Java多线程~~~使用信号量来控制资源获取

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在多线程开发中，有一个很经典的名词，那就是信号量。信号量就是用来衡量一个资源的可利用数目的，根据信号

量的多少来控制在多线程中各个资源之间的冲突问题，在Java中也提供了对信号量的支持。

而且在创建信号量的时候，第二个参数用来指定采取何种分配策略，比如当有很多线程被阻塞，但有一个机会的时

候，信号量应该选择谁去运行呢，如果选择true，就采用公平模式，到时候看哪个线程等待的时间最久，就把机会给那

个等待最久的线程，这样的就是公平分配策略。

下面就用代码来说明一下问题

package com.bird.concursey.charpet4;import java.util.concurrent.Semaphore;public class PrintQueue {// It initializes the semaphore object that will protect the access from the// print queue.private final Semaphore semaphore = new Semaphore(1,true);public void printJob(Object document) {//you must acquire the semaphore callingtry {semaphore.acquire();long duration = (long)(Math.random()*10);System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n",Thread.currentThread().getName(),duration);Thread.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();}finally{//free the semaphore by calling the release() method of the semaphore.semaphore.release();}}}

package com.bird.concursey.charpet4;public class Job implements Runnable {private PrintQueue printQueue;public Job(PrintQueue printQueue) {this.printQueue = printQueue;}@Overridepublic void run() {System.out.printf("%s: Going to print a job\n",Thread.currentThread().getName());printQueue.printJob(new Object());System.out.printf("%s: The document has been printed\n",Thread.currentThread().getName());}public static void main(String[] args) {PrintQueue printQueue = new PrintQueue();Thread thread[] = new Thread[10];for(int i = 0; i < 10; i++) {thread[i] = new Thread(new Job(printQueue), "Thread " + i);}for(int i = 0; i < 10; i++) {thread[i].start();}}}

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