java基础知识回顾之java Thread类学习（十一）--join方法的理解

* 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.
       
     */
    // 加锁当前线程

    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) {
            //A线程是start，在运行中
            while (isAlive()) {
　　　　　　　　　　//main线程等待
                wait(0);
            }
        } else {
            //join(timeOut)的情况
            while (isAlive()) {
　　　　　　　　　　//根据当前timeout的时间-now 是否<=0进行判断，主线程是否继续阻塞等待
                long delay = millis - now;
                //等待超时时间到了，则主线程不阻塞了，等待结束
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                //子线程从循环开始到现在运行的时间
                now = System.currentTimeMillis() - base;
            }
        }
    }

package concurrentMy.joins;

/**
 * 
 * @author Administrator
 * 
 * main 线程 和 A线程，A线程是main线程创建并且启动的，main线程优先级比较高，正在执行；
 * 这个时候main线程调用A.join()之后，main线程一直等待，直到A线程运行完毕，main线程才运行
 * 
 */
class ThreadA extends Thread {
    
    public ThreadA(String name){
        super(name);
    }

    @Override
    public void run() {
       
        for (int i = 0; i < 20; i++) {
            // 复写父类的toString方法， 返回该线程的字符串表示形式，包括线程名称、优先级和线程组。
            System.out.println(Thread.currentThread().getName() + "-" + i);

        }
    }

}

public class JoinDemo {

    /**
     * @param args
     * @throws InterruptedException 
     */
    public static void main(String[] args) throws InterruptedException {
        ThreadA A = new ThreadA("线程A");
        A.start();
        A.join(); //A线程加入到“main线程”中，main线程一直等待，直到A线程执行完毕，main线程才运行
        System.out.println(Thread.currentThread().getName() + "执行");
        System.out.println(Thread.currentThread().getName() + "执行终止");

    }

}

线程A-0
线程A-1
线程A-2
线程A-3
线程A-4
线程A-5
线程A-6
线程A-7
线程A-8
线程A-9
线程A-10
线程A-11
线程A-12
线程A-13
线程A-14
线程A-15
线程A-16
线程A-17
线程A-18
线程A-19
main执行
main执行终止