void doReleaseShared() {
for (;;) {
Node h = head; //头节点
if (h != null && h != tail) {//同步队列中存在线程等待
int ws = h.waitStatus; //头节点线程状态
if (ws == Node.SIGNAL) {//头节点线程状态为SIGNAL 唤醒后续线程节点
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h); //唤醒下个节点
}
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使用AQS同步状态来保存信号量的当前计数。它里面定义的acquireSharedInterruptibly方法会减少计数，当计数为非正值时阻塞线程，releaseShared方法会增加计数，在计数不超过信号量限制时要解除线程的阻塞。
参考资料:
https://github.com/lingjiango/ConcurrentProgramPractice
https://www.caveofprogramming.com/java-multithreading/java-multithreading-semaphores-part-12.html
https://java2blog.com/java-semaphore-example/
http://tutorials.jenkov.com/java-util-concurrent/semaphore.html