Android -- Thread

创建线程的方法：

官网的说法：

There are basically two main ways of having a Thread execute application code.
One is providing a new class that extends Thread and overriding its run() method.
The other is providing a new Thread instance with a Runnable object during its creation. In both cases, the start() method must be called to actually execute the new Thread.

扩展Thread的示例代码：

public class CommonTestActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        new Thread(){
            public void run(){
                System.out.println("Thread is running.");
            }
        }.start();
    }
}

使用Runnable的示例代码：

public class CommonTestActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        new Thread(r).start();
    }

    Runnable r = new Runnable() {
        @Override
        public void run() {
            System.out.println("Runnable running ");
        }
    };

}

经典实例：

多个窗口一起卖火车票问题。假设有3个窗口同时售票，共有10张火车票代售。启动三个线程卖共同的10张票。

1. 使用下面的代码试图实现功能

public class CommonTestActivity extends Activity{

    private Button mybutton;
    private TextView mytext;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        mybutton = (Button) findViewById(R.id.button);
        mytext = (TextView) findViewById(R.id.text);

        mybutton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                MyThread myThread1 = new MyThread();
                MyThread myThread2= new MyThread();
                MyThread myThread3 = new MyThread();
                myThread1.start();
                myThread2.start();
                myThread3.start();
            }
        });

    }

    class MyThread extends Thread {
        private int tickets = 10;

        public void run() {
            for (int i = 0; i < 200; i++) {
                if (tickets > 0) {
                    System.out.println(Thread.currentThread().getName() + "==>"
                            + tickets--);
                }
            }
        }
    }

}

输出如下

11-17 22:45:01.234: I/System.out(672): Thread-10==>10
11-17 22:45:01.234: I/System.out(672): Thread-10==>9
11-17 22:45:01.234: I/System.out(672): Thread-10==>8
11-17 22:45:01.234: I/System.out(672): Thread-10==>7
11-17 22:45:01.234: I/System.out(672): Thread-10==>6
11-17 22:45:01.234: I/System.out(672): Thread-10==>5
11-17 22:45:01.234: I/System.out(672): Thread-10==>4
11-17 22:45:01.234: I/System.out(672): Thread-10==>3
11-17 22:45:01.244: I/System.out(672): Thread-10==>2
11-17 22:45:01.244: I/System.out(672): Thread-10==>1
11-17 22:45:01.244: I/System.out(672): Thread-11==>10
11-17 22:45:01.244: I/System.out(672): Thread-11==>9
11-17 22:45:01.244: I/System.out(672): Thread-11==>8
11-17 22:45:01.244: I/System.out(672): Thread-11==>7
11-17 22:45:01.244: I/System.out(672): Thread-11==>6
11-17 22:45:01.254: I/System.out(672): Thread-11==>5
11-17 22:45:01.254: I/System.out(672): Thread-11==>4
11-17 22:45:01.254: I/System.out(672): Thread-11==>3
11-17 22:45:01.254: I/System.out(672): Thread-11==>2
11-17 22:45:01.254: I/System.out(672): Thread-11==>1
11-17 22:45:01.264: I/System.out(672): Thread-12==>10
11-17 22:45:01.264: I/System.out(672): Thread-12==>9
11-17 22:45:01.264: I/System.out(672): Thread-12==>8
11-17 22:45:01.264: I/System.out(672): Thread-12==>7
11-17 22:45:01.264: I/System.out(672): Thread-12==>6
11-17 22:45:01.274: I/System.out(672): Thread-12==>5
11-17 22:45:01.274: I/System.out(672): Thread-12==>4
11-17 22:45:01.274: I/System.out(672): Thread-12==>3
11-17 22:45:01.274: I/System.out(672): Thread-12==>2
11-17 22:45:01.274: I/System.out(672): Thread-12==>1

分析：

运行结果与预期不一致，分析可以看出3个线程各种卖各自的10张票，而不是共同的10张票。

在上面的代码中，只能保证：每个线程都将启动，每个线程都将运行直到完成。一系列线程以某种顺序启动并不意味着将按该顺序执行。对于任何一组启动的线程来说，调度程序不能保证其执行次序，持续时间也无法保证。

2. 只修改onClick里面的代码，并分析运行结果

public void onClick(View v) {
    //实例化线程对象
    MyThread myThread = new MyThread();
    new Thread(myThread, "窗口1").start();
    new Thread(myThread, "窗口2").start();
    new Thread(myThread, "窗口3").start();
}

