android.os.NetworkOnMainThreadException异常处理

这个异常情况是在访问网络的时候如果你把操作放在了Activity的主线程当中的话就会产生，并且导致访问网络失败。

在Android2.3之后才会有这个问题，主要是为了防止网络操作阻塞UI线程。

package com.example.asynctask_download;import org.apache.http.HttpEntity;import org.apache.http.HttpResponse;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.DefaultHttpClient;import org.apache.http.util.EntityUtils;import android.app.Activity;import android.app.ProgressDialog;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.ImageView;public class MainActivity extends Activity {private Button button1 = null;private ImageView imageView1 = null;private String imagePath = "http://img1.gtimg.com/news/pics/hv1/229/20/1529/99428554.jpg";private Bitmap bitmap = null;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);button1 = (Button)findViewById(R.id.button1);imageView1 = (ImageView)findViewById(R.id.imageView1);button1.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {//HttpClient用于发起GET或者POST请求HttpClient httpClient = new DefaultHttpClient();//创建HttpGet或HttpPost对象，将要请求的URL通过构造方法传入HttpGet或HttpPost对象。HttpGet httpGet = new HttpGet(imagePath);try{//发起GET请求第2步：使用execute方法发送HTTP,GET请求，并返回HttpResponse对象  HttpResponse httpResponse= httpClient.execute(httpGet);//判断请求响应状态码，状态码为200表示服务端成功响应了客户端的请求 if(httpResponse.getStatusLine().getStatusCode() == 200){//HttpEntity实体类  即可以是流也可以是字符串形式。 下面是一个流的形式HttpEntity  httpEntity  = httpResponse.getEntity();byte data[] = EntityUtils.toByteArray(httpEntity);bitmap = BitmapFactory.decodeByteArray(data,0,data.length);imageView1.setImageBitmap(bitmap);}}catch(Exception e){e.printStackTrace();}}});}} 

上面的程序要从网络上下载一张图片到手机屏幕上面，没有开新的线程或者使用异步任务类出现警告并且访问失败

02-25 14:44:44.222: W/System.err(21134): android.os.NetworkOnMainThreadException

package com.example.asynctask_download;import org.apache.http.HttpEntity;import org.apache.http.HttpResponse;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.DefaultHttpClient;import org.apache.http.util.EntityUtils;import android.app.Activity;import android.app.ProgressDialog;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.AsyncTask;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.ImageView;public class MainActivity extends Activity {private Button button1 = null;private ImageView imageView1 = null;private String imagePath = "http://img1.gtimg.com/news/pics/hv1/229/20/1529/99428554.jpg";private ProgressDialog dialog = null;private Bitmap bitmap = null;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);button1 = (Button)findViewById(R.id.button1);imageView1 = (ImageView)findViewById(R.id.imageView1);dialog = new ProgressDialog(this);dialog.setTitle("提示信息");dialog.setMessage("正在下载，请稍后......");button1.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {new MyTask().execute(imagePath);}});}/** * 使用异步任务的规则 * 1.声明一个类继承AsyncTask，标注三个参数的类型 * 2.第一个参数表示 我们要执行的任务，通常是网络路径 * 3.第二个参数表示进入的刻度 * 4.第三个参数表示任务执行的返回结果 * @author Administrator * */public class MyTask extends AsyncTask<String,Void,Bitmap>{//表示任务执行之前的操作@Overrideprotected void onPreExecute() {// TODO Auto-generated method stubsuper.onPreExecute();dialog.show();}//主要是完成耗时操作@Overrideprotected Bitmap doInBackground(String... params) {//通常使用网络连接类HttpClient类完成对网络数据的提取//HttpClient用于发起GET或者POST请求HttpClient httpClient = new DefaultHttpClient();//创建HttpGet或HttpPost对象，将要请求的URL通过构造方法传入HttpGet或HttpPost对象。HttpGet httpGet = new HttpGet(imagePath);try{//发起GET请求第2步：使用execute方法发送HTTP,GET请求，并返回HttpResponse对象  HttpResponse httpResponse= httpClient.execute(httpGet);//判断请求响应状态码，状态码为200表示服务端成功响应了客户端的请求 if(httpResponse.getStatusLine().getStatusCode() == 200){//HttpEntity实体类  即可以是流也可以是字符串形式。 下面是一个流的形式HttpEntity  httpEntity  = httpResponse.getEntity();byte data[] = EntityUtils.toByteArray(httpEntity);bitmap = BitmapFactory.decodeByteArray(data,0,data.length);}}catch(Exception e){e.printStackTrace();}return bitmap;}//主要是更新IO操作@Overrideprotected void onPostExecute(Bitmap result) {// TODO Auto-generated method stubsuper.onPostExecute(result);imageView1.setImageBitmap(bitmap);dialog.dismiss();}  }}

使用异步任务类完成了下载任务