【Java多线程系列随笔一】浅析 Java Thread.join()
一、在研究join的用法之前，先明确两件事情。
1、join方法定义在Thread类中，则调用者必须是一个线程，
例如：
Thread t = new CustomThread(); //这里一般是自定义的线程类 t.start();//线程起动 t.join();//此处会抛出InterruptedException异常
2、上面的两行代码也是在一个线程里面执行的。
以上出现了两个线程，一个是我们自定义的线程类，我们实现了run方法，做一些我们需要的工作；另外一个线程，生成我们自定义线程类的对象，然后执行。
customThread.start();
customThread.join();
在这种情况下，两个线程的关系是一个线程由另外一个线程生成并起动，所以我们暂且认为第一个线程叫做“子线程”，另外一个线程叫做“主线程”。
二、为什么要用join()方法
主线程生成并起动了子线程，而子线程里要进行大量的耗时的运算(这里可以借鉴下线程的作用)，当主线程处理完其他的事务后，需要用到子线程的处理结果，这个时候就要用到join();方法了。
三、join方法的作用
在网上看到有人说“将两个线程合并”。这样解释我觉得理解起来还更麻烦。不如就借鉴下API里的说法： “等待该线程终止。”
解释一下，是主线程(我在“一”里已经命名过了)等待子线程的终止。也就是在子线程调用了join()方法后面的代码，只有等到子线程结束了才能执行。(Waits for this thread to die.)
四、用实例来理解
写一个简单的例子来看一下join()的用法，一共三个类：
1.CustomThread 类
2. CustomThread1类
3. JoinTestDemo 类，main方法所在的类。
代码一、
class CustomThread1 extends Thread { public CustomThread1() { super("[CustomThread1] Thread"); } @Override public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); try { for (int i = 0; i < 5; i++) { System.out.println(threadName + " loop at " + i); Thread.sleep(1000); } System.out.println(threadName + " end."); } catch (Exception e) { System.out.println("Exception from " + threadName + ".run"); } } } class CustomThread extends Thread { CustomThread1 t1; public CustomThread(CustomThread1 t1){ super("[CustomThread] Thread"); this.t1 = t1; } @Override public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start!"); try { t1.join(); System.out.println(threadName + " end!"); } catch (Exception e) { System.out.println("Exception from " + threadName + " run!"); } } } public class JoinTest { public static void main(String[] args) { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); CustomThread1 t1 = new CustomThread1(); CustomThread t = new CustomThread(t1); try { t1.start(); Thread.sleep(1000L); t.start(); t.join(); //在代码2里，注释掉此行 } catch (InterruptedException e) { System.out.println("Exception from main"); } System.out.println(threadName + " end!"); } }
打印结果：
main start. [CustomThread1] Thread start. [CustomThread1] Thread loop at 0 [CustomThread] Thread start! [CustomThread1] Thread loop at 1 [CustomThread1] Thread loop at 2 [CustomThread1] Thread loop at 3 [CustomThread1] Thread loop at 4 [CustomThread1] Thread end. [CustomThread] Thread end! main end!
代码二、
package com.concurrent.test.join; class CustomThread1 extends Thread { public CustomThread1() { super("[CustomThread1] Thread"); } @Override public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); try { for (int i = 0; i < 5; i++) { System.out.println(threadName + " loop at " + i); Thread.sleep(1000); } System.out.println(threadName + " end."); } catch (Exception e) { System.out.println("Exception from " + threadName + ".run"); } } } class CustomThread extends Thread { CustomThread1 t1; public CustomThread(CustomThread1 t1){ super("[CustomThread] Thread"); this.t1 = t1; } @Override public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start!"); try { t1.join(); System.out.println(threadName + " end!"); } catch (Exception e) { System.out.println("Exception from " + threadName + " run!"); } } } public class JoinTest { public static void main(String[] args) { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start."); CustomThread1 t1 = new CustomThread1(); CustomThread t = new CustomThread(t1); try { t1.start(); Thread.sleep(1000L); t.start(); //t.join(); } catch (InterruptedException e) { System.out.println("Exception from main"); } System.out.println(threadName + " end!"); } }
打印结果：
main start. [CustomThread1] Thread start. [CustomThread1] Thread loop at 0 main end! [CustomThread1] Thread loop at 1 [CustomThread] Thread start! [CustomThread1] Thread loop at 2 [CustomThread1] Thread loop at 3 [CustomThread1] Thread loop at 4 [CustomThread1] Thread end. [CustomThread] Thread end!
五、从源码看join()方法
JDK源码：
/** * Waits for this thread to die. * * <p> An invocation of this method behaves in exactly the same * way as the invocation * * <blockquote> * {@linkplain #join(long) join}{@code (0)} * </blockquote> * * @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. */ public final void join() throws InterruptedException { join(0); }
进入join(0) 方法，查看源码：
/** * 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. * * millis 主线程等待时间 */ 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; } } }
如果线程被生成了，但还未被起动，调用它的join()方法是没有作用的。
