作用：
主线程等待子线程执行完成后结束，join()的作用是等待线程对象的销毁

1、join中断

join期间主线程被中断则不会等待子线程执行完毕，所以应该在主线程被中断时调用interrupt方法中断子线程

  1. package com.imooc.thread_demo.join;
  3. import lombok.Setter;
  5. /**
  6.  * @Author: zhangjx
  7.  * @Date: 2020/9/12 16:41
  8.  * @Description:
  9.  */
  10. public class JoinInterrupt {
  12.     public static void main(String[] args) {
  13.         Thread thread = new Thread(new SubThread(Thread.currentThread()));
  15.         thread.start();
  16.         try {
  17.             thread.join();
  18.         } catch (InterruptedException e) {
  19.             System.out.println("主线程被中断========");;
  20.             thread.interrupt();
  21.         }
  22.         System.out.println("主线程运行完毕=========");
  23.     }
  25.     static class SubThread implements Runnable{
  27.         private Thread mainThread;
  29.         public SubThread(Thread mainThread) {
  30.             this.mainThread = mainThread;
  31.         }
  33.         @Override
  34.         public void run() {
  36.             try {
  37.                 mainThread.interrupt();
  38.                 Thread.sleep(5000);
  39.             } catch (InterruptedException e) {
  40.                 System.out.println("子线程被中断=========");
  41.             }
  42.             System.out.println("子线程运行完毕======");
  43.         }
  44.     }
  45. }

此时主线程被中断后子线程也会被中断。

  1. 主线程被中断========
  2. 主线程运行完毕=========
  3. 子线程被中断=========
  4. 子线程运行完毕======

2、join期间，主线程的状态

主线程运行状态为Waiting

  1. package com.imooc.thread_demo.join;
  3. /**
  4.  * @Author: zhangjx
  5.  * @Date: 2020/9/12 17:39
  6.  * @Description:
  7.  */
  8. public class JoinState {
  10.     public static void main(String[] args) {
  11.         Thread thread = new Thread(new SubThread(Thread.currentThread()));
  13.         thread.start();
  14.         try {
  15.             thread.join();
  16.         } catch (InterruptedException e) {
  17.             System.out.println("主线程被中断========");;
  18.         }
  19.         System.out.println("主线程运行完毕=========");
  20.     }
  22.     static class SubThread implements Runnable{
  24.         private Thread mainThread;
  26.         public SubThread(Thread mainThread) {
  27.             this.mainThread = mainThread;
  28.         }
  30.         @Override
  31.         public void run() {
  33.             try {
  34.                 Thread.sleep(5000);
  35.                 System.out.println("主线程状态：" + mainThread.getState());
  36.             } catch (InterruptedException e) {
  37.                 System.out.println("子线程被中断=========");
  38.             }
  39.             System.out.println("子线程运行完毕======");
  40.         }
  41.     }
  42. }
  1. "C:\Program Files\Java\jdk1.8.0_171\bin\java.exe" "-javaagent:D:\idea2020\IntelliJ IDEA 2020.1.2\lib\idea_rt.jar=4155:D:\idea2020\IntelliJ IDEA 2020.1.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_171\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_171\jre\lib\rt.jar;C:\Users\z\Desktop\mooc\多线程\thread_demo\target\classes;E:\repository\org\springframework\boot\spring-boot-starter\2.2.2.RELEASE\spring-boot-starter-2.2.2.RELEASE.jar;E:\repository\org\springframework\boot\spring-boot\2.2.2.RELEASE\spring-boot-2.2.2.RELEASE.jar;E:\repository\org\springframework\spring-context\5.2.2.RELEASE\spring-context-5.2.2.RELEASE.jar;E:\repository\org\springframework\spring-aop\5.2.2.RELEASE\spring-aop-5.2.2.RELEASE.jar;E:\repository\org\springframework\spring-beans\5.2.2.RELEASE\spring-beans-5.2.2.RELEASE.jar;E:\repository\org\springframework\spring-expression\5.2.2.RELEASE\spring-expression-5.2.2.RELEASE.jar;E:\repository\org\springframework\boot\spring-boot-autoconfigure\2.2.2.RELEASE\spring-boot-autoconfigure-2.2.2.RELEASE.jar;E:\repository\org\springframework\boot\spring-boot-starter-logging\2.2.2.RELEASE\spring-boot-starter-logging-2.2.2.RELEASE.jar;E:\repository\ch\qos\logback\logback-classic\1.2.3\logback-classic-1.2.3.jar;E:\repository\ch\qos\logback\logback-core\1.2.3\logback-core-1.2.3.jar;E:\repository\org\apache\logging\log4j\log4j-to-slf4j\2.12.1\log4j-to-slf4j-2.12.1.jar;E:\repository\org\apache\logging\log4j\log4j-api\2.12.1\log4j-api-2.12.1.jar;E:\repository\org\slf4j\jul-to-slf4j\1.7.29\jul-to-slf4j-1.7.29.jar;E:\repository\jakarta\annotation\jakarta.annotation-api\1.3.5\jakarta.annotation-api-1.3.5.jar;E:\repository\org\springframework\spring-core\5.2.2.RELEASE\spring-core-5.2.2.RELEASE.jar;E:\repository\org\springframework\spring-jcl\5.2.2.RELEASE\spring-jcl-5.2.2.RELEASE.jar;E:\repository\org\yaml\snakeyaml\1.25\snakeyaml-1.25.jar;E:\repository\org\springframework\boot\spring-boot-devtools\2.2.2.RELEASE\spring-boot-devtools-2.2.2.RELEASE.jar;E:\repository\org\springframework\boot\spring-boot-configuration-processor\2.2.2.RELEASE\spring-boot-configuration-processor-2.2.2.RELEASE.jar;E:\repository\org\projectlombok\lombok\1.18.10\lombok-1.18.10.jar;E:\repository\org\slf4j\slf4j-api\1.7.29\slf4j-api-1.7.29.jar" com.imooc.thread_demo.join.JoinState
  2. 主线程状态：WAITING
  3. 子线程运行完毕======
  4. 主线程运行完毕=========

3、join注意点

线程执行完毕后会主动调用notify_all方法，而join实质上就是使用wait()实现的，当子线程执行完毕唤醒主线程

  1.  public final synchronized void join(long millis)
  2.     throws InterruptedException {
  3.         long base = System.currentTimeMillis();
  4.         long now = 0;
  6.         if (millis < 0) {
  7.             throw new IllegalArgumentException("timeout value is negative");
  8.         }
  10.         if (millis == 0) {
  11.             while (isAlive()) {
  12.                 wait(0);
  13.             }
  14.         } else {
  15.             while (isAlive()) {
  16.                 long delay = millis - now;
  17.                 if (delay <= 0) {
  18.                     break;
  19.                 }
  20.                 wait(delay);
  21.                 now = System.currentTimeMillis() - base;
  22.             }
  23.         }
  24.     }

join等价于

  1. synichronized(thread.wait();){
  2.     thread.wait();
  3. }

3.1 、CountDownLatch
3.2 、CyclicBarrier