semaphore是java.util.concurrent包下的并發工具類。

A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each acquire() blocks if necessary until a permit is available, and then takes it. Each release() adds a permit, potentially releasing a blocking acquirer. However, no actual permit objects are used; the semaphore just keeps a count of the number available and acts accordingly.Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource.

大致意思是semaphore可以持有許可證，可以在初始化時指定，執行一次acquire()，獲取一個許可證，線程進入，當許可證為0，則不再允許線程進入，執行一次release釋放一個許可證。

package com.smikevon.concurrent.tools;import java.util.Random; import java.util.concurrent.Semaphore;/*** @description: 有A,B,C三個線程, A線程輸出A, B線程輸出B, C線程輸出C.要求, 同時啟動三個線程, 按順序輸出ABC, 循環10次* @date : 2014年9月17日 下午8:23:05*/ public class TestSemaphore {public static void main(String[] args) {final Semaphore sa = new Semaphore(1);final Semaphore sb = new Semaphore(0);final Semaphore sc = new Semaphore(0);final Random random = new Random(System.currentTimeMillis());new Thread(new Runnable() {public void run() {try {while(true){Thread.sleep(random.nextInt(3000));sa.acquire();doing("A");sb.release();}} catch (InterruptedException e) {e.printStackTrace();}}},"線程a").start();new Thread(new Runnable() {public void run() {try {while(true){Thread.sleep(random.nextInt(3000));sb.acquire();doing("B");sc.release();}} catch (InterruptedException e) {e.printStackTrace();}}},"線程b").start();new Thread(new Runnable() {public void run() {try {while(true){Thread.sleep(random.nextInt(3000));sc.acquire();doing("C");sa.release();}} catch (InterruptedException e) {e.printStackTrace();}}},"線程c").start();}/*** @Description:打印出線程的名字* @param msg* @returType:void*/public static void doing(String msg){System.out.println(Thread.currentThread().getName()+":"+msg);}}

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轉載于:https://www.cnblogs.com/seanvon/p/4072050.html

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