join() 和 sleep() 区别

来源于《Java多线程编程核心技术》

一、join() 作用

在很多情况，主线程创建并启动子线程，如果子线程中需要进行大量的耗时计算，主线程往往早于子线程结束。这时，如果主线程想等待子线程执行结束之后再结束，比如子线程处理一个数据，主线程要取得这个数据，就要用待jion() 方法。

方法join()的作用是等待线程对象的销毁

二、区别

　　sleep(long)方法在睡眠时不释放对象锁

　　join(long)方法在等待的过程中释放对象锁

三、实例

　　创建join_sleep_1项目

　　

　　类ThreadA.java代码如下

package extthread;

public class ThreadA extends Thread {

    private ThreadB b;

    public ThreadA(ThreadB b) {
        super();
        this.b = b;
    }

    @Override
    public void run() {
        try {
            synchronized (b) {
                b.start();
                Thread.sleep(6000);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

类ThreadB.java代码如下

package extthread;

public class ThreadB extends Thread {

    @Override
    public void run() {
        try {
            System.out.println("   b run begin timer="
                    + System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("   b run   end timer="
                    + System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    synchronized public void bService() {
        System.out.println("��ӡ��bService timer=" + System.currentTimeMillis());
    }

}

类ThreadC.java代码如

package extthread;

public class ThreadC extends Thread {

    private ThreadB threadB;

    public ThreadC(ThreadB threadB) {
        super();
        this.threadB = threadB;
    }

    @Override
    public void run() {
        threadB.bService();
    }
}

Run.java 代码

package test.run;

import extthread.ThreadA;
import extthread.ThreadB;
import extthread.ThreadC;

public class Run {
    public static void main(String[] args) {

        try {
            ThreadB b = new ThreadB();

            ThreadA a = new ThreadA(b);
            a.start();

            Thread.sleep(1000);

            ThreadC c = new ThreadC(b);
            c.start();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

结果

由于线程ThreadA使用Thread.sleep(long)方法一直持有ThreadB对象的锁，时间达到6秒，所以线程C只有在ThreadA时间到达6秒后释放ThreadB的锁时，才可以调用ThreadB中的同步方法Synchronized public void bService()

上面实验证明Thread.sleep(long)不释放锁

---

 下面实验修改ThreadA.java

package extthread;

public class ThreadA extends Thread {

    private ThreadB b;

    public ThreadA(ThreadB b) {
        super();
        this.b = b;
    }

    @Override
    public void run() {
        try {
            synchronized (b) {
                b.start();
                b.join();
                for (int i = 0; i < Integer.MAX_VALUE; i++) {
                    String newString = new String();
                    Math.random();
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

结果如下

由于线程ThreadA释放了ThreadB的锁，所以线程ThreadC可以调用ThreadB中的同步方法synchronized public void bService()

实验证明join(long)方法具有释放锁的特点