Android中handler的作用与线程

Android handler 的主要用途根据官网上的解释，可以概括为两点：

         There are two main uses for a Handler: (1) to schedule messages and runnables to be executed as some point in the future; and (2) to enqueue an action to be performed on a different thread than your own. 

         也就是说handler保存了两个队列，一个消息队列，一个runnable队列，handler负责顺序一个一个执行两个队列里需要处理的元素。首先需要强调一下，handler并不是开辟了一个新的线程去执行，而是在其调用线程中，这个可以用以下代码去测试：

package com.relin.handler;    import android.app.Activity;  import android.os.Bundle;  import android.os.Handler;    public class AndroidHandlerActivity extends Activity {      /** Called when the activity is first created. */      private Handler handler = new Handler();         @Override      public void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          setContentView(R.layout.main);          handler.post(r);    System.out.println("Activity-------->"+Thread.currentThread().getId());    System.out.println("Activityname----->"+Thread.currentThread().getName());      }            Runnable r = new Runnable(){       @Override       public void run(){        System.out.println("handler-------->"+Thread.currentThread().getId());        System.out.println("hanldername----->"+Thread.currentThread().getName());        try{         Thread.sleep(5000);        }catch(Exception e){                 }       }      };  }

           上述代码首先定义了一个Runable对象，但是handler.post(r)并没有开辟新的线程，很容易发现，handler实际上使用的线程就是主线程。当然我们也可以用自定义一个线程，让handler在另一个线程中执行，这样主线程不会因为handler中有大量工作需要处理而进入阻塞状态：

 

package com.relin.bundle;    import android.app.Activity;  import android.os.Bundle;  import android.os.Handler;  import android.os.HandlerThread;  import android.os.Looper;  import android.os.Message;    public class AndroidBundleActivity extends Activity {      /** Called when the activity is first created. */      @Override      public void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          setContentView(R.layout.main);          System.out.println("activity----->" + Thread.currentThread().getId());          //由android系统内核提供，其中有一个looper专门用于处理消息队列          HandlerThread handlerThread = new HandlerThread("handler_thread");          //在使用getLooper之前，必须先调用start，不然looper是null          handlerThread.start();          MyHandler myHandler = new MyHandler(handlerThread.getLooper());          Message msg = myHandler.obtainMessage();          Bundle b =  new Bundle();          b.putInt("age", 20);          b.putString("name", "relin");          msg.setData(b);          msg.sendToTarget();      }            class MyHandler extends Handler{       public MyHandler(){         }              public MyHandler(Looper looper){        super(looper);       }           @Override    public void handleMessage(Message msg) {     // TODO Auto-generated method stub     super.handleMessage(msg);     Bundle b = msg.getData();     int age = b.getInt("age");     String name = b.getString("name");     System.out.println("age is " + age + ", name is " + name);     System.out.println("handler----->" + Thread.currentThread().getId());    }             }  }