解析Android应用程序发送的JSON数据

Question

The application published by hmkcode.com takes some data from user and parse & convert , and send to a java servlet called jsonservlet . I changed the input values parsed but I'm receiving everytime NullPointerException .

java.lang.NullPointerException
    java.io.StringReader.<init>(StringReader.java:50)
    com.fasterxml.jackson.core.JsonFactory.createParser(JsonFactory.java:796)
    com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2034)
    iaau.uimsmobile.getData.getDataServlet.doPost(getDataServlet.java:48)
    iaau.uimsmobile.getData.getDataServlet.doGet(getDataServlet.java:65)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)

---

LogCat trace

02-01 10:31:38.232      424-424/com.iaaum D/dalvikvm﹕ GC_EXTERNAL_ALLOC freed 81K, 52% free 2630K/5379K, external 904K/1038K, paused 72ms
02-01 10:32:03.392      424-424/com.iaaum W/KeyCharacterMap﹕ No keyboard for id 0
02-01 10:32:03.392      424-424/com.iaaum W/KeyCharacterMap﹕ Using default keymap: /system/usr/keychars/qwerty.kcm.bin
02-01 10:32:08.962      424-433/com.iaaum D/InputStream﹕ Connection to http://localhost:8080 refused
02-01 10:34:28.142      424-434/com.iaaum D/InputStream﹕ Connection to http://localhost:8080 refused

---

At last, I changed the strings in JSON format inside the toString() methods in both sides. The failure what @SotiriosDelimanolis talked about is appeared .

com.fasterxml.jackson.databind.JsonMappingException: No content to map due to end-of-input
 at [Source: java.io.StringReader@30b3b5; line: 1, column: 1]
    com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:164)
    com.fasterxml.jackson.databind.ObjectMapper._initForReading(ObjectMapper.java:2931)
    com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2873)
    com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2034)
    iaau.uimsmobile.getData.getDataServlet.doPost(getDataServlet.java:48)
    iaau.uimsmobile.getData.getDataServlet.doGet(getDataServlet.java:65)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)

---

Client side --> User.java

public class User{
private String IDnumber;
private String Password;

public String getIDnumber() {
    return IDnumber;
}

public void setIDnumber(String IDnumber) {
    this.IDnumber = IDnumber;
}

public String getPassword() {
    return Password;
}

public void setPassword(String password) {
    Password = password;
}

@Override
public String toString()
{
 // { "IDnumber":"" , "Password":"" }
    return "{\"IDnumber\":"+"\""+IDnumber+"\""+",\"Password\":"+"\""+Password+"\"}";
}
}

---

Client side --> LoginActivity.java

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.util.Log;
import android.widget.Toast;

import com.iaaum.user.User;

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

public class LoginActivity extends Activity implements OnClickListener
{
    TextView IsConnected;
    EditText id_number;
    EditText password;
    Button sign_button;

    User current_user;

    public String urlAddress = "http://192.168.1.101:8080/UIMSMobile/getDataServlet";
  //public String urlAddress = "http://127.0.0.1:8080/UIMSMobile/getDataServlet"; 
  //public String urlAddress = "http://localhost:8080/UIMSMobile/getDataServlet";

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_login);

        // get reference to the views
        IsConnected = (TextView) findViewById(R.id.isConnected);

        id_number = (EditText) findViewById(R.id.id_number);
        password = (EditText) findViewById(R.id.password);

        sign_button= (Button) findViewById(R.id.sign_in_button);

        // check if you are connected or not
        if(IsConnectedMethod())
        {
            IsConnected.setBackgroundColor(0xFF00CC00);
            IsConnected.setText("You are connected");
        }
        else{
            IsConnected.setText("You are NOT connected");
        }

        // add click listener to Button "POST"
        sign_button.setOnClickListener(this);
    }

    @Override
    public void onClick(View view)
    {

        switch(view.getId()){
            case R.id.sign_in_button:
                if(!validate())
                    Toast.makeText(getBaseContext(), "Enter some data!", Toast.LENGTH_LONG).show();
                // call AsyncTask to perform network operation on separate thread
                new HttpAsyncTask().execute(urlAddress);
                break;
        }

    }

    public boolean IsConnectedMethod()
    {
        ConnectivityManager connMgr = (ConnectivityManager) getSystemService(Activity.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
        return networkInfo != null && networkInfo.isConnected();
   }

    public static String Post(String url, User current_user)
    {
        InputStream inputStream = null;
        String result = "";

        try
        {
            // 1. create HttpClient
            HttpClient httpclient = new DefaultHttpClient();

            // 2. make POST request to the given URL
            HttpPost httpPost = new HttpPost(url);

            String json = "";


            // 3. build jsonObject
            JSONObject jsonObject = new JSONObject();

            jsonObject.accumulate("IDnumber", current_user.getIDnumber());
            jsonObject.accumulate("Password", current_user.getPassword());

            // 4. convert JSONObject to JSON in String
            json = jsonObject.toString();

            // ** Alternative way to convert Person object to JSON string using Jackson Lib
            // ObjectMapper mapper = new ObjectMapper();
            // json = mapper.writeValueAsString(person);

            // 5. set json to StringEntity
            StringEntity se = new StringEntity(json);

            // 6. set httpPost Entity
            httpPost.setEntity(se);

            // 7. Set some headers to inform server about the type of the content
            httpPost.setHeader("Accept", "application/json");
            httpPost.setHeader("Content-type", "application/json");

            // 8. Execute POST request to the given URL
            HttpResponse httpResponse = httpclient.execute(httpPost);

            // 9. receive response as inputStream
            inputStream = httpResponse.getEntity().getContent();


