看了代码一切就明白了。做下记录：

package com.bupt.test;public class JoinTest {		public static void main(String[] args) {		Thread t = new Thread(new RunThread());		t.start();		try {			t.join();  //等到t线程执行结束后，再继续执行			System.out.println("main end");		} catch (InterruptedException e) {			e.printStackTrace();		}	}}class RunThread implements Runnable {	@Override	public void run() {		System.out.println("Thread begin");		try {			Thread.sleep(2000);		} catch (InterruptedException e) {			e.printStackTrace();		}		System.out.println("Thread end");	}	}

package com.bupt.test;public class JoinTest {		public static void main(String[] args) {		Thread t = new Thread(new RunThread());		t.start();		try {			t.join(1000); //只等1秒，就继续执行。			System.out.println("main end");		} catch (InterruptedException e) {			e.printStackTrace();		}	}}class RunThread implements Runnable {	@Override	public void run() {		System.out.println("Thread begin");		try {			Thread.sleep(2000);		} catch (InterruptedException e) {			e.printStackTrace();		}		System.out.println("Thread end");	}	}