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。
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