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方法的注釋上寫著：阻塞當前線程，直到收到結束執行或者死亡。當接收者的interrupt方法被調用，并且它被調用了join方法的時候，就會拋出InteruptedException。
舉個例子：線程A的run方法中調用了線程B的join，此時線程A處于阻塞狀態，直到線程B執行完畢或者死亡的時候，線程A才會繼續執行。如果此時線程A處于阻塞狀態，而線程B調用線程A的interrupt的時候，就會拋出InterruptedException。
下面舉兩個例子
示例1：
有兩個線程A，B，在A的run方法中，會調用ThreadB.join，也就是說，在ThreadA啟動之后，打印出來"ThreadA Started"之后，就會被線程B阻塞。然后線程B開始執行，打印出來"ThreadB started"之后，會休眠5S，之后再打印出"ThreadB Sleep End"，之后線程B執行結束，于是線程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");
    }
}
}

運行結果：

示例1結果

示例2
和上例不一樣的地方是，在ThreadB的Run方法中，Sleep完后，調用ThreadA.interrupt，試一下官方文檔上注釋對不對。

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");
    }
}
}

運行結果：
打印出InterruptedException

示例2運行結果