Question

我正在开发一个使用java-servlet访问数据库的Andriod应用程序。由于我正在使用sdk 23，以前的一些模块已被弃用，所以我使用URLconnection类来获取与servlet的连接。但是通过运行下面的代码，我的应用程序停止工作。

App基于Navigation抽屉活动构建，下面的代码在一个片段类中实现。是的，我已经为网络

设置了权限

package com.example.nirmal.gaminghub;


import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.os.AsyncTask;
import android.widget.Toast;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.lang.Object;

public class GameList extends Fragment {


TextView gm_lst=(TextView)getView().findViewById(R.id.game_list);
Button show;
String str ="" ;

public GameList() {
    // Required empty public constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    new AsyncTask<String, String, String>() {
        protected String doInBackground(String... url) {
            try {
                URL openUrl = new URL("http://localhost:8080/GamingHub/ShowDataServlet");
                HttpURLConnection connection = (HttpURLConnection) openUrl.openConnection();
                connection.setDoInput(true);
              //  Toast.makeText(getApplicationContext(),"hello",Toast.LENGTH_LONG).show();
                BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                String line = "";
                StringBuilder getOutput = new StringBuilder();
                while ((line = br.readLine()) != null) {
                    getOutput.append(line);
                }
                br.close();
                str=line;
            } catch (Exception e) {
                e.printStackTrace();
            }
            return str;
        }

        protected void OnPostExecute(String otpt)
        {
            gm_lst.setText(otpt);
        }
    }.execute();
    gm_lst.setText(str);
    return inflater.inflate(R.layout.fragment_game_list, container, false);
   }
 }

下面的代码是针对servlet的，它工作正常。

package com.gaming_hub;

import java.io.DataOutputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


 public class ShowDataServlet extends HttpServlet {


protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException, ClassNotFoundException, SQLException {
    response.setContentType("text/html;charset=UTF-8");
    try (PrintWriter out = response.getWriter()) {
         Class.forName("com.mysql.jdbc.Driver"); 

        Connection con=DriverManager.getConnection(  
            "jdbc:mysql://localhost:3306/gaming_hub","root","");
        String sql="SELECT * from games";
        PreparedStatement ps=con.prepareStatement(sql);
        ResultSet rs=ps.executeQuery();

       // DataOutputStream dout=new DataOutputStream();
        while(rs.next())
        {
            out.println(rs.getString(1));
        }

    }
}


@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    try {
        processRequest(request, response);
    } catch (ClassNotFoundException ex) {
        Logger.getLogger(ShowDataServlet.class.getName()).log(Level.SEVERE, null, ex);
    } catch (SQLException ex) {
        Logger.getLogger(ShowDataServlet.class.getName()).log(Level.SEVERE, null, ex);
    }
}


@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    try {
        processRequest(request, response);
    } catch (ClassNotFoundException ex) {
        Logger.getLogger(ShowDataServlet.class.getName()).log(Level.SEVERE, null, ex);
    } catch (SQLException ex) {
        Logger.getLogger(ShowDataServlet.class.getName()).log(Level.SEVERE, null, ex);
    }
}


@Override
public String getServletInfo() {
    return "Short description";
}// </editor-fold>

}

Answer 1

堆栈跟踪可能会清楚地指出问题出在代码行中：

TextView gm_lst=(TextView)getView().findViewById(R.id.game_list);

问题与Fragment生命周期有关。你不能（成功地）在那时调用findViewById()作为Fragment并且它的视图还不存在。

例如，onViewCreated()方法可以安全地调用findViewById()。所以你可以尝试类似的东西：

public class GameList extends Fragment {


TextView gm_lst;
Button show;
String str ="" ;

public GameList() {
    // Required empty public constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_game_list, container, false);
   }
 }

 @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

    gm_lst = (TextView)getView().findViewById(R.id.game_list);

    new AsyncTask<String, String, String>() {
        protected String doInBackground(String... url) {
            try {
                URL openUrl = new URL("http://localhost:8080/GamingHub/ShowDataServlet");
                HttpURLConnection connection = (HttpURLConnection) openUrl.openConnection();
                connection.setDoInput(true);
              //  Toast.makeText(getApplicationContext(),"hello",Toast.LENGTH_LONG).show();
                BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                String line = "";
                StringBuilder getOutput = new StringBuilder();
                while ((line = br.readLine()) != null) {
                    getOutput.append(line);
                }
                br.close();
                str=line;
            } catch (Exception e) {
                e.printStackTrace();
            }
            return str;
        }

        protected void OnPostExecute(String otpt)
        {
            gm_lst.setText(otpt);
        }
    }.execute();
    gm_lst.setText(str);

    }

然后另一个问题是，您指定了str = line;并且仅在您可能想要br.readLine()的内容时获取最后getOutput返回的内容。

如何使用URLconnection将servlet连接到Android应用程序。

1 个答案: