join(long)方法和sleep(long)方法的比较

join(long)方法的源代码

    public final synchronized void join(long millis)
        throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

sleep(long)方法的源代码

 public static native void sleep(long millis) throws InterruptedException; 

从源代码中可以发现，join(long)方法内部使用wait(long)实现，所以join(long)方法执行后会释放锁，所以其他线程就可以调用此线程中的同步方法

验证sleep(long)方法会不会释放锁

--------------------------------------------------------线程类--------------------------------------------------------

package com.qf.test04.thread;

/**
 * @author qf
 * @create 2018-09-20 15:22
 */
public class ThreadA extends Thread {
    private ThreadB threadB;

    public ThreadA(ThreadB threadB) {
        this.threadB = threadB;
    }

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

package com.qf.test04.thread;

/**
 * @author qf
 * @create 2018-09-20 15:22
 */
public class ThreadB extends Thread {
    @Override
    public void run() {
        try {
            System.out.println("b run begin time = "+System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("b run --end time = "+System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

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

package com.qf.test04.thread;

/**
 * @author qf
 * @create 2018-09-20 15:22
 */
public class ThreadC extends Thread {
    private ThreadB threadB;

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

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

--------------------------------------------------------测试类--------------------------------------------------------

package com.qf.test04;

import com.qf.test04.thread.ThreadA;
import com.qf.test04.thread.ThreadB;
import com.qf.test04.thread.ThreadC;

/**
 * @author qf
 * @create 2018-09-20 15:22
 */
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();
        }
    }
}

-------------------------------------------------------打印输出------------------------------------------------------

b run begin time = 1537429005098
b run --end time = 1537429010098
bService time = 1537429011098

结果显示只有a线程执行完毕(sleep了6000ms)，c线程才能执行，所以sleep方法并没有释放锁

验证join(long)方法释放锁

修改ThreadA.java中的代码

-----------------------------------------------------ThreadA.java----------------------------------------------------

package com.qf.test04.thread;

/**
 * @author qf
 * @create 2018-09-20 15:22
 */
public class ThreadA extends Thread {
    private ThreadB threadB;

    public ThreadA(ThreadB threadB) {
        this.threadB = threadB;
    }

    @Override
    public void run() {
        try {
            synchronized (threadB){
                threadB.start();
                //Thread.sleep(6000);
                threadB.join();
                for (int i = 0; i < Integer.MAX_VALUE ; i++) {
                    //用于耗时
                    String newString = new String();
                    Math.random();
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

-------------------------------------------------------打印输出------------------------------------------------------

b run begin time = 1537429346791
bService time = 1537429347790
b run --end time = 1537429351793

bService在1000后执行打印，说明c线程获得了threadB的锁，所以a线程在join后释放了锁