join()是Thread类的一个方法。根据jdk文档的定义:
public final void join()throws InterruptedException: Waits for this thread to die. join()方法的作用,是等待这个线程结束
但显然,这样的定义并不清晰。个人认为"Java 7 Concurrency Cookbook"的定义较为清晰:
join() method suspends the execution of the calling thread until the object called finishes its execution.
也就是说,t.join()方法阻塞调用此方法的线程(calling thread),直到线程t完成,此线程再继续;通常用于在main()主线程内,等待其它线程完成再结束main()主线程。例如:
package com.bijian.test; import java.util.Date; import java.util.concurrent.TimeUnit; public class JoinTester01 implements Runnable { private String name; public JoinTester01(String name) { this.name = name; } public void run() { System.out.printf("%s begins: %s\n", name, new Date()); try { TimeUnit.SECONDS.sleep(4); } catch (InterruptedException e) { e.printStackTrace(); } System.out.printf("%s has finished: %s\n", name, new Date()); } public static void main(String[] args) { Thread thread1 = new Thread(new JoinTester01("One")); Thread thread2 = new Thread(new JoinTester01("Two")); thread1.start(); thread2.start(); try { thread1.join(); thread2.join(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Main thread is finished"); } }
上述代码如果没有join()方法,输出如下:
Main thread is finished One begins: Wed Aug 28 10:21:36 CST 2013 Two begins: Wed Aug 28 10:21:36 CST 2013 Two has finished: Wed Aug 28 10:21:40 CST 2013 One has finished: Wed Aug 28 10:21:40 CST 2013
但有了join()方法,输出如下:
One begins: Sun Jan 20 17:53:38 CST 2019 Two begins: Sun Jan 20 17:53:38 CST 2019 Two has finished: Sun Jan 20 17:53:42 CST 2019 One has finished: Sun Jan 20 17:53:42 CST 2019 Main thread is finished
可以看出主线程main比其它两个线程先结束。
最后来深入了解一下join(),请看其源码:
/** * Waits at most {@code millis} milliseconds for this thread to * die. A timeout of {@code 0} means to wait forever. * * <p> This implementation uses a loop of {@code this.wait} calls * conditioned on {@code this.isAlive}. As a thread terminates the * {@code this.notifyAll} method is invoked. It is recommended that * applications not use {@code wait}, {@code notify}, or * {@code notifyAll} on {@code Thread} instances. * * @param millis * the time to wait in milliseconds * * @throws IllegalArgumentException * if the value of {@code millis} is negative * * @throws InterruptedException * if any thread has interrupted the current thread. The * <i>interrupted status</i> of the current thread is * cleared when this exception is thrown. * 此处A timeout of 0 means to wait forever 字面意思是永远等待,其实是等到t结束后 */ 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; } } }
可以看出,Join方法实现是通过wait(小提示:Object 提供的方法)。 当main线程调用t.join时候,main线程会获得线程对象t的锁(wait 意味着拿到该对象的锁),调用该对象的wait(等待时间),直到该对象唤醒main线程 ,比如退出后。这就意味着main 线程调用t.join时,必须能够拿到线程t对象的锁。
package com.bijian.test; public class JoinTester02 implements Runnable { Thread thread; public JoinTester02(Thread thread) { this.thread = thread; } public void run() { synchronized (thread) { System.out.println("getObjectLock"); try { Thread.sleep(9000); } catch (InterruptedException ex) { ex.printStackTrace(); } System.out.println("ReleaseObjectLock"); } } public static void main(String[] args) { Thread thread = new Thread(new JoinTester01("Three")); Thread getLockThread = new Thread(new JoinTester02(thread)); getLockThread.start(); thread.start(); try { thread.join(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Main finished!"); } }
输出如下:
getObjectLock Three begins: Sun Jan 20 18:05:31 CST 2019 Three has finished: Sun Jan 20 18:05:35 CST 2019 ReleaseObjectLock Main finished!
getLockThread通过 synchronized(thread),获取线程对象t的锁,并Sleep(9000)后释放,这就意味着,即使main方法t.join(1000)等待一秒钟,它必须等待ThreadTest 线程释放t锁后才能进入wait方法中。
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