Thread中join()使用

一. 使用方式

Thread thread = new Thread("[test] thread");
thread.start();
thread.join();

二. 为啥要用join()

比如当子线程中业务逻辑处理的时间很长时，那么主线程就会先于子线程提前结束，而如果想要主线程在子线程处理完以后再结束（比如需要子线程中返回的数据），那就可以使用 Thread threadSon.join()；

三. 作用

join()上面的解释：

Waits for this thread to die.

当前线程指得是子线程，即阻塞主线程继续执行，直到子线程处理结束；

四. 实例

public class ThreadTest {
    public static void main(String[] args) throws InterruptedException {
        String mainThread = Thread.currentThread().getName();
        System.out.println(mainThread + " start..");
        ThreadB threadB = new ThreadB();
        threadB.start();
        threadB.join();
        System.out.println(mainThread + " end..");
    }
}


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

    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start..");
        for(int i=0;i<5;i++) {
            System.out.println(threadName + " loop at 0" + i);
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(threadName + " end..");
    }
}

执行结果：

main start..
[ThreadB] thread start..
[ThreadB] thread loop at 00
[ThreadB] thread loop at 01
[ThreadB] thread loop at 02
[ThreadB] thread loop at 03
[ThreadB] thread loop at 04
[ThreadB] thread end..
main end..

五. 源码解析

jdk中join()的源码：

public final void join() throws InterruptedException {
	join(0);
}

主要是接下来的 join(long millis)

/**
* Waits at most {@code millis} milliseconds for this thread to
* die. A timeout of {@code 0} means to wait forever.
*/
public final synchronized void join(long millis)    //注意这里的synchronized
throws InterruptedException {
	long base = System.currentTimeMillis();
	long now = 0;

	if (millis < 0) {
		throw new IllegalArgumentException("timeout value is negative");
	}
    //如果millis为0的话，只要线程（上面的子线程ThreadB）还活着，就会调用wait(0)阻塞当前线程，
    //一直到threadB执行结束；（wait(0)相当于持有threadB线程的锁然后等待）
	if (millis == 0) {
		while (isAlive()) {
			wait(0);
		}
	} else {
		while (isAlive()) {
			long delay = millis - now;
			if (delay <= 0) {
				break;
			}
			wait(delay);
			now = System.currentTimeMillis() - base;
		}
	}
}

六. 我的感想

我刚刚是在springboot启动过程中的源码才注意到的（）：

private void refreshContext(ConfigurableApplicationContext context) {
	refresh(context);
	if (this.registerShutdownHook) {
		try {
        //看到了这里
			context.registerShutdownHook();
		}
		catch (AccessControlException ex) {
			// Not allowed in some environments.
		}
	}
}

然后跟着context.registerShutdownHook();继续挖掘，直到看到了这个类ApplicationShutdownHooks，它里面的runHooks()方法如下：

static void runHooks() {
	Collection<Thread> threads;
	synchronized(ApplicationShutdownHooks.class) {
		threads = hooks.keySet();
		hooks = null;
	}

	for (Thread hook : threads) {
		hook.start();
	}
	for (Thread hook : threads) {
		try {
        //看到了这个join()方法
			hook.join();
		} catch (InterruptedException x) { }
	}
}