Question

我正在尝试设计一个基本的聊天应用。应用程序构建没有任何错误，但只要我点击模拟器上的任何按钮，应用程序崩溃，日志显示以下错误：

07-01 22:50:39.367  30016-30032/com.example.ibm_admin.chat E/Buffer Error﹕ Error converting result java.lang.NullPointerException: lock == null
07-01 22:50:39.367  30016-30032/com.example.ibm_admin.chat E/JSON Parser﹕ Error parsing data org.json.JSONException: End of input at character 0 of
07-01 22:50:39.370  30016-30016/com.example.ibm_admin.chat D/AndroidRuntime﹕ Shutting down VM
07-01 22:50:39.370  30016-30016/com.example.ibm_admin.chat E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.ibm_admin.chat, PID: 30016
    java.lang.NullPointerException: Attempt to invoke virtual method 'int org.json.JSONArray.length()' on a null object reference
            at com.example.ibm_admin.chat.UserFragment$Load.onPostExecute(UserFragment.java:84)
            at com.example.ibm_admin.chat.UserFragment$Load.onPostExecute(UserFragment.java:71)
            at android.os.AsyncTask.finish(AsyncTask.java:632)
            at android.os.AsyncTask.access$600(AsyncTask.java:177)
            at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
07-01 22:50:40.975  30016-30023/com.example.ibm_admin.chat W/art﹕ Suspending all threads took: 15.530ms
07-01 22:51:02.766  30016-30023/com.example.ibm_admin.chat W/art﹕ Suspending all threads took: 6.893ms

MainActivity.java

import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.Context;
import android.content.SharedPreferences;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;

import com.google.android.gms.common.ConnectionResult;
import com.google.android.gms.common.GooglePlayServicesUtil;
import com.google.android.gms.gcm.GoogleCloudMessaging;

import java.io.IOException;


public class MainActivity extends Activity {

    private final static int PLAY_SERVICES_RESOLUTION_REQUEST = 9000;

    String SENDER_ID = "Your Project Number";


    static final String TAG = "L2C";

    GoogleCloudMessaging gcm;
    SharedPreferences prefs;
    Context context;
    String regid;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        prefs = getSharedPreferences("Chat", 0);
        context = getApplicationContext();
        if(!prefs.getString("REG_FROM","").isEmpty()){
            Fragment user = new UserFragment();
            FragmentTransaction ft = getFragmentManager().beginTransaction();
            ft.replace(R.id.content_frame, user);
            ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
            ft.addToBackStack(null);
            ft.commit();
        }else  if(!prefs.getString("REG_ID", "").isEmpty()){
            Fragment reg = new LoginFragment();
            FragmentTransaction ft = getFragmentManager().beginTransaction();
            ft.replace(R.id.content_frame, reg);
            ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
            ft.addToBackStack(null);
            ft.commit();

        }else if(checkPlayServices()){

            new Register().execute();

        }else{
            Toast.makeText(getApplicationContext(),"This device is not supported",Toast.LENGTH_SHORT).show();
        }
    }
    private boolean checkPlayServices() {
        int resultCode = GooglePlayServicesUtil.isGooglePlayServicesAvailable(this);
        if (resultCode != ConnectionResult.SUCCESS) {
            if (GooglePlayServicesUtil.isUserRecoverableError(resultCode)) {
                GooglePlayServicesUtil.getErrorDialog(resultCode, this,
                        PLAY_SERVICES_RESOLUTION_REQUEST).show();
            } else {
                Log.i(TAG, "This device is not supported.");
                finish();
            }
            return false;
        }
        return true;
    }


    private class Register extends AsyncTask<String, String, String> {

        @Override
        protected String doInBackground(String... args) {
            try {
                if (gcm == null) {
                    gcm = GoogleCloudMessaging.getInstance(context);
                    regid = gcm.register(SENDER_ID);
                    Log.e("RegId",regid);

                    SharedPreferences.Editor edit = prefs.edit();
                    edit.putString("REG_ID", regid);
                    edit.commit();

                }

                return  regid;

            } catch (IOException ex) {
                Log.e("Error", ex.getMessage());
                return "Fails";

            }
        }
        @Override
        protected void onPostExecute(String json) {
            Fragment reg = new LoginFragment();
            FragmentTransaction ft = getFragmentManager().beginTransaction();
            ft.replace(R.id.content_frame, reg);
            ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
            ft.addToBackStack(null);
            ft.commit();
        }
    }
}

