Android Thread

Android中很重要的一個機制就是線程+消息，當然線程並不是android獨有的，下面，簡單的說說使用線程的時候應該注意的地方

我們采用最簡單的方法來建立一個android的線程+消息的例子

1.Thread + Handler

[java] package com.example.test_thread; 
 
import android.app.Activity; 
import android.os.Bundle; 
import android.os.Handler; 
import android.os.Message; 
import android.widget.TextView; 
 
public class MainActivity extends Activity { 
 
    TextView mTextView = null; 
//  static TextView mTextView = null;  
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState); 
        setContentView(R.layout.activity_main); 
        mTextView = (TextView)findViewById(R.id.textview); 
        Thread th = new Thread(new Runnable() { 
             
            @Override 
            public void run() { 
                // TODO Auto-generated method stub  
                for(int i = 0;i<1000;i++) 
                { 
                    try { 
                        Thread.sleep(500); 
                        System.out.println("Thread running :"+i+"!"); 
                        Message msg = new Message(); 
                        msg.what = i; 
                        mHandler.sendMessage(msg); 
                    } catch (InterruptedException e) { 
                        // TODO Auto-generated catch block  
                        e.printStackTrace(); 
                    } 
                } 
            } 
        }); 
        th.start(); 
         
    } 
    public Handler mHandler = new Handler(){ 
//    public static Handler mHandler = new Handler(){  
 
        @Override 
        public void handleMessage(Message msg) { 
            // TODO Auto-generated method stub  
            super.handleMessage(msg); 
             
            mTextView.setText(String.valueOf(msg.what)); 
        } 
         
    }; 
     
} 

package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

 TextView mTextView = null;
// static TextView mTextView = null;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView)findViewById(R.id.textview);
        Thread th = new Thread(new Runnable() {
   
   @Override
   public void run() {
    // TODO Auto-generated method stub
    for(int i = 0;i<1000;i++)
    {
     try {
      Thread.sleep(500);
      System.out.println("Thread running :"+i+"!");
      Message msg = new Message();
      msg.what = i;
      mHandler.sendMessage(msg);
     } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
     }
    }
   }
  });
        th.start();
       
    }
    public Handler mHandler = new Handler(){
//    public static Handler mHandler = new Handler(){

  @Override
  public void handleMessage(Message msg) {
   // TODO Auto-generated method stub
   super.handleMessage(msg);
   
   mTextView.setText(String.valueOf(msg.what));
  }
     
    };
   
}
當我們用以上方式建立線程時，進入應用之後，線程開始運行，Handler接收消息改變UI中的TextView，此時一切正常

當按下退出時，程序退出，但是程序進程還在stack中，因此主線程之子線程，也就是我們定義的th（th_1）不會退出，此時，在log信息中可以看到，system.out還在print數字

當再次進入程序的時候，可以看到，log中打印的信息double，但是UI會按照新線程（th_2）的次序改變

此時th_1仍在運行，th_1使用的 handler_1也在運行，只不過上一個Activity的狀態已經是finish，因此不會改變UI  this ->mFinished= true

其實只要th_1中有關於上一個Activity的引用，那麼Activity就不會銷毀，java的機制就是這樣，這是我們推薦的線程機制，下面著重說一下可能遇到的問題

2.同樣是剛剛的例子，我們將Handler定義成static

[java]  public static Handler mHandler = new Handler(){ 

public static Handler mHandler = new Handler(){此時，在退出應用再重新進入時，由於Handler並不會有新的實例，因此，th_1與th_2同時發消息給一個static Handler 或者說是指向了同一塊內存區域，這時就會出現TextView上的數字來回跳的現象

3.這樣也可以

使用static定義Handler也不是不可以，只要在Activity的onCreate()中重新實例一個Handler，這樣，JVM分配另一塊內存給新的Handler，這樣運行就正常了

[java]  package com.example.test_thread; 
 
import android.app.Activity; 
import android.os.Bundle; 
import android.os.Handler; 
import android.os.Message; 
import android.widget.TextView; 
 
public class MainActivity extends Activity { 
 
    public Handler mHandler = null; 
    TextView mTextView = null; 
//  static TextView mTextView = null;  
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState); 
        setContentView(R.layout.activity_main); 
        mTextView = (TextView)findViewById(R.id.textview); 
        mHandler = new TestHandler(); 
        Thread th = new Thread(new Runnable() { 
             
            @Override 
            public void run() { 
                // TODO Auto-generated method stub  
                for(int i = 0;i<1000;i++) 
                { 
                    try { 
                        Thread.sleep(500); 
                        System.out.println("Thread running :"+i+"!"); 
                        Message msg = new Message(); 
                        msg.what = i; 
                        mHandler.sendMessage(msg); 
                    } catch (InterruptedException e) { 
                        // TODO Auto-generated catch block  
                        e.printStackTrace(); 
                    } 
                } 
            } 
        }); 
        th.start(); 
         
    } 
    class TestHandler extends Handler 
    { 
        @Override 
        public void handleMessage(Message msg) { 
            // TODO Auto-generated method stub  
            super.handleMessage(msg); 
            System.out.println("Handler running :"+msg.what+"!"); 
            mTextView.setText(String.valueOf(msg.what)); 
        } 
         
    } 
} 

package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

    public Handler mHandler = null;
 TextView mTextView = null;
// static TextView mTextView = null;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView)findViewById(R.id.textview);
        mHandler = new TestHandler();
        Thread th = new Thread(new Runnable() {
   
   @Override
   public void run() {
    // TODO Auto-generated method stub
    for(int i = 0;i<1000;i++)
    {
     try {
      Thread.sleep(500);
      System.out.println("Thread running :"+i+"!");
      Message msg = new Message();
      msg.what = i;
      mHandler.sendMessage(msg);
     } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
     }
    }
   }
  });
        th.start();
       
    }
    class TestHandler extends Handler
    {
  @Override
  public void handleMessage(Message msg) {
   // TODO Auto-generated method stub
   super.handleMessage(msg);
   System.out.println("Handler running :"+msg.what+"!");
   mTextView.setText(String.valueOf(msg.what));
  }
     
    }
}

當然，總的來說Java還是不推薦使用static變量的，這本身也不符合面向對象的變成思想，所以，建議除了一些final值，盡量還是多使用消息機制來解決問題，維護也輕松些