join()方法的使用

1.join()方法介绍

join():使所属的线程对象x正常执行run()方法中的任务，而使当前线程z进行无限期的阻塞，等待线程x销毁后再继续执行z后面的代码。

join(long):等待一定时间。

• 在join过程中，如果当前线程被中断，则当前线程出现异常，但join所属的线程继续运行。
• join()在内部使用wait()方法进行等待，所以会释放锁

package chapter3.join;

public class MyThread extends Thread{
	
	@Override
	public void run() {
		super.run();
		try {
			int secondValue = (int) (Math.random()*10000);
			System.out.println(secondValue);
			Thread.sleep(secondValue);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}

package chapter3.join;

public class Test {

    public static void main(String[] args) {
        try {
            MyThread myThread = new MyThread();
            myThread.start();
            myThread.join();
            System.out.println("MyThread 执行完之后执行！");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

运行结果:

9574
MyThread 执行完之后执行！

2.join()方法后面的代码提前运行

join()方法源码:

//不带参数
public final void join() throws InterruptedException {
     join(0);
} 
//带参数
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;
            }
        }
    }

package chapter3.join;

public class ThreadA extends Thread{
	
	private ThreadB threadB;
	
	public ThreadA(ThreadB threadB) {
		this.threadB = threadB;
	}
	
	@Override
	public void run() {
		super.run();
		try {
			synchronized (threadB) {
				System.out.println("begin A ThreadName="+Thread.currentThread().getName()+"--"+System.currentTimeMillis());
				Thread.sleep(3000);
				System.out.println("end A ThreadName="+Thread.currentThread().getName()+"--"+System.currentTimeMillis());
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}

package chapter3.join;

public class ThreadB extends Thread{
	
	@Override
	synchronized public void run() {
		try {
			super.run();
			System.out.println("begin B ThreadName="+Thread.currentThread().getName()+"--"+System.currentTimeMillis());
			Thread.sleep(3000);
			System.out.println("end B ThreadName="+Thread.currentThread().getName()+"--"+System.currentTimeMillis());
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}

package chapter3.join;

public class Test {

	public static void main(String[] args) {
		try {
			ThreadB threadB = new ThreadB();
			ThreadA threadA = new ThreadA(threadB);
			threadA.start();
			threadB.start();
			threadB.join(2000);
			System.out.println("main end:"+System.currentTimeMillis());
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

 运行结果1:

begin A ThreadName=Thread-1--1561019085673
end A ThreadName=Thread-1--1561019088673
main end:1561019088673
begin B ThreadName=Thread-0--1561019088673
end B ThreadName=Thread-0--1561019091673

• threadB.join(2000)先抢到threadB锁,然后释放；
• ThreadA抢到锁，打印begin，sleep;
• 打印end,释放锁;
• threadB.join(2000)和ThreadB争抢锁，join再次抢到，发现时间已过，释放然后打印main end;
• ThreadB抢到锁，打印begin,end;

运行结果2:

begin A ThreadName=Thread-1--1561019572226
end A ThreadName=Thread-1--1561019575226
begin B ThreadName=Thread-0--1561019575226
end B ThreadName=Thread-0--1561019578226
main end:1561019578226

• threadB.join(2000)先抢到threadB锁,然后释放；
• ThreadA抢到锁，打印begin，sleep;
• 打印end,释放锁;
• threadB.join(2000)和ThreadB争抢锁，ThreadB抢到，打印begin,sleep,打印end后释放锁；
• mian end最后输出;