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());
		}
    	
    }
}