http://blog.163.com/fanning_7213/blog/static/2496505201071255157497/
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1：
- package wxhx.csdn2;
- /**
- *
- * @author bzwm
- *
- */
- class CustomThread1 extends Thread {
- public CustomThread1() {
- super("[CustomThread1] Thread");
- };
- 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;
- }
- 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 JoinTestDemo {
- 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(2000);
- t.start();
- t.join();//在代碼2里，將此處注釋掉
- } catch (Exception e) {
- System.out.println("Exception from main");
- }
- System.out.println(threadName + " end!");
- }
- }
package wxhx.csdn2; /** * * @author bzwm * */ class CustomThread1 extends Thread { public CustomThread1() { super("[CustomThread1] Thread"); }; 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; } 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 JoinTestDemo { 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(2000); t.start(); t.join();//在代碼2里，將此處注釋掉 } catch (Exception e) { System.out.println("Exception from main"); } System.out.println(threadName + " end!"); } }
打印结果：
main start.//main方法所在的线程起动，但没有马上结束，因为调用t.join();，所以要等到t结束了，此线程才能向下执行。
[CustomThread1] Thread start.//线程CustomThread1起动
[CustomThread1] Thread loop at 0//线程CustomThread1执行
[CustomThread1] Thread loop at 1//线程CustomThread1执行
[CustomThread] Thread start.//线程CustomThread起动，但没有马上结束，因为调用t1.join();，所以要等到t1结束了，此线程才能向下执行。
[CustomThread1] Thread loop at 2//线程CustomThread1继续执行
[CustomThread1] Thread loop at 3//线程CustomThread1继续执行
[CustomThread1] Thread loop at 4//线程CustomThread1继续执行
[CustomThread1] Thread end. //线程CustomThread1结束了
[CustomThread] Thread end.// 线程CustomThread在t1.join();阻塞处起动，向下继续执行的结果
main end!//线程CustomThread结束，此线程在t.join();阻塞处起动，向下继续执行的结果。
修改一下代码，得到代码2：(这里只写出修改的部分)
- public class JoinTestDemo {
- 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(2000);
- t.start();
- // t.join();//在代碼2里，將此處注釋掉
- } catch (Exception e) {
- System.out.println("Exception from main");
- }
- System.out.println(threadName + " end!");
- }
- }
public class JoinTestDemo { 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(2000); t.start(); // t.join();//在代碼2里，將此處注釋掉 } catch (Exception e) { System.out.println("Exception from main"); } System.out.println(threadName + " end!"); } }
打印结果：
main start. // main方法所在的线程起动，但没有马上结束，这里并不是因为join方法，而是因为Thread.sleep(2000);
[CustomThread1] Thread start. //线程CustomThread1起动
[CustomThread1] Thread loop at 0//线程CustomThread1执行
[CustomThread1] Thread loop at 1//线程CustomThread1执行
main end!// Thread.sleep(2000);结束，虽然在线程CustomThread执行了t1.join();，但这并不会影响到其他线程(这里main方法所在的线程)。
[CustomThread] Thread start. //线程CustomThread起动，但没有马上结束，因为调用t1.join();，所以要等到t1结束了，此线程才能向下执行。
[CustomThread1] Thread loop at 2//线程CustomThread1继续执行
[CustomThread1] Thread loop at 3//线程CustomThread1继续执行
[CustomThread1] Thread loop at 4//线程CustomThread1继续执行
[CustomThread1] Thread end. //线程CustomThread1结束了
[CustomThread] Thread end. // 线程CustomThread在t1.join();阻塞处起动，向下继续执行的结果
五、从源码看join()方法
在CustomThread的run方法里，执行了t1.join();，进入看一下它的JDK源码：
- public final void join() throws InterruptedException {
- n(0);
- }
public final void join() throws InterruptedException { join(0); }
然后进入join(0)方法：
- /**
- * Waits at most <code>millis</code> milliseconds for this thread to
- * die. A timeout of <code>0</code> means to wait forever. //注意这句
- *
- * @param millis the time to wait in milliseconds.
- * @exception InterruptedException if another thread has interrupted
- * the current thread. The <i>interrupted status</i> of the
- * current thread is cleared when this exception is thrown.
- */
- public final synchronized void join(long millis) //参数millis为0.
- throws InterruptedException {
- long base = System.currentTimeMillis();
- long now = 0;
- if (millis < 0) {
- throw new IllegalArgumentException("timeout value is negative");
- }
- if (millis == 0) {//进入这个分支
- while (isAlive()) {//判断本线程是否为活动的。这里的本线程就是t1.
- wait(0);//阻塞
- }
- } else {
- while (isAlive()) {
- long delay = millis - now;
- if (delay <= 0) {
- break;
- }
- wait(delay);
- now = System.currentTimeMillis() - base;
- }
- }
- }
/** * Waits at most <code>millis</code> milliseconds for this thread to * die. A timeout of <code>0</code> means to wait forever. //注意这句 * * @param millis the time to wait in milliseconds. * @exception InterruptedException if another thread has interrupted * the current thread. The <i>interrupted status</i> of the * current thread is cleared when this exception is thrown. */ public final synchronized void join(long millis) //参数millis为0. throws InterruptedException { long base = System.currentTimeMillis(); long now = 0; if (millis < 0) { throw new IllegalArgumentException("timeout value is negative"); } if (millis == 0) {//进入这个分支 while (isAlive()) {//判断本线程是否为活动的。这里的本线程就是t1. wait(0);//阻塞 } } else { while (isAlive()) { long delay = millis - now; if (delay <= 0) { break; } wait(delay); now = System.currentTimeMillis() - base; } } }
单纯从代码上看，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。将直接继续向下执行，这里就不写代码验证了。