初识多线程

在java中 创建一个线程可以有两种方法：1：实现Runnable接口 2：派生Thread类本身

1 实现Runnable接口

   实现Runnable接口类只需要一个run（）方法 run方法是程序中的另一个执行线程的入口点，这个线程在run方法返回时结束。

   构造函数Thread（Runnable threadOb， String threadName）中的threadOb定义了线程的执行从哪开始。线程的名字由threadName决定。新线程创建之后，只有调用了他的start（）方法才执行，本质上，start（）方法执行对run（）方法的调用。

例子：

ThreadDemo：

package thread;

public class ThreadDemo {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub

        new NewThread();
        try {
            for (int n = 5; n > 0; n--) {
                System.out.println("Main Thread:" + n);
                Thread.sleep(5000);
            }
        } catch (Exception e) {
            // TODO: handle exception
            System.out.println("Main interrupted");
        }
    }

}

NewThread：

package thread;

public class NewThread implements Runnable{
    Thread t;
    NewThread(){
        t = new Thread(this,"Demo Thread");
        //t = new Thread();
        System.out.println("this:"+this);
        System.out.println("Child Thread:"+t);
        t.start();
    }
    public void run()
    {
        try {
            for (int n = 5; n > 0; n--) {
                System.out.println("Child Thread:" + n);
                Thread.sleep(1000);
            }
        } catch (Exception e) {
            // TODO: handle exception
            System.out.println("Child interrupted");
        }
        System.out.println("Exiting child thread");
    }

}

运行结果：

Child Thread:Thread[Demo Thread,5,main]

Main Thread:5

Child Thread:5

Child Thread:4

Child Thread:3

Main Thread:4

Child Thread:2

Main Thread:3

Child Thread:1

Exiting child thread

Main Thread:2

Main Thread:1

在上诉代码中， 如果将NewThread中的t = new Thread(this,"Demo Thread");改为t = new Thread();则子线程将不会执行，结果如下：

this:thread.NewThread@c17164

Child Thread:Thread[Thread-0,5,main]

Main Thread:5

Main Thread:4

Main Thread:3

Main Thread:2

Main Thread:1

将t.start()改为t.run()。执行效果如下：

this:thread.NewThread@c17164

Child Thread:Thread[Demo Thread,5,main]

Child Thread:5

Child Thread:4

Child Thread:3

Child Thread:2

Child Thread:1

Exiting child thread

Main Thread:5

Main Thread:4

Main Thread:3

Main Thread:2

Main Thread:1

可以看出t = new Thread(this,"Demo Thread") 第一个参数this表示希望新线程调用this对象上的run方法，然后再调用start（），也就是从run（）方法开始启用线程的执行。调用start后，NewThread的构造函数返回到main函数中，主线程重新激活并进入for循环。

2 派生Thread类本身，扩展线程

  第二种创建线程的方法是创建一个扩展Thread类的新类，然后创建了一个该类的实例。这个扩展类必须重写run（）方法，这是新线程的进入点，同时也必须调用start（）方法来执行新线程。

NewThread1：

package thread;

public class NewThread1 extends Thread{

    NewThread1()
    {
        super("Demo Thread");
        System.out.println("Child thread:"+this);
        start();
    }
    public void run()
    {
        try {
            for (int n = 5; n > 0; n--) {
                System.out.println("Child Thread:" + n);
                Thread.sleep(500);
            }
        } catch (Exception e) {
            // TODO: handle exception
            System.out.println("Child interrupted");
        }
        System.out.println("Exiting child thread");
    }

}

  super("Demo Thread");super方法的调用，它调用的线程构造函数是：public Thread（String threadname）

ExtendThread：

package thread;

public class ExtendThread {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        new NewThread1();
        try {
            for (int n = 5; n > 0; n--) {
                System.out.println("Main Thread:" + n);
                Thread.sleep(1000);
            }
        } catch (Exception e) {
            // TODO: handle exception
            System.out.println("Main interrupted");
        }
        System.out.println("Exiting Main thread");
    }

}

运行结果输出：

Child thread:Thread[Demo Thread,5,main]

Main Thread:5

Child Thread:5

Child Thread:4

Child Thread:3

Main Thread:4

Child Thread:2

Main Thread:3

Child Thread:1

Exiting child thread

Main Thread:2

Main Thread:1

Exiting Main thread

两种创建子线程的方法，如果需要以某种方式增强或修改Thread类时才应该派生Thread，如果不想重写Thread类的其他方法，还是简单的实现Runnable接口吧。