thread.join
java多线程编程join的作用是等待线程结束，这个作用可以产生很多特定的场景。
1）A线程中调用B线程的join方法，那么A线程需要等待B线程执行完成后才能完成
2）主线程中依次调用A线程的join方法，B线程的join方法，可以保证A，B线程顺序执行；
是主线程进入等待状态，子线程在运行，子线程运行完成后会通知主线程继续运行，或者join也可以设置主线程的等待时间，当主线程等待超时时，即使子线程没有运行完，主线程也会开始继续执行，
实例1：
public class jointest1 {
public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(new MyRunnable(), "t1");
t1.start();
new JoinTest1().listAllThread();
// t1.join();//等价于t1.join(0);一直等待到线程t1结束，主线程才继续运行
// t1.join(1000);//线程t1等待1s后，会超时，此时t1没结束，主线程此时会继续运行
t1.join(6000);//线程t1的等待时间大于运行时间，t1运行结束后，主线程才继续运行，不必等待6s
// t1.join(3000);//两个时间一致，线程t1运行结束后，主线程才继续运行
new JoinTest1().listAllThread();
System.out.println(new Date() + "主线程");
}
/**
* 显示该程序的所有线程
*/
public void listAllThread() {
System.out.println(new Date() + " listAllThread...");
ThreadGroup currentGroup =
Thread.currentThread().getThreadGroup();
int noThreads = currentGroup.activeCount();
Thread[] lstThreads = new Thread[noThreads];
currentGroup.enumerate(lstThreads);
for (int i = 0; i < noThreads; i++)
System.out.println("线程号：" + i + " = " + lstThreads[i].getName());
}
}
class MyRunnable implements Runnable {
@Override
public void run() {
try {
Thread.sleep(3000);//模拟使线程运行3s
} catch (InterruptedException e) {
e.printstacktrace();
}
}
}
输出：
t1.join();
Sun Sep 30 11:22:01 CST 2018 listAllThread...
线程号：0 = main
线程号：1 = t1
Sun Sep 30 11:22:04 CST 2018 listAllThread...
线程号：0 = main
Sun Sep 30 11:22:04 CST 2018主线程
//线程t1已结束
t1.join(1000);
Sun Sep 30 11:22:42 CST 2018 listAllThread...
线程号：0 = main
线程号：1 = t1
Sun Sep 30 11:22:43 CST 2018 listAllThread...
线程号：0 = main
线程号：1 = t1 //线程t1仍然存活
Sun Sep 30 11:22:43 CST 2018主线程
t1.join(6000);
Sun Sep 30 11:25:18 CST 2018 listAllThread...
线程号：0 = main
线程号：1 = t1
Sun Sep 30 11:25:21 CST 2018 listAllThread...
线程号：0 = main
Sun Sep 30 11:25:21 CST 2018主线程
//线程t1已结束
t1.join(3000);
Sun Sep 30 11:36:43 CST 2018 listAllThread...
线程号：0 = main
线程号：1 = t1
Sun Sep 30 11:36:46 CST 2018 listAllThread...
线程号：0 = main
Sun Sep 30 11:36:46 CST 2018主线程
//线程t1已结束
join源码，java1.8:
/**
* Waits at most {@code millis} milliseconds for this thread to
* die. A timeout of {@code 0} means to wait forever.
*
* <p> This implementation uses a loop of {@code this.wait} calls
* conditioned on {@code this.isAlive}. As a thread terminates the
* {@code this.notifyAll} method is invoked. It is recommended that
* APPlications not use {@code wait}, {@code notify}, or
* {@code notifyAll} on {@code Thread} instances.
*
* @param millis
* the time to wait in milliseconds
*
* @throws illegalargumentException
* if the value of {@code millis} is negative
*
* @throws InterruptedException
* if any thread has interrupted the current thread. The
* <i>interrupted status</i> of the current thread is
* cleared when this exception is thrown.
*/
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);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
可见另一篇博客：
《thread.join，理解与实例（二）》https://blog.csdn.net/u010002184/article/details/82910229
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