java等待线程执行终止的join方法

join方法介绍

join方法是Thread类的方法，不是Object对象的方法，是无参的且返回值为void的方法。

等待子线程执行完成，再继续执行主线程

来看一个示范：

package com.tim.base.easystart.thread.base; public class JoinTest { public static void main(String[] args) throws InterruptedException { System.out.println("main thread start"); Thread threadA = new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("child thread: threadA is over"); } }); Thread threadB = new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("child thread: threadB is over"); } }); threadA.start(); threadB.start(); System.out.println("threadA and threadB has been started"); //等待threadA和threadB执行完毕返回 threadA.join(); threadB.join(); System.out.println("all child thread over!"); } }

输出结果：

main thread start threadA and threadB has been started child thread: threadA is over child thread: threadB is over all child thread over!

可以看出2个子线程threadA和threadB启动之后，main线程继续执行，等执行完join方法之后，main线程暂停执行，等待2个线程执行完成，才继续执行join方法之后的程序代码。

线程中断与join方法

public class JoinInterruptTest { public static void main(String[] args) { System.out.println("main thread start"); Thread threadA = new Thread(new Runnable() { @Override public void run() { System.out.println("child thread: threadA begin"); for (;;) { try { Thread.sleep(500); System.out.println("child thread:threadA is executing"); } catch (InterruptedException e) { e.printStackTrace(); } } } }); Thread mainThread = Thread.currentThread(); Thread threadB = new Thread(new Runnable() { @Override public void run() { System.out.println("child thread: threadB begin"); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("child thread: threadB will interrupt mainThread"); mainThread.interrupt(); } }); threadA.start(); threadB.start(); try { threadA.join(); } catch (InterruptedException e) { System.out.println("main thread:" + e); } } }

输出结果：

main thread start child thread: threadA begin child thread: threadB begin child thread:threadA is executing child thread:threadA is executing child thread: threadB will interrupt mainThread main thread:java.lang.InterruptedException child thread:threadA is executing child thread:threadA is executing

…

child thread:threadA is executing child thread:threadA is executing

threadB执行过程中调用了主线程mainThread的interrupt方法，为主线程设置了中断标志，这时在threadA.join等待threadA返回的try{}catch{}代码块中会捕获到InterruptedException，但是threadA并没有被中断，还是继续执行。

一直执行的线程休眠中被中断的不会导致线程退出

还发现一个有趣的现象：在上面的代码中将mainThread.interrupt();换成threadA.interrupt();

package com.tim.base.easystart.thread.base; public class JoinInterruptTest { public static void main(String[] args) { System.out.println("main thread start"); Thread threadA = new Thread(new Runnable() { @Override public void run() { System.out.println("child thread: threadA begin"); for (;;) { try { Thread.sleep(1000); } catch (InterruptedException e) { System.out.println("threadA is interrupted..." + e); } System.out.println("threadA is still executing..."); } } }); Thread mainThread = Thread.currentThread(); Thread threadB = new Thread(new Runnable() { @Override public void run() { System.out.println("child thread: threadB begin"); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("child thread: threadB will interrupt mainThread"); threadA.interrupt(); } }); threadA.start(); threadB.start(); try { threadA.join(); } catch (InterruptedException e) { System.out.println("main thread:" + e); } } }

输出结果：

main thread start child thread: threadA begin child thread: threadB begin threadA is still executing… child thread: threadB will interrupt mainThread threadA is interrupted…java.lang.InterruptedException: sleep interrupted threadA is still executing… threadA is still executing…

…

threadA is still executing… threadA is still executing…

可以看出threadA在sleep的时候被设置中断标志后，线程并没有停止运行，还是在继续执行。