【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。