java线程

zyme007

Java中的线程包括前台线程(用户线程)和后台线程(守护线程)。当Java程序中只剩下后台线程运行时，整个Java程序随即变退出。相反，但还有前台线程在运行前，整个Java程序仍保持运行。
当线程产生而开始进行的时候，默认是作为前台线程运行的。让线程成为后台线程的方法是：在Thread对象调用start()方法前，先调用Thread对象的setDaemon(true)方法。其中若参数加false则让其成为前台线程之意。
一、创建线程的两种方法
    1、继承Thread类，并覆盖Thread类的run()方法，要执行的代码写入子类的run()方法体内。如：
class MyThread extends Thread {
    public void run() {
        //要执行的代码
    }
}
然后创建MyThread的一个对象，并调用它的start()方法（从父类Thread继承而得来的）：
Thread t = new MyThread();
t.setDaemon(false); //若想让其成为后台进程则传递true
t.start();
    2、Thread(Runnable target) 创建Thread对象时，给Thread构造函数传递一个实现Runnable接口的类。
    Thread(Runnable target)
Runnalbe接口只有一个方法，即run()方法。通过new Thread(Runnable target) 创建的Thread对象start()时调用的对象参数对应的run(),而不是Thread本身默认的run()。
具体做法如下：
class MyThread implements Runnable {
    public void run() {
        //要执行的代码
    }
}
...
Thread t = new Thread(new MyThread());
t.start();
二、两种线程创建方法的比较
   使用Runnable接口适合多个相同程序代码的线程去处理同一资源的情况。
   举个例子：比如有4个售票窗口要共同卖10张票，两种创建线程的方法，执行的结果是不一样的：
   1、采用Runnable接口方法
    class Aa
    {
     public static void main(String[] args)
     {
      TestThread tt = new TestThread();
      new Thread(tt).start();
      new Thread(tt).start();
      new Thread(tt).start();
      new Thread(tt).start();
     }
    }
   
    class TestThread implements Runnable
    {
     int tickets = 10;
   
     public void run()
     {
      while (tickets > 0)
      {
       System.out.println("TestThread: "
         + Thread.currentThread().getName() + "the tickets = "
         + tickets--);
      }
     }
    }
   
    执行结果：
    TestThread: Thread-0the tickets = 10
    TestThread: Thread-1the tickets = 9
    TestThread: Thread-1the tickets = 8
    TestThread: Thread-1the tickets = 7
    TestThread: Thread-1the tickets = 6
    TestThread: Thread-1the tickets = 5
    TestThread: Thread-0the tickets = 4
    TestThread: Thread-1the tickets = 3
    TestThread: Thread-0the tickets = 2
    TestThread: Thread-2the tickets = 1
    就相当于采用了线程同步
   
    2、采用继承Thread类的方法
    class Aa
    {
     public static void main(String[] args)
     {
      TestThread tt1 = new TestThread();
      tt1.start();
      TestThread tt2 = new TestThread();
      tt2.start();
      TestThread tt3 = new TestThread();
      tt3.start();
      TestThread tt4 = new TestThread();
      tt4.start();
     }
    }
   
    class TestThread extends Thread
    {
     int tickets = 10;
   
     public void run()
     {
      while (tickets > 0)
      {
       System.out.println("TestThread: "
         + Thread.currentThread().getName() + "the tickets = "
         + tickets--);
      }
     }
    }           
    执行结果：
    TestThread: Thread-0the tickets = 10
    TestThread: Thread-0the tickets = 9
    TestThread: Thread-0the tickets = 8
    TestThread: Thread-0the tickets = 7
    TestThread: Thread-0the tickets = 6
    TestThread: Thread-0the tickets = 5
    TestThread: Thread-0the tickets = 4
    TestThread: Thread-0the tickets = 3
    TestThread: Thread-0the tickets = 2
    TestThread: Thread-1the tickets = 10
    TestThread: Thread-0the tickets = 1
    TestThread: Thread-2the tickets = 10
    TestThread: Thread-3the tickets = 10
    TestThread: Thread-1the tickets = 9
    TestThread: Thread-2the tickets = 9
    TestThread: Thread-3the tickets = 9
    TestThread: Thread-1the tickets = 8
    TestThread: Thread-2the tickets = 8
    TestThread: Thread-3the tickets = 8
    TestThread: Thread-1the tickets = 7
    TestThread: Thread-2the tickets = 7
    TestThread: Thread-3the tickets = 7
    TestThread: Thread-1the tickets = 6
    TestThread: Thread-2the tickets = 6
    TestThread: Thread-3the tickets = 6
    TestThread: Thread-1the tickets = 5
    TestThread: Thread-2the tickets = 5
    TestThread: Thread-1the tickets = 4
    TestThread: Thread-3the tickets = 5
    TestThread: Thread-2the tickets = 4
    TestThread: Thread-1the tickets = 3
    TestThread: Thread-3the tickets = 4
    TestThread: Thread-2the tickets = 3
    TestThread: Thread-3the tickets = 3
    TestThread: Thread-2the tickets = 2
    TestThread: Thread-1the tickets = 2
    TestThread: Thread-3the tickets = 2
    TestThread: Thread-2the tickets = 1
    TestThread: Thread-1the tickets = 1
    TestThread: Thread-3the tickets = 1
   
    可见用Runnable接口方式才能让4个售票点共同销售，而继承Thread类，4个线程的数据各自独立
三、联合线程
    t.join()的作用是将线程t合并到调用join()方法的当前线程中。说白了就是，t线程的代码不执行完，当前线程中的代码就只能一直等待。
    join()方法可带一表示合并时间的参数N，单位为毫秒。意思是N毫秒之后又恢复到合并前的状态。


本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u2/67175/showart_682512.html