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());
    }
}

    上述程序的输出结果为：

Thread started:::t1Thread started:::t2Thread ended:::t1Thread started:::t3Thread ended:::t2Thread ended:::t3All threads are dead, exiting main thread