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