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.

package com.smikevon.concurrent.tools;import java.util.Random;import java.util.concurrent.Semaphore;/** * @description: 有A,B,C三个线程, A线程输出A, B线程输出B, C线程输出C.要求, 同时启动三个线程, 按顺序输出ABC, 循环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();                }            }        },"线程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();                }            }        },"线程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();                }            }        },"线程c").start();    }    /**     * @Description:打印出线程的名字     * @param msg     * @returType:void     */    public static void doing(String msg){        System.out.println(Thread.currentThread().getName()+":"+msg);    }}