?????技术信号量（Counting Semaphore)用来控制同时访问某个特定资源的操作数量，或者执行某个特定操作的数量。计数信号量还可以用来实现某种资源池或者对容器加边界。
??Semaphore中管理着一组虚拟许可（permit）,许可的初始数量可以通过在构造方法中指定。在执行操作时首先获取许可（permit）,在使用完成后释放许可。如果没有许可，那么acquire则一直阻塞到有许可或者超时中断。
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/**
* 计数信号量（Counting Semaphore)用来控制同时访问某个特定资源的操作数量，或者执行某个指定操作的数量。
* 计数信号量还可以用来实现某种资源池或者对容器进行加边界
* Semaphore 中管理着一组虚拟的许可（permit）,许可的初始化数量可以通过构造函数来指定。在执行操作时可以
* 首先获取许可（只要还有剩余的许可），并在使用后释放许可（permit）。 如果没有许可（permit）那么aquire将阻塞
* 直到有许可或者直到被中断、操作超时。release方法将释放一个许可，返回给信号量（semaphore）
*
* @author zhangwei_david
* @version $Id: BoundedHashSet.java, v 0.1 2014年11月11日 下午1:49:24 zhangwei_david Exp $
*/
public class BoundedHashSet<T> {
//
private final Set<T> set;
// 信号量
private final Semaphore sem;
public BoundedHashSet(int bound) {
//初始化一个同步Set
this.set = Collections.synchronizedSet(new HashSet<T>());
//初始化信号量大小
sem = new Semaphore(bound);
}
public boolean add(T o) throws InterruptedException {
// 获取许可
sem.acquire();
System.out.println("获取许可 " + sem.availablePermits());
boolean wasAdded = false;
try {
wasAdded = set.add(o);
return wasAdded;
} finally {
if (!wasAdded) {
// 如果增加失败
System.out.println("添加失败，释放许可");
sem.release();
}
}
}
public boolean remove(Object o) {
boolean wasRemoved = set.remove(o);
if (wasRemoved) {
// 删除后释放许可
System.out.println("删除成功，释放许可");
sem.release();
}
return wasRemoved;
}
}
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/**
* 资源池
* @author zhangwei_david
* @version $Id: ResourcePool.java, v 0.1 2015年7月5日 上午9:53:18 zhangwei_david Exp $
*/
public class ResourcePool<T> {
private int size;
private List<T> items = new ArrayList<T>();
private volatile boolean[] checkOut;
private Semaphore available;
public ResourcePool(Class<T> clazz, int max) {
size = max;
checkOut = new boolean[size];
available = new Semaphore(size);
for (int i = 0; i < size; i++) {
try {
items.add(clazz.newInstance());
} catch (Exception e) {
e.printStackTrace();
}
}
}
public T checkOut() throws InterruptedException {
available.acquire();
return getItem();
}
public void checkIn(T t) {
if (releaseItem(t)) {
available.release();
}
}
/**
* 释放对象，如果该对象在对象池中不存在则返回false
* 如果该对象尚未被checkOut则返回fasle
* 将checkOut标准置为false，返回true
*
* @param t
* @return
*/
private synchronized boolean releaseItem(T t) {
int index = items.indexOf(t);
if (index == -1) {
return false;
}
if (checkOut[index]) {
checkOut[index] = false;
return true;
}
return false;
}
/**6
* 如果还有对象没有checkOut则返回该对象，如果没有则返回null
*
* @return
*/
private synchronized T getItem() {
for (int i = 0; i < size; i++) {
if (!checkOut[i]) {
checkOut[i] = true;
return items.get(i);
}
}
return null;
}
}
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/**
*
* @author zhangwei_david
* @version $Id: Link.java, v 0.1 2015年7月5日 上午10:11:03 zhangwei_david Exp $
*/
public class Link {
private static int counter = 0;
private final int id = counter++;
public Link() {
try {
TimeUnit.MILLISECONDS.sleep(300);
} catch (Exception e) {
}
}
/**
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return "Link [ id=" + id + "]";
}
}
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/**
*
* @author zhangwei_david
* @version $Id: SemaphoreDemo.java, v 0.1 2015年7月5日 上午10:20:16 zhangwei_david Exp $
*/
public class SemaphoreDemo {
/**
*
* @param args
*/
public static void main(String[] args) {
final ResourcePool<Link> linkes = new ResourcePool<Link>(Link.class, 30);
ExecutorService exec = Executors.newCachedThreadPool();
for (int i = 0; i < 60; i++) {
exec.execute(new CheckOutTask<Link>(linkes));
}
exec.shutdown();
}
}
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原文：http://zhangwei-david.iteye.com/blog/2224357