Java线程的join方法可用于暂停当前线程的执行直至目标线程死亡。Thread中一共有三个join的重载方法。

public final void join()：该方法将当前线程放入等待队列中，直至被它调用的线程死亡为止。如果该线程被中断，则会抛出InterruptedException异常。

public final synchronized void join(long millis)：该方法用于让当前线程进入等待状态，直至被它调用的线程死亡或是经过millis毫秒。由于线程的执行依赖于操作系统，所以该方法无法保证当前线程等待的时间和指定时间一致。

public final synchronized void join(long millis, int nanos)：该方法用于让线程暂停millis毫秒nanos纳秒。

下面的例子演示了join方法的使用。该段代码的目的是确保main线程最后结束，并且仅当第一个线程死亡才能启动第三个线程。

ThreadJoinExample.java

package com.journaldev.threads;
public class ThreadJoinExample {
Thread t1 = new Thread(new MyRunnable(), "t1");
Thread t2 = new Thread(new MyRunnable(), "t2");
Thread t3 = new Thread(new MyRunnable(), "t3");

t1.start();

//start second thread after waiting for 2 seconds or if it's dead
try {
t1.join(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}

t2.start();

//start third thread only when first thread is dead
try {
t1.join();
} catch (InterruptedException e) {
e.printStackTrace();
}

t3.start();

//let all threads finish execution before finishing main thread
try {
t1.join();
t2.join();
t3.join();
} catch (InterruptedException e) {
e.printStackTrace();
}

System.out.println("All threads are dead, exiting main thread");
}

class MyRunnable implements Runnable {
@Override
public void run() {
System.out.println("Thread started:::" + Thread.currentThread.getName());
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
e.printStackTrace();
}

System.out.println("Thread ended:::" +　Thread.currentThread.getName());
}
}