输出如下

11-17 22:49:17.314: I/System.out(708): 窗口1==>10
11-17 22:49:17.314: I/System.out(708): 窗口1==>9
11-17 22:49:17.314: I/System.out(708): 窗口1==>8
11-17 22:49:17.314: I/System.out(708): 窗口1==>7
11-17 22:49:17.314: I/System.out(708): 窗口1==>6
11-17 22:49:17.324: I/System.out(708): 窗口1==>5
11-17 22:49:17.324: I/System.out(708): 窗口1==>4
11-17 22:49:17.324: I/System.out(708): 窗口1==>3
11-17 22:49:17.324: I/System.out(708): 窗口1==>2
11-17 22:49:17.324: I/System.out(708): 窗口1==>1

分析：

我认为此处3个窗口已经是在卖共同的10张票了，只不过由于没有进行线程同步才造成数据混乱。

线程的同步是为了防止多个线程访问一个数据对象时，对数据造成的破坏。

3. 下面继续修改代码，验证我刚得猜测。

public void onClick(View v) { 
    //实例化线程对象 
    MyThread myThread = new MyThread(); 
    new Thread(myThread, "窗口1").start(); 
    new Thread(myThread, "窗口2").start(); 
    new Thread(myThread, "窗口3").start(); 
}

class MyThread extends Thread{ 
    private int tickets = 10; 
    public void run() { 
        for(int i = 0; i< 200; i++){ 
                sell(); 
        } 
    } 
    public synchronized  void sell(){ 
            if(tickets > 0) 
            { 
                System.out.println(Thread.currentThread().getName() + "==>" + tickets--); 
            } 
    } 
}

运行结果：

05-11 08:53:31.986: INFO/System.out(7116): 窗口1==>10 
05-11 08:53:32.006: INFO/System.out(7116): 窗口1==>9 
05-11 08:53:32.016: INFO/System.out(7116): 窗口1==>8 
05-11 08:53:32.066: INFO/System.out(7116): 窗口1==>7 
05-11 08:53:32.086: INFO/System.out(7116): 窗口1==>6 
05-11 08:53:32.106: INFO/System.out(7116): 窗口1==>5 
05-11 08:53:32.106: INFO/System.out(7116): 窗口1==>4 
05-11 08:53:32.126: INFO/System.out(7116): 窗口1==>3 
05-11 08:53:32.146: INFO/System.out(7116): 窗口1==>2 
05-11 08:53:32.146: INFO/System.out(7116): 窗口1==>1

分析：

一个对象只有一个锁。所以，如果一个线程获得该锁，就没有其他线程可以获得锁，直到第一个线程释放（或返回）锁。这也意味着任何其他线程都不能进入该对象上的synchronized方法或代码块，直到该锁被释放。

释放锁是指持锁线程退出了synchronized同步方法或代码块。

上述代码没有Thread.sleep(10)，换句话说线程一一直处于运行状态，没有释放锁，没有给其他线程运行的机会。

4. 根据上述分析，修改代码如下：

public void onClick(View v) { 
        //实例化线程对象 
        MyThread myThread = new MyThread(); 
        new Thread(myThread, "窗口1").start(); 
        new Thread(myThread, "窗口2").start(); 
        new Thread(myThread, "窗口3").start(); 
    } 
    class MyThread extends Thread{ 
        private int tickets = 10; 
        public void run() { 
            for(int i = 0; i< 200; i++){ 
                try{ 
                    sell(); 
                    Thread.sleep(10); 
                }catch (InterruptedException e) { 
                    System.out.println("我被打断了" + Thread.currentThread().getName()); 
                    e.printStackTrace(); 
                }

            } 
        } 
        public synchronized  void sell(){ 
                if(tickets > 0) 
                { 
                    System.out.println(Thread.currentThread().getName() + "==>" + tickets--); 
                } 
        } 
    }

运行结果：

05-11 09:17:07.496: INFO/System.out(7898): 窗口1==>10 
05-11 09:17:07.528: INFO/System.out(7898): 窗口1==>9 
05-11 09:17:07.546: INFO/System.out(7898): 窗口1==>8 
05-11 09:17:07.577: INFO/System.out(7898): 窗口1==>7 
05-11 09:17:07.626: INFO/System.out(7898): 窗口3==>6 
05-11 09:17:07.626: INFO/System.out(7898): 窗口2==>5 
05-11 09:17:07.636: INFO/System.out(7898): 窗口1==>4 
05-11 09:17:07.646: INFO/System.out(7898): 窗口2==>3 
05-11 09:17:07.646: INFO/System.out(7898): 窗口1==>2 
05-11 09:17:07.656: INFO/System.out(7898): 窗口3==>1

分析：

此次得出的正是我们想要的结果