            // 10. convert inputstream to string
            if(inputStream != null)
                result = convertInputStreamToString(inputStream);
            else
                result = "Did not work!";

        } catch (Exception e) {
            Log.d("InputStream", e.getLocalizedMessage());
        }

        // 11. return result
        return result;
    }


    private class HttpAsyncTask extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... urls)
        {
            current_user = new User();
            current_user.setIDnumber(id_number.getText().toString());
            current_user.setPassword(password.getText().toString());

            return Post(urls[0], current_user);
        }
        // onPostExecute displays the results of the AsyncTask.
        @Override
        protected void onPostExecute(String result) {
            Toast.makeText(getBaseContext(), "Data Sent!", Toast.LENGTH_LONG).show();
        }
    }


    private boolean validate()
    {
        if(id_number.getText().toString().trim().equals(""))
            return false;
        else return !password.getText().toString().trim().equals("");
    }

    private static String convertInputStreamToString(InputStream inputStream) throws IOException {
        BufferedReader bufferedReader = new BufferedReader( new InputStreamReader(inputStream));
        String line = "";
        String result = "";
        while((line = bufferedReader.readLine()) != null)
            result += line;

        inputStream.close();
        return result;
    }

}

Client side --> AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.iaaum" >

    <uses-sdk
        android:minSdkVersion="8"
        android:maxSdkVersion="19"
        />

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.INTERNET"/>


    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

        <activity
            android:name="com.iaaum.LoginActivity"
            android:label="@string/title_activity_login" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Server side --> User.java

public class User 
{
    private String idnumber;
    private String password;

    public User(String idnumber, String password) {
        this.idnumber = idnumber;
        this.password = password;
    }

    public String getIDnumber() {
        return idnumber;
    }

    public void setIDnumber(String idnumber) {
        this.idnumber = idnumber;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString()
    {
        return "{\"IDnumber\":"+"\""+idnumber+"\""+",\"Password\":"+"\""+password+"\"}";
    }        
}

Server side --> getDataServlet.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.fasterxml.jackson.databind.ObjectMapper;


public class getDataServlet extends HttpServlet 
{

    private static final long serialVersionUID = 1L;

    // This will store all received articles
    List<User> _users = new LinkedList<User>();

    /***************************************************
     * URL: /getDataServlet
     * doPost(): receives JSON data, parse it, map it and send back as JSON
     * @param request
     * @param response
     * @throws javax.servlet.ServletException
     * @throws java.io.IOException
     ****************************************************/
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
        {

        // 1. get received JSON data from request
        BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));
        String json = "";

            if(br.readLine() != null)
            {
                 json = br.readLine();
            }

        // 2. initiate jackson mapper
                ObjectMapper mapper = new ObjectMapper();

                // 3. Convert received JSON to User
                User _user = mapper.readValue(json, User.class);

        // 4. Set response type to JSON
        response.setContentType("application/json");            

                // 5. Add article to List<Article>
        _users.add(_user);

        // 6. Send List<Article> as JSON to client
                mapper.writeValue(response.getOutputStream(), _users);
    }



    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
    {
        doPost(request, response);

    }
    @Override
    public String getServletInfo() 
    {
        return "Short description";
    }

}

Answer 1

The only way that exception could be thrown is if the String json you pass in

mapper.readValue(json, User.class);

is null .

You're doing something wrong here

if(br != null)
{
    json = br.readLine();
}

br could never be null there. Maybe you meant

String temp;
if ((temp = br.readLine()) != null)
{
    json = temp;
}

Or even better, pass the stream directly to one of the ObjectMapper 's overloaded readValue(..) methods and handle any errors.

In any case, it seems you aren't sending anything in the body of the request, so readLine() returns null which you assign to json and that causes a failure in the ObjectMapper .

Your doGet() is delegating to your doPost() . Is it possible you are sending GET requests?

Answer 2

By handling your suggessions, I made some changes in codes and the application is working now. Here is the changes:

Server Side ---> getDataServlet.java

---

package com.****;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.****.dao.User;

public class getDataServlet extends HttpServlet 
{
    private static final long serialVersionUID = 1L;
    List<User> users = new LinkedList<User>();

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {

        BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));
        String json = "";
        if(br != null){
            json = br.readLine();}
        ObjectMapper mapper = new ObjectMapper();
        User user = mapper.readValue(json, User.class);
        response.setContentType("application/json");            
        if(users.size() > 20){
            users.remove(0);}
        users.add(user);
        mapper.writeValue(response.getOutputStream(), users);
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException 
    {

        ObjectMapper mapper = new ObjectMapper();
        resp.setContentType("application/json");            
        mapper.writeValue(resp.getOutputStream(), users);
    }
}

Server Side --> User.java

---

package com.*****.dao;

public class User {

    private String idnumber;
    private String password;

    public String getIdnumber() {
        return idnumber;
    }

    public void setIdnumber(String idnumber) {
        this.idnumber = idnumber;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString() {
        return "User [idnumber=" + idnumber + ", password=" + password  + "]";
    }
}

Client Side ---> MainActivity.java

JSONObject jsonObject = new JSONObject();
jsonObject.accumulate("idnumber", user.getIdnumber());
jsonObject.accumulate("password", user.getPassword());

Client Side ---> User.java

@Override
public String toString() 
{
    return "User [idnumber=" + idnumber + ", password=" + password + "]";
}

And after post data output appeared:

[{"idnumber":"987654321 ","password":"123456789"}]

Answer 3

Solution 1:

May be its problem of you given URL, if you working on localhost then use your IP Address instead of localhost

you can check your IP in command prompt using ipconfig command.

Solution 2:

In your User.java You may have to use JSONObject and JSONArray to create JSON String in toString() method.