java并发:Semaphore 的使用

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package com;import java.util.concurrent.Semaphore;/** *  Semapore 类在并发中的作用 * */public class SemaporeDemo {private static Semaphore mSemaphore;public static void print(String msg){try {mSemaphore.acquire();Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + ":" + msg);mSemaphore.release();}/** */public static void main(String[] args) { // 如果此信号量保证在争用时按先进先出的顺序授予许可mSemaphore = new Semaphore(2,false); // true:表示按循序先进先执行,false:表示不需要遵守这样的规则for(int i = 0 ; i < 5 ; i++){Thread th = new Thread(new Runnable() {@Overridepublic void run() {print("d");}});th.start();}}}

package com;import java.util.ArrayList;import java.util.List;import java.util.concurrent.Semaphore;/** *Semaphore模拟实现一个数据库连接池 * * 1.一个对象保存若干个连接  * 2.开启若干个线程来过去这些连接 * 3.获取到连接的执行完成之后,需要释放连接, 让其他的线程来获取到连接 */public class DataPoolDemo {private List<Conn> pool = new ArrayList<DataPoolDemo.Conn>();private static Semaphore semaphore;/** * 初始化多少的连接 * @param num */public DataPoolDemo(int num){for(int i = 0 ; i < num ; i++){pool.add(new Conn(i));}}/** * @param args */public static void main(String[] args) {final DataPoolDemo demo = new DataPoolDemo(3);//需要将连接数量和信号量设置为一致semaphore = new Semaphore(3);Thread th1 = new Thread(new Runnable() {@Overridepublic void run() {try {Conn conn = demo.getConn();System.out.println(conn.toString()); Thread.sleep(3000);  demo.realse(conn);} catch (InterruptedException e) {e.printStackTrace();}}});th1.start();for(int i = 0 ; i < 8 ; i ++){new Thread(new Runnable() {@Overridepublic void run() {try {Conn conn = demo.getConn();System.out.println(conn.toString());demo.realse(conn);} catch (InterruptedException e) {e.printStackTrace();}}}).start();}}/** 释放连接  */protected void realse(Conn conn) {pool.add(conn);System.out.println(Thread.currentThread().getName()+" release a conn " + conn.toString());  semaphore.release();}private Conn getConn() throws InterruptedException {semaphore.acquire();synchronized (pool) {Conn conn = pool.remove(0);System.out.println(Thread.currentThread().getName()+" get a conn " + conn.toString());  return conn;}}private class Conn{private int num;public Conn(int num){this.num = num;}@Overridepublic String toString() {return "con: num-" + num;}}}

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