Question

我试图通过使用php连接到mysql的一些教程从URL获取数据。

但是，当我运行代码时，我得到一个nullpointerexception。

Database_demo.java

 package com.database_demo;

 import java.io.BufferedReader;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.util.ArrayList;
 import java.util.List;

 import org.apache.http.HttpEntity;
 import org.apache.http.HttpResponse;
 import org.apache.http.NameValuePair;
 import org.apache.http.client.HttpClient;
 import org.apache.http.client.entity.UrlEncodedFormEntity;
 import org.apache.http.client.methods.HttpPost;
 import org.apache.http.impl.client.DefaultHttpClient;
 import org.json.JSONArray;
 import org.json.JSONException;
 import org.json.JSONObject;

 import android.app.ListActivity;
 import android.net.ParseException;
 import android.os.Bundle;
 import android.widget.ArrayAdapter;
 import android.widget.Toast;

 public class Database_demo extends ListActivity {

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    String result = null;
    InputStream is = null;
    StringBuilder sb = null;
    ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
    List<String> r = new ArrayList<String>();

    try{

    //http post
    HttpClient httpclient = new DefaultHttpClient();
    HttpPost httppost = new HttpPost("http://uragon-dev.com/synergy/webservices.php?action=displayAll");
    httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
    HttpResponse response = httpclient.execute(httppost);
    HttpEntity entity = response.getEntity();
    is = entity.getContent();
    }
    catch(Exception e){
        Toast.makeText(getBaseContext(),e.toString() ,Toast.LENGTH_LONG).show();
   }

    //Convert response to string  
    try
    {
      BufferedReader reader = new BufferedReader(new InputStreamReader(is,"UTF-8"));

      sb = new StringBuilder();

      String line = null;

      while ((line = reader.readLine()) != null) 
      {
         sb.append(line + "\n");
      }

      is.close();

      result = sb.toString();
    }
    catch(Exception e)
    {
        Toast.makeText(getBaseContext(),e.toString() ,Toast.LENGTH_LONG).show();
    }
    //END Convert response to string   
    try{
            JSONArray jArray = new JSONArray(result);
            JSONObject json_data=null;
            for(int i=0;i<jArray.length();i++)
            {
               json_data = jArray.getJSONObject(i);
               r.add(json_data.getString("users"));
           }
           setListAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_expandable_list_item_1, r));
        }
        catch(JSONException e1){
            Toast.makeText(getBaseContext(),e1.toString() ,Toast.LENGTH_LONG).show();
        } catch (ParseException e1) {
            Toast.makeText(getBaseContext(),e1.toString() ,Toast.LENGTH_LONG).show();
    }

}
 }

logcat中的错误指向此行。

 try{
            JSONArray jArray = new JSONArray(result);

以下是在网址http://uragon-dev.com/synergy/webservices.php?action=displayAll

上提取的数据

{"users":[{"username":"jokerxvier","firstname":"jason","lastname":"javier"},{"username":"migz","firstname":"miguel","lastname":"rivera"}],"success":1}

造成这种情况的原因是什么？另外我想知道如何将数据传递给url而不仅仅是从中获取数据。 Android新手在这里。谢谢！

Answer 1

看起来您正尝试在主线程上尝试网络操作。看看这篇关于如何处理这个问题的帖子：https://stackoverflow.com/a/6343299/2045570

阅读docs

编辑：

您收到JSON错误的原因是因为您正在捕获您的第一个例外：HERE

    try{

    //http post
    HttpClient httpclient = new DefaultHttpClient();
    HttpPost httppost = new HttpPost("http://uragon-dev.com/synergy/webservices.php?action=displayAll");
    httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
    HttpResponse response = httpclient.execute(httppost);
    HttpEntity entity = response.getEntity();
    is = entity.getContent();
    }
    catch(Exception e){
        Toast.makeText(getBaseContext(),e.toString() ,Toast.LENGTH_LONG).show();

   }

所以这里：

JSONArray jArray = new JSONArray(result);

result将始终为null。这是您需要解决的第一个问题。您无法从主线程访问网络。

要查看您的错误消息，请在catch块中添加以下行：e.printStackTrace();，它将在您的logcat输出中打印android.os.NetworkOnMainThreadException。

Answer 2

你说错误发生在JSONArray jArray = new JSONArray(result);。如果是这种情况，您可以尝试检查result的值吗？也许使用Log在Logcat上显示它。

如果错误为NetworkOnMainThread，您需要查看@ user2045570的答案

希望这有帮助，祝你好运

里德

Answer 3

你有几个问题。您尝试读取的JSON不是一个对象的数组。所以代码应该是

JSONObject result = new JSONObject(result);
JSONArray users = result.getJSONArray("users");

并且，请不要在主线程上打开网络连接。

使用php和在android中连接到mysql的NullPointerException

3 个答案: