thread join

简单解释下，join就是调用wait()使线程进入WAIT状态。使线程完全执行完成。

/**
 * 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;
        }
    }
}

join使用时注意几点：

1、join与start调用顺序问题

　　上面的讨论大概知道了join的作用了，那么，入股 join在start前调用，会出现什么后果呢？先看下面的测试结果。

package com.dxz.join;
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 < 5; 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");
        }
    }
}

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

执行结果：

main start.
[BThread] Thread start.
[BThread] Thread loop at 0
[BThread] Thread loop at 1
main end!
[BThread] Thread loop at 2
[BThread] Thread loop at 3
[BThread] Thread loop at 4
[BThread] Thread end.

main线程没有等待[BThread]执行完再执行。join方法必须在线程start方法调用之后调用才有意义。这个也很容易理解：如果一个线程都没有start，那它也就无法同步了。

2、join()与异常

在join()过程中，如果当前线程被中断，则当前线程出现异常。(注意是调用thread.join()的线程被中断才会进入异常，比如a线程调用b.join()，a中断会报异常而b中断不会异常)

如下:threadB中启动threadA，并且调用其方法等待threadA完成，此时向threadB发出中断信号，会进入中断异常代码。

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

执行结果：

main start.
[BThread] Thread start.
[BThread] Thread loop at 0
Exception in thread "main" java.lang.InterruptedException
    at java.lang.Object.wait(Native Method)
    at java.lang.Thread.join(Thread.java:1249)
    at java.lang.Thread.join(Thread.java:1323)
    at com.dxz.join.TestDemo.main(TestDemo.java:46)
[BThread] Thread loop at 1
[BThread] Thread loop at 2
[BThread] Thread loop at 3
[BThread] Thread loop at 4
[BThread] Thread end.