Java核心知识点学习----线程中的Semaphore学习,公共厕所排队策略

1.什么是Semaphore?A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each acquire blocks ifnecessary until a permit is available, and then takes it. Each release adds a p ermit, potentially releasing ablocking acquirer. However, no actual permit objects are used; the Semaphore just keeps a count of thenumber available and acts accordingly.Semaphores are often used to restrict the number of threads than can access so me (physical or logical)resource.上面是官方给予该类的介绍,信号量(Semaphore)，有时被称为信号灯，是在多线程环境下使用的一种设施, 它负责协调各个线程, 以保证它们能够正确、合理的使用公共资源。

举个例子,公共厕所只有3个位子,有10个人要上厕所.一开始3个位都是空的,那么将有3个人先后占得坑,其它人只要在外面等待.如果有一个人上完厕所,然后等待中的7人中将有一个可以去上厕所.也有可能同时有两人OK,那么也将会同时有两人补位.简单说就是你必须有空位了,才能去上厕所.上厕所的例子虽不雅观,但令人印象深刻,本来想举ATM机取钱的例子的,但意思一样,还是举上厕所吧.Semaphore的作用就是控制位置的分配,一般情况下位置的分配是随机的,可以在实例化对象时设置规则进行排序.2.实例先看效果:再来看代码:package com.amos.concurrent;import java.util.Random;import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Semaphore;/*** @ClassName: SemaPhoreTest* @Description: 线程通信中的"信号灯"* @author: amosli* @email:hi_amos@* @date Apr 25, 2014 12:06:22 AM*/public class SemaPhoreTest {public static void main(String[] args) {ExecutorService threadPool =Executors.newCachedThreadPool();final Semaphore semaphore=new Semaphore(3);for(int i=0;i<10;i++){threadPool.execute(new Runnable() {public void run() {try{semaphore.acquire();//获取一个可用的permits} catch(Exception e) {e.printStackTrace();}System.out.println("线程" + Thread.currentThread().getN ame()+" 已进入. " + "目前已经有"+(3-semaphore.availablePermits())+" 个线程进入");try{Thread.sleep(new Random().nextInt(1000));} catch(InterruptedException e) {e.printStackTrace();} System.out.println("线程"+Thread.currentThread().getNam e()+ " 即将离开...");semaphore.release();//释放一个线程System.out.println("线程"+Thread.currentThread().getNam e()+ " 已离开. 当前有"+(3-semaphore.availablePermits())+"并发");}});}}}怎么保证是有序的?上面的代码是不保证顺序的,如果要有个先后顺序,即FIFO,先进先出原则,有个排队,上厕所要排队,只需要将上面的代码改掉一行即可.final Semaphore semaphore=new Semaphore(3, true);这样就可以了,其它代码不变.效果如下图所示:３.代码说明1).如何创建Semaphore?new Semaphore(3);//无序默认创建是无序的.new Semaphore(3,true);//有序2)如何获取一个"坑位"?首先,程序会先检查,"坑"是否已经满了,如果没满,那么可以按需分配,具体代码如下:try{semaphore.acquire();//获取一个可用的permits} catch(Exception e) {e.printStackTrace();}然后,分配完以后,将会把此坑位置为不可用了.3)如何释放一个"坑位"?semaphore.release();//释放一个线程释放完成以后将会把此坑位置为可用的.4.扩展--官方例子For example, here is a class that uses a semaphore to control access to a pool of items:class Pool {private static final int MAX_AVAILABLE = 100;private final Semaphore available = new Semaphore(MAX_AVAILABLE, tr ue);public Object getItem() throws InterruptedException {available.acquire();return getNextAvailableItem();}public void putItem(Object x) {if(markAsUnused(x))available.release();}// Not a particularly efficient data structure; just for demoprotected Object[] items =... whatever kinds of items being managedprotected boolean[] used = new boolean[MAX_AVAILABLE];protected synchronized Object getNextAvailableItem() {for(int i = 0; i < max_available; ++i) {if(!used[i]) {used[i] = true;return items[i];}}return null; // not reached}protected synchronized boolean markasunused(object item) {for(int i = 0; i < max_available; ++i) {if(item ==items[i]) {if(used[i]) {used[i] = false;return true;} elsereturn false;}}return false;}}java核心知识点学习----创建线程的第三种方式Callable和Future CompletionService前面已经指出通过实现Runnable时,Thread类的作用就是将run()方法包装成线程执行体,那么是否可以直接把任意方法都包装成线程执行体呢?Java目前不行,但其模仿者C#中是可以的.Callabel接口可以看成是Runnable接口的增强版,只不过其线程执行体call()方法比run 方法更加强大罢了:>>call()方法中可以有返回值>>call()方法中可以声明抛出异常.一.创建线程的第三种方式----使用Callable对象进行创建package com.amos.concurrent;import java.util.concurrent.Callable;import java.util.concurrent.ExecutionException;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;/*** @ClassName: CallableAndFuture* @Description: 多线程中的Callable和Future学习* @author: amosli* @email:hi_amos@* @date Apr 22, 2014 12:07:26 AM*/public class CallableAndFuture {public static void main(String[] args) throws Exception, ExecutionException {ExecutorService newSingleThreadExecutor =Executors.newSingleThrea dExecutor();Future<String> future = newSingleThreadExecutor.submit(new Call able<String>() {public String call() throws Exception {Thread.sleep(20);return"hi,amos";}});// System.out.println("future:"+future.get(1,LISECONDS));/ /等待指定的时间System.out.println("future:" +future.get());}}效果如下:注:1.这里要注意的是,创建线程时执行任务不是用execute()方法去执行了,而是用submi t()方法.2.同时要注意,这里call()方法返回值,要和上面的保持一致.3.另外,可以设置最大等待时间,就是等待程序的返回值,这里使用get()方法.4.其常用的其它方法有cancel(),isCancelled(),isDone(),分别表示取消关联的任务,是否已经取消,任务是否已经完成.二.CompeltionServiceCompeltionService主要用于提交一组Callable对象,其take方法用于返回已完成的c allable任务的Future对象.可以用麦子收割来作比喻,种了10亩地的麦子,哪一块先成熟先收割哪一块.举例:package com.amos.concurrent;import java.util.Random;import java.util.concurrent.Callable;import pletionService;import java.util.concurrent.ExecutionException;import java.util.concurrent.ExecutorCompletionService;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CallableAndFuture {public static void main(String[] args) throws Exception, ExecutionException {ExecutorService newFixedThreadPool = Executors.newFixedThreadPool(10);CompletionService<Integer> completionService = new ExecutorCompl etionService<Integer>(newFixedThreadPool);for(int i=0;i<11;i++){//创建10个任务final int task=i;completionService.submit(new Callable<Integer>() {//提交任务public Integer call() throws Exception {Thread.sleep(new Random().nextInt(3000));//最多3秒return task;}});}//takefor(int i=0;i<11;i++){System.out.println("已完成的任务:"+completionService.take().get());}}效果如下图所示:注:由结果也可以看出来,其随机结果是根据任务的先后完成顺序来的,使用其take()方法可以获取其返回结果.。