一、join()方法，官方描述 waits for this thread to die 等待当前线程死亡；
源码：
//无参，默认调用join(0)
public final void join() throws InterruptedException {
join(0);
}
//传入两时间millis 毫秒+nanos纳秒，表示等等millis+nanos,最终还是调用了方法3
public final synchronized void join(long millis, int nanos)
throws InterruptedException {
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
}
join(millis);
}
//方法3：传入等时间，单位为毫秒，底层调用Object的wait(time)
public final synchronized void join(long millis)
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (millis == 0) {
while (isAlive()) {
wait(0);//表示一直等待，指导线程死亡
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
二、应用，比如几个线程要按一定顺序执行
public class ThreadJoin extends Thread {
public void run(){
try {
this.sleep(500);
System.out.println("["+new Date()+"]"+this.getName());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) throws Exception {
int length = 5;
Thread[] threads = new Thread[length];
for(int i=0; i<length;i++){
threads[i] = new ThreadJoin();
threads[i].start();
threads[i].join();//每个线程开启后都调用join
}
long endTime=System.currentTimeMillis();
}
}
/*
output:
[Sun Jun 11 13:40:42 CST 2017]Thread-0
[Sun Jun 11 13:40:43 CST 2017]Thread-1
[Sun Jun 11 13:40:43 CST 2017]Thread-2
[Sun Jun 11 13:40:44 CST 2017]Thread-3
[Sun Jun 11 13:40:44 CST 2017]Thread-4
如果把join()注释
结果可能每次都不一样
[Sun Jun 11 13:51:09 CST 2017]Thread-2
[Sun Jun 11 13:51:09 CST 2017]Thread-4
[Sun Jun 11 13:51:09 CST 2017]Thread-1
[Sun Jun 11 13:51:09 CST 2017]Thread-3
[Sun Jun 11 13:51:09 CST 2017]Thread-0
*/
做事不能急，给自己一个大方向后总得一步一步来