Java:多线程,Semaphore同步器
1. 背景
类java.util.concurrent.Semaphore提供了一个计数信号量。通过Semaphore类,可以控制某个资源可被同时访问的个数,通过 acquire() 获取一个许可,如果没有就等待,而 release() 释放一个许可。
Semaphore的构造函数中带有一个fairness的参数,用于设置是否“公平”。当fairness为true时,Semaphore保证各线程以后进先出(FIFO)的方式获得信号量。如果fairness为false,则不保证这种顺序。
2. 示范代码
下面这段代码演示了某种资源只限于5个线程访问:
package com.clzhang.sample.thread; import java.text.SimpleDateFormat; import java.util.Date; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Semaphore; public class SyncSemaphore { private static final SimpleDateFormat MY_SDF = new SimpleDateFormat("mm:ss"); // 只能5个线程同时访问 private final Semaphore available = new Semaphore(5); class Mythread implements Runnable { @Override public void run() { try { // 获取许可 available.acquire(); // 业务处理 Thread.sleep((long) (Math.random() * 10000)); System.out.println("[" + MY_SDF.format(new Date()) + "]" + Thread.currentThread().getName() + " ending process!"); // 访问完后,释放 available.release(); } catch (InterruptedException e) { e.printStackTrace(); } } } public static void main(String[] args) { // 创建线程池 ExecutorService executorPool = Executors.newCachedThreadPool(); // 加入线程 SyncSemaphore ins = new SyncSemaphore(); for (int index = 0; index < 10; index++) { executorPool.execute(ins.new Mythread()); } // 关闭线程池 executorPool.shutdown(); } }
输出
[43:04]pool-1-thread-5 ending process!
[43:04]pool-1-thread-7 ending process!
[43:08]pool-1-thread-9 ending process!
[43:11]pool-1-thread-3 ending process!
[43:11]pool-1-thread-4 ending process!
[43:13]pool-1-thread-1 ending process!
[43:13]pool-1-thread-2 ending process!
[43:14]pool-1-thread-6 ending process!
[43:16]pool-1-thread-10 ending process!
[43:21]pool-1-thread-8 ending process!