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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);
}
}
关于Http请求出现NetworkOnMainThreadException。
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原文地址：http://www.cnblogs.com/mydomainlistentome/p/4701617.html