JSONParser.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.util.List;

import org.apache.http.HttpEntity;
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.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.util.Log;


public class JSONParser {
    static InputStream is = null;
    static JSONObject jObj = null;
    static JSONArray jAry = null;
    static String json = "";
    // constructor
    public JSONParser() {
    }
    public JSONObject getJSONFromUrl(String url,List<NameValuePair> params) {
        // Making HTTP request
        try {
            // defaultHttpClient
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpPost httpPost = new HttpPost(url);
            if(params != null){
                httpPost.setEntity(new UrlEncodedFormEntity(params));
            }
            HttpResponse httpResponse = httpClient.execute(httpPost);
            HttpEntity httpEntity = httpResponse.getEntity();
            is = httpEntity.getContent();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "n");
            }
            is.close();
            json = sb.toString();
        } catch (Exception e) {
            Log.e("Buffer Error", "Error converting result " + e.toString());
        }
        // try parse the string to a JSON object
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }
        // return JSON String
        return jObj;
    }

    public JSONArray getJSONArray(String url,List<NameValuePair> params) {
        // Making HTTP request
        try {
            // defaultHttpClient
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpPost httpPost = new HttpPost(url);
            httpPost.setEntity(new UrlEncodedFormEntity(params));
            HttpResponse httpResponse = httpClient.execute(httpPost);
            HttpEntity httpEntity = httpResponse.getEntity();
            is = httpEntity.getContent();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
    }
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "n");
            }
            is.close();
            json = sb.toString();
            Log.e("JSS",json);
        } catch (Exception e) {
            Log.e("Buffer Error", "Error converting result " + e.toString());
        }
        // try parse the string to a JSON object
        try {
            jAry = new JSONArray(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }
        // return JSON String
        return jAry;
    }
}

LoginFragment.java

import android.app.Fragment;
import android.app.FragmentTransaction;
import android.app.ProgressDialog;
import android.content.SharedPreferences;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;


import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.List;


public class LoginFragment extends Fragment {
    SharedPreferences prefs;
    EditText name, mobno;
    Button login;
    List<NameValuePair> params;
    ProgressDialog progress;
    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.login_fragment, container, false);
        prefs = getActivity().getSharedPreferences("Chat", 0);

        name = (EditText)view.findViewById(R.id.name);
        mobno = (EditText)view.findViewById(R.id.mobno);
        login = (Button)view.findViewById(R.id.log_btn);
        progress = new ProgressDialog(getActivity());
        progress.setMessage("Registering ...");
        progress.setProgressStyle(ProgressDialog.STYLE_SPINNER);
        progress.setIndeterminate(true);



        login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                progress.show();
                SharedPreferences.Editor edit = prefs.edit();
                edit.putString("REG_FROM", mobno.getText().toString());
                edit.putString("FROM_NAME", name.getText().toString());
                edit.commit();
                new Login().execute();
            }
        });

        return view;
    }
    private class Login extends AsyncTask<String, String, JSONObject> {

        @Override
        protected JSONObject doInBackground(String... args) {
            JSONParser json = new JSONParser();
            params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("name", name.getText().toString()));
            params.add(new BasicNameValuePair("mobno", mobno.getText().toString()));
            params.add((new BasicNameValuePair("reg_id",prefs.getString("REG_ID",""))));

            JSONObject jObj = json.getJSONFromUrl("http://10.0.2.2:8080/login",params);
            return jObj;



        }
        @Override
        protected void onPostExecute(JSONObject json) {
            progress.dismiss();
            try {
                String res = json.getString("response");
                if(res.equals("Sucessfully Registered")) {
                    Fragment reg = new UserFragment();
                    FragmentTransaction ft = getFragmentManager().beginTransaction();
                    ft.replace(R.id.content_frame, reg);
                    ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                    ft.addToBackStack(null);
                    ft.commit();
                }else{
                    Toast.makeText(getActivity(),res,Toast.LENGTH_SHORT).show();

                }

            } catch (JSONException e) {
                e.printStackTrace();
            }

        }
    }

}

UserFragment.java

import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;


public class UserFragment extends Fragment {
    ListView list;
    ArrayList<HashMap<String, String>> users = new ArrayList<HashMap<String, String>>();
    Button refresh,logout;
    List<NameValuePair> params;
    SharedPreferences prefs;


    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container, Bundle savedInstanceState) {
        View view =inflater.inflate(R.layout.user_fragment, container, false);
        prefs = getActivity().getSharedPreferences("Chat", 0);

        list = (ListView)view.findViewById(R.id.listView);
        refresh = (Button)view.findViewById(R.id.refresh);
        logout = (Button)view.findViewById(R.id.logout);
        logout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new  Logout().execute();

            }
        });
        refresh.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                getFragmentManager().beginTransaction().remove(getFragmentManager().findFragmentById(R.id.content_frame)).commit();
                Fragment reg = new UserFragment();
                FragmentTransaction ft = getFragmentManager().beginTransaction();
                ft.replace(R.id.content_frame, reg);
                ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                ft.addToBackStack(null);
                ft.commit();

