Java Thread.join()具有什么功能呢？

Thread.join()方法的功能:
  用于等待进程
 如:主线程创建子线程
      当子线程运行时间非常长时,主线程通常会在子线程之前结束,
	   当主线程需等待子线程的处理结果时，此时我们需要使用join()方法 

Thread.join()方法的语法

//无参数的join()等价于join(0),作用是一直等待该线程死亡
join() throws InterruptedException;  
//最多等待该线程死亡millis毫秒
join(long millis, int nanos) throws InterruptedException;  
//最多等待该线程死亡millis毫秒加nanos纳秒
join(long millis, int nanos) throws InterruptedException ; 

 package com.java265.other;

public class Test3 {
	public static void main(String[] args) {
		String threadName = Thread.currentThread().getName();
		System.out.println(threadName + " start.");
		BThread bt = new BThread();
		AThread at = new AThread(bt);
		try {
			bt.start();
			Thread.sleep(2000);
			at.start();
			at.join();
		} catch (Exception e) {
			System.out.println("Exception from main");
		}
		System.out.println(threadName + " end!");
	}
}

class BThread extends Thread {
	public BThread() {
		super("[BThread] Thread");
	};

	public void run() {
		String threadName = Thread.currentThread().getName();
		System.out.println(threadName + " start.");
		try {
			for (int i = 0; i < 6; i++) {
				System.out.println(threadName + " loop at " + i);
				Thread.sleep(1000);
			}
			System.out.println(threadName + " end.");
		} catch (Exception e) {
			System.out.println("Exception from " + threadName + ".run");
		}
	}
}

class AThread extends Thread {
	BThread bt;

	public AThread(BThread bt) {
		super("[AThread] Thread");
		this.bt = bt;
	}

	public void run() {
		String threadName = Thread.currentThread().getName();
		System.out.println(threadName + " start.");
		try {
			bt.join();
			System.out.println(threadName + " end.");
		} catch (Exception e) {
			System.out.println("Exception from " + threadName + ".run");
		}
	}
}
------运行以上代码，将输出以下信息------
main start.
[BThread] Thread start.
[BThread] Thread loop at 0
[BThread] Thread loop at 1
[BThread] Thread loop at 2
[AThread] Thread start.
[BThread] Thread loop at 3
[BThread] Thread loop at 4
[BThread] Thread loop at 5
[BThread] Thread end.
[AThread] Thread end.
main end!

join()方法注意事项:
  1. 当一个线程s在另一个线程t上调用t.join()
       此线程s将被挂起，直到目标线程t结束才恢复（此时t.isAlive()返回为假）
      也可以在调用join()时带上一个超时参数（单位可以是毫秒，或者毫秒和纳秒）
      当目标线程在这段时间内还没有结束的话，join()方法总能返回
       对join()方法的调用可以被中断，做法是在调用线程上调用interrupt()方法。
 2. 当线程生成后，但还未被起动，isAlive()将返回false，调用它的join()方法是没有作用的
      将直接继续向下执行