Android handler详解（面试百分之100问到）

handler在Android中被称为“消息处理者”，在多线程中比较常用。

handler内部实现原理

package com.example.uri;

import java.io.IOException;
import java.io.InputStream;
import java.lang.ref.WeakReference;
import java.net.MalformedURLException;
import java.net.URL;

import android.net.Uri;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.Menu;
import android.view.View;
import android.widget.ImageView;

/**
 *
 * 访问网络的操作，必须放在工作线程中完成
 *
 */
public class MainActivity extends Activity {

    private static final int LOADSUCCESS=0x1;
    private static ImageView iv;
    private final myhandler handler=new myhandler(this);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iv=(ImageView) findViewById(R.id.imageView1);
    }

    private static class myhandler extends Handler{

        private final WeakReference<MainActivity> weakReference;
        public myhandler(MainActivity mainActivity){
            weakReference=new WeakReference<MainActivity>(mainActivity);
        }

        public void handleMessage(Message msg) {
            MainActivity mainActivity=weakReference.get();
            if(mainActivity!=null){
                switch (msg.what) {
                case LOADSUCCESS:
                    MainActivity.iv.setImageBitmap((Bitmap) msg.obj);
                    break;

                }
            }
        }
    }
    public void geturl(View v){
        /*Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.baidu.com"));
        startActivity(intent);  */
        new Thread(new Runnable() {

            @Override
            public void run() {
                try {
                    URL url=new URL("http://img2.3lian.com/img2007/10/28/123.jpg");
                    InputStream in=url.openStream();
                    Bitmap bitmap=BitmapFactory.decodeStream(in);
                    Message message= handler.obtainMessage(LOADSUCCESS, bitmap);
                    handler.sendMessage(message);
                } catch (MalformedURLException e) {
                    // TODO 自动生成的 catch 块
                    e.printStackTrace();
                } catch (IOException e) {
                    // TODO 自动生成的 catch 块
                    e.printStackTrace();
                }
            }
        }).start();

    }

}