            }
        });
        new Load().execute();

        return view;
    }

    private class Load extends AsyncTask<String, String, JSONArray> {

        @Override
        protected JSONArray doInBackground(String... args) {
            JSONParser json = new JSONParser();
            params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("mobno", prefs.getString("REG_FROM","")));
            JSONArray jAry = json.getJSONArray("http://10.0.2.2:8080/getuser",params);

            return jAry;
        }
        @Override
        protected void onPostExecute(JSONArray json) {
            for(int i = 0; i < json.length(); i++){
                JSONObject c = null;
                try {
                    c = json.getJSONObject(i);
                    String name = c.getString("name");
                    String mobno = c.getString("mobno");
                    HashMap<String, String> map = new HashMap<String, String>();
                    map.put("name", name);
                    map.put("mobno", mobno);
                    users.add(map);
                } catch (JSONException e) {
                    e.printStackTrace();
                }

            }
            ListAdapter adapter = new SimpleAdapter(getActivity(), users,
                    R.layout.user_list_single,
                    new String[] { "name","mobno" }, new int[] {
                    R.id.name, R.id.mobno});
            list.setAdapter(adapter);
            list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view,
                                        int position, long id) {
                    Bundle args = new Bundle();
                    args.putString("mobno", users.get(position).get("mobno"));
                    Intent chat = new Intent(getActivity(), ChatActivity.class);
                    chat.putExtra("INFO", args);
                    startActivity(chat);
                }
            });
        }
    }
    private class Logout extends AsyncTask<String, String, JSONObject> {

        @Override
        protected JSONObject doInBackground(String... args) {
            JSONParser json = new JSONParser();
            params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("mobno", prefs.getString("REG_FROM","")));
            JSONObject jObj = json.getJSONFromUrl("http://10.0.2.2:8080/logout",params);

            return jObj;
        }
        @Override
        protected void onPostExecute(JSONObject json) {

            String res = null;
            try {
                res = json.getString("response");
                Toast.makeText(getActivity(),res,Toast.LENGTH_SHORT).show();
                if(res.equals("Removed Sucessfully")) {
                    Fragment reg = new LoginFragment();
                    FragmentTransaction ft = getFragmentManager().beginTransaction();
                    ft.replace(R.id.content_frame, reg);
                    ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                    ft.addToBackStack(null);
                    ft.commit();
                    SharedPreferences.Editor edit = prefs.edit();
                    edit.putString("REG_FROM", "");
                    edit.commit();
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }



        }
    }

}

ChatActivity.java

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.SharedPreferences;
import android.graphics.Color;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.content.LocalBroadcastManager;
import android.text.Html;

import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TableLayout;
import android.widget.TableRow;
import android.widget.TextView;
import android.widget.Toast;

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.List;


public class ChatActivity extends Activity {
    SharedPreferences prefs;
    List<NameValuePair> params;
    EditText chat_msg;
    Button send_btn;
    Bundle bundle;
    TableLayout tab;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_chat);
        tab = (TableLayout)findViewById(R.id.tab);

        prefs = getSharedPreferences("Chat", 0);
        bundle = getIntent().getBundleExtra("INFO");
        SharedPreferences.Editor edit = prefs.edit();
        edit.putString("CURRENT_ACTIVE", bundle.getString("mobno"));
        edit.commit();
        LocalBroadcastManager.getInstance(this).registerReceiver(onNotice, new IntentFilter("Msg"));


        if(bundle.getString("name") != null){
            TableRow tr1 = new TableRow(getApplicationContext());
            tr1.setLayoutParams(new TableRow.LayoutParams( TableRow.LayoutParams.WRAP_CONTENT, TableRow.LayoutParams.WRAP_CONTENT));
            TextView textview = new TextView(getApplicationContext());
            textview.setTextSize(20);
            textview.setTextColor(Color.parseColor("#0B0719"));
            textview.setText(Html.fromHtml("<b>"+bundle.getString("name")+" : </b>"+bundle.getString("msg")));
            tr1.addView(textview);
            tab.addView(tr1);

        }

        chat_msg = (EditText)findViewById(R.id.chat_msg);
        send_btn = (Button)findViewById(R.id.sendbtn);

        send_btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                TableRow tr2 = new TableRow(getApplicationContext());
                tr2.setLayoutParams(new TableRow.LayoutParams( TableRow.LayoutParams.WRAP_CONTENT, TableRow.LayoutParams.WRAP_CONTENT));
                TextView textview = new TextView(getApplicationContext());
                textview.setTextSize(20);
                textview.setTextColor(Color.parseColor("#A901DB"));
                textview.setText(Html.fromHtml("<b>You : </b>" + chat_msg.getText().toString()));
                tr2.addView(textview);
                tab.addView(tr2);
                new Send().execute();
            }
        });


