java同步工具类--Semaphores
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文章链接:http://blog.csdn.net/owillll/article/details/8882562
作者:azibug 邮箱: azibug#163.com
信号量(Semaphares)
概述
用来控制同时访问某个资源池的操作数量。当permits为零时使用acquire会一直阻塞,直至permits不为零;如果permits不为零,使用acquire则会将permits减一。使用完资源可以使用release释放许可,这样permits就会加一
用法:
- 使用new Semaphare(permits) 初始化可同时操作自由的数量
- 使用semaphare.acquire() 获取一个允许,如果permits值为零,则会一直阻塞,直至permits不为零
- 工作完使用semaphare.release()(一般在finally里面),释放许可,使其他线程可以使用资源。
使用场所:
通常用于限制可以访问某些资源的线程数量。比如打印机中将permits设为1,则多个线程打印时,打印队列也只会有一个在工作。
代码示例
PrintQueue.java打印队列,执行打印的操作
package com.chaokuaidi.java.concurrency.utils.semaphores;import java.util.concurrent.Semaphore;import java.util.concurrent.TimeUnit;/** * This class implements the PrintQueue using a Semaphore to control the * access to it. * */public class PrintQueue {/** * Semaphore to control the access to the queue */private final Semaphore semaphore;/** * Constructor of the class. Initializes the semaphore */public PrintQueue(){semaphore=new Semaphore(1);}/** * Method that simulates printing a document * @param document Document to print */public void printJob (Object document){try {// Get the access to the semaphore. If other job is printing, this// thread sleep until get the access to the semaphoresemaphore.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);TimeUnit.SECONDS.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();} finally {// Free the semaphore. If there are other threads waiting for this semaphore,// the JVM selects one of this threads and give it the access.semaphore.release();}}}
Job.java 工作线程,调用打印操作
package com.chaokuaidi.java.concurrency.utils.semaphores;/** * This class simulates a job that send a document to print. * */public class Job implements Runnable {/** * Queue to print the documents */private PrintQueue printQueue;/** * Constructor of the class. Initializes the queue * @param printQueue */public Job(PrintQueue printQueue){this.printQueue=printQueue;}/** * Core method of the Job. Sends the document to the print queue and waits * for its finalization */@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());}}
Main.java 初始化打印机并开始打印线程
package com.chaokuaidi.java.concurrency.utils.semaphores;/** * Main class of the example. * */public class Main {/** * Main method of the class. Run ten jobs in parallel that * send documents to the print queue at the same time. */public static void main (String args[]){// Creates the print queuePrintQueue printQueue=new PrintQueue();// Creates ten ThreadsThread thread[]=new Thread[10];for (int i=0; i<10; i++){thread[i]=new Thread(new Job(printQueue),"Thread "+i);}// Starts the Threadsfor (int i=0; i<10; i++){thread[i].start();}}}
注:java同步工具类的所有代码全部托管在github上:https://github.com/azibug/concurrency 你可以很容易地构建并测试他
参考资料:
1. 《Java 7 Concurrency Cookbook》,绝大部分示例来源于此
2. 《Java 并发编程实战》
3. 《Thinking in Java》
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