关于Http请求出现NetworkOnMainThreadException。

　　连不上网并不是一定Ip，tomcat错误，因为在Android4.0以后，只要是写在主线程（就是Activity）中的HTTP请求，运行时都会下面这样错，这是因为Android在4.0以后为了防止应用的ANR（aplication Not Response）异常。

java.lang.RuntimeException: Unable to start activity ComponentInfo

android.os.NetworkOnMainThreadException 

   有两种解决的方法：

1.可以再Activity的onCreate()方法中加入这样一段代码，如下：

  if (Build.VERSION.SDK_INT >= 11) {
      StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder().detectDiskReads    ().detectDiskWrites().detectNetwork().penaltyLog().build());
   StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder().detectLeakedSqlLiteObjects().detectLeakedClosableObjects().penaltyLog().penaltyDeath().build());
  }
后就可以在主线程中进行网络操作了；

2.但是我们应该这样做

  启动一条子线程进行你的网络请求。

 

建议使用第二种方法，养成规范的习惯；

 示例：

package com.example.postandroid;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.util.ArrayList;
import java.util.List;

import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.EntityUtils;

import com.example.postandroid.R.id;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class MainActivity extends Activity {
    private TextView show;
    private String URL = "http://192.168.156.1/lyl/android.jsp";
    private boolean flag = false;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        show = (TextView) findViewById(id.show);
        Thread thread = new Thread(null, threadInBack, "HttpRequest");
       //启动线程 
　　　　thread.start();
    }

    private Runnable threadInBack = new Runnable() {
        public void run() {
            httpRequest();
        }

        private void httpRequest() {
            HttpPost httpPost = new HttpPost(URL);
            List<NameValuePair> params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("id", "lyl"));
            params.add(new BasicNameValuePair("password", "123"));
            try {
                httpPost.setEntity(new UrlEncodedFormEntity(params, "UTF-8"));
                HttpResponse httpResponse = new DefaultHttpClient()
                        .execute(httpPost);
                if (httpResponse.getStatusLine().getStatusCode()!= 404) {
                    flag = Boolean.parseBoolean(EntityUtils.toString(
                            httpResponse.getEntity()).trim());
                }
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            } catch (ClientProtocolException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            if(flag){
                show.setText("登陆成功");
            }else{
                show.setText("登陆失败");
            }
        }

    };

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}