    }

    private BroadcastReceiver onNotice= new BroadcastReceiver() {

        @Override
        public void onReceive(Context context, Intent intent) {
            String str = intent.getStringExtra("msg");
            String str1 = intent.getStringExtra("fromname");
            String str2 = intent.getStringExtra("fromu");
            if(str2.equals(bundle.getString("mobno"))){

                TableRow tr1 = new TableRow(getApplicationContext());
                tr1.setLayoutParams(new TableRow.LayoutParams( TableRow.LayoutParams.WRAP_CONTENT, TableRow.LayoutParams.WRAP_CONTENT));
                TextView textview = new TextView(getApplicationContext());
                textview.setTextSize(20);
                textview.setTextColor(Color.parseColor("#0B0719"));
                textview.setText(Html.fromHtml("<b>"+str1+" : </b>"+str));
                tr1.addView(textview);
                tab.addView(tr1);
            }



        }
    };
    private class Send extends AsyncTask<String, String, JSONObject> {

        @Override
        protected JSONObject doInBackground(String... args) {
            JSONParser json = new JSONParser();
            params = new ArrayList<NameValuePair>();

            params.add(new BasicNameValuePair("from", prefs.getString("REG_FROM","")));
            params.add(new BasicNameValuePair("fromn", prefs.getString("FROM_NAME","")));
            params.add(new BasicNameValuePair("to", bundle.getString("mobno")));
            params.add((new BasicNameValuePair("msg",chat_msg.getText().toString())));

            JSONObject jObj = json.getJSONFromUrl("http://10.0.2.2:8080/send",params);
            return jObj;



        }
        @Override
        protected void onPostExecute(JSONObject json) {
            chat_msg.setText("");

            String res = null;
            try {
                res = json.getString("response");
                if(res.equals("Failure")){
                    Toast.makeText(getApplicationContext(),"The user has logged out. You cant send message anymore !",Toast.LENGTH_SHORT).show();

                }
            } catch (JSONException e) {
                e.printStackTrace();
            }

        }
    }
}

有人可以帮助我吗？

Answer 1

此：

java.lang.NullPointerException: Attempt to invoke virtual method 'int org.json.JSONArray.length()' on a null object reference
        at com.example.ibm_admin.chat.UserFragment$Load.onPostExecute(UserFragment.java:84)

表示在类 com.example.ibm_admin.chat.UserFragment 的内部类加载的第84行的文件UserFragment.java中有方法 length 在某个对象上调用，该对象为null。

我想，就是这一行：

        for(int i = 0; i < json.length(); i++){

在此方法参数中 json 等于 null 。

或许，在 doInBackroung 方法中， jObj.getJSONArray 会返回 null 。

很难知道到底发生了什么，因为你的JSONParser.getJSONArray会抑制所有异常并返回null，如果有的话。

看下两行logcat：

07-01 22:50:39.367  30016-30032/com.example.ibm_admin.chat E/Buffer Error﹕ Error converting result java.lang.NullPointerException: lock == null
07-01 22:50:39.367  30016-30032/com.example.ibm_admin.chat E/JSON Parser﹕ Error parsing data org.json.JSONException: End of input at character 0 of

这意味着getJSONArray中该块内的某个位置有一个空引用，它可能来自getJSONArray的最顶层，其中其他异常被抑制，变量变为 null

您必须仔细调试所有代码。

Answer 2

这是您主要获得的例外

java.lang.NullPointerException: Attempt to invoke virtual method 'int org.json.JSONArray.length()' on a null object reference
    at com.example.ibm_admin.chat.UserFragment$Load.onPostExecute(UserFragment.java:84)

要检查数组是否为空，这在解析之前更好。就像这个

JSONArray dataArray = new JSONArray(data);
        if(!(dataArray.length() == 0)){
            for(int i = 0 ;i < dataArray.length();i++){
                JSONObject chainObj = dataArray.getJSONObject(i);
                String chain_name = chainObj.getString("chain_name");
                String chain_id = chainObj.getString("retailerid");
                String chain_location = chainObj.getString("area");
            }
        }

Answer 3

这是后退null JSONArray jAry = json.getJSONArray（“http://10.0.2.2:8080/getuser”，params）; 在userFragment中由于服务器响应无效json和当你调用json.length（）时。 json对象为null UserFragment $ Load.onPostExecute（Jsonarray json）。

空指针异常：调用虚方法

3 个答案: