Blocks the current Thread (<code>Thread.currentThread()</code>) until the receiver finishes its execution and dies.
@throws InterruptedException if <code>interrupt()</code> was called for the receiver while it was in the <code>join()</code> call
@see Object#notifyAll
@see java.lang.ThreadDeath

join方法的注释上写着：阻塞当前线程，直到收到结束执行或者死亡。当接收者的interrupt方法被调用，并且它被调用了join方法的时候，就会抛出InteruptedException。
举个例子：线程A的run方法中调用了线程B的join，此时线程A处于阻塞状态，直到线程B执行完毕或者死亡的时候，线程A才会继续执行。如果此时线程A处于阻塞状态，而线程B调用线程A的interrupt的时候，就会抛出InterruptedException。
下面举两个例子
示例1：
有两个线程A，B，在A的run方法中，会调用ThreadB.join，也就是说，在ThreadA启动之后，打印出来"ThreadA Started"之后，就会被线程B阻塞。然后线程B开始执行，打印出来"ThreadB started"之后，会休眠5S，之后再打印出"ThreadB Sleep End"，之后线程B执行结束，于是线程A阻塞解除，打印出"ThreadA End"。

public class Test {
public static void main(String[] args) {
    ThreadB threadB = new ThreadB();
    ThreadA threadA = new ThreadA(threadB);
    threadA.start();
    threadB.start();
}

private static class ThreadA extends Thread {
    ThreadB joinThread;

    ThreadA(ThreadB threadB) {
        joinThread = threadB;
    }

    @Override
    public void run() {
        super.run();
        System.out.println("ThreadA Started");
        try {
            joinThread.join();
        } catch (InterruptedException e) {
        }
        System.out.println("ThreadA End");
    }
}

private static class ThreadB extends Thread {
    @Override
    public void run() {
        super.run();
        System.out.println("ThreadB started");
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("ThreadB Sleep End");
    }
}
}

运行结果：

示例1结果

示例2
和上例不一样的地方是，在ThreadB的Run方法中，Sleep完后，调用ThreadA.interrupt，试一下官方文档上注释对不对。

public class Test {
public static void main(String[] args) {
    ThreadB threadB = new ThreadB();
    ThreadA threadA = new ThreadA(threadB);
    threadB.setThreadA(threadA);
    threadA.start();
    threadB.start();
}

private static class ThreadA extends Thread {
    ThreadB joinThread;

    ThreadA(ThreadB threadB) {
        joinThread = threadB;
    }

    @Override
    public void run() {
        super.run();
        System.out.println("ThreadA Started");
        try {
            joinThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("ThreadA End");
    }
}

private static class ThreadB extends Thread {
    ThreadA threadA;

    public void setThreadA(ThreadA threadA) {
        this.threadA = threadA;
    }

    @Override
    public void run() {
        super.run();
        System.out.println("ThreadB started");
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("ThreadB Call ThreadA.interrupt");
        threadA.interrupt();
        System.out.println("ThreadB Sleep End");
    }
}
}

运行结果：
打印出InterruptedException

示例2运行结果