在 Android Studio 中使用 Volley 从 Android 应用程序对 JSON 数据对象进行 asmx Web 服务调用时出现 NullPointerException

Question

我正在研究 Arnab's tutorial on Volley from this site。我在 Android Studio 中使用 Volley 从 Android 应用程序对 JSON 数据对象进行 asmx Web 服务调用时收到 NullPointerException。 "req" 变量导致了第 84 行的错误，但我在有问题的代码行运行之前明确声明了它。这是监视器的错误：

FATAL EXCEPTION: main
Process: com.caduceususa.app.myApp, PID: 14603
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.caduceususa.app.myApp.ApplicationController.addToRequestQueue(com.android.volley.Request)' on a null object reference
at com.caduceususa.app.myApp.ConsumeWS.onClick(ConsumeWS.java:84)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

我在这个项目中有三个 JAVA 文件。第一个包含 onCreate 在这里：

package com.caduceususa.app.myApp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.android.volley.VolleyError;
import com.android.volley.VolleyLog;
import com.android.volley.toolbox.HttpHeaderParser;
import com.android.volley.toolbox.JsonRequest;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.UnsupportedEncodingException;
import java.util.HashMap;

import com.android.volley.NetworkResponse;
import com.android.volley.ParseError;
import com.android.volley.Response;
import com.android.volley.Response.ErrorListener;
import com.android.volley.Response.Listener;


public class ConsumeWS extends AppCompatActivity {

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

        final Button submit = (Button) findViewById(R.id.submit);
        final TextView status = (TextView)findViewById(R.id.connStatus);
        final TextView response = (TextView)findViewById(R.id.serverResp);


        submit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                status.setText("Connection Requested...");
                response.setText("waiting...");
                JSONObject oCreds = new JSONObject();

                final String URL = "https://myApp.caduceususa.com/ws/myApp.asmx/myAppLogin";

                HashMap<String, String> params = new HashMap<String, String>();
                params.put("sName", "email@email.com");
                params.put("sPass", "password");

                JsonObjectRequest req = new JsonObjectRequest(URL, new JSONObject(params),
                        new Response.Listener<JSONObject>() {
                            @Override
                            public void onResponse(JSONObject response) {
                                try {
                                    VolleyLog.v("Response:%n %s", response.toString(4));
                                } catch (JSONException e) {
                                    e.printStackTrace();
                                }
                            }
                        }, new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        VolleyLog.e("Error: ", error.getMessage());
                    }
                });

// add the request object to the queue to be executed
                ApplicationController.getInstance().addToRequestQueue(req);

            }
        });
    }



    public class JsonObjectRequest extends JsonRequest<JSONObject> {
        /**
         * Creates a new request.
         * @param method the HTTP method to use
         * @param url URL to fetch the JSON from
         * @param jsonRequest A {@link JSONObject} to post with the request. Null is allowed and
         *   indicates no parameters will be posted along with request.
         * @param listener Listener to receive the JSON response
         * @param errorListener Error listener, or null to ignore errors.
         */
        public JsonObjectRequest(int method, String url, JSONObject jsonRequest,
                                 Listener<JSONObject> listener, ErrorListener errorListener) {
            super(method, url, (jsonRequest == null) ? null : jsonRequest.toString(), listener,
                    errorListener);
        }
        /**
         * Constructor which defaults to <code>GET</code> if <code>jsonRequest</code> is
         * <code>null</code>, <code>POST</code> otherwise.
         *
         * @see #JsonObjectRequest(int, String, JSONObject, Listener, ErrorListener)
         */
        public JsonObjectRequest(String url, JSONObject jsonRequest, Listener<JSONObject> listener,
                                 ErrorListener errorListener) {
            this(jsonRequest == null ? Method.GET : Method.POST, url, jsonRequest,
                    listener, errorListener);
        }
        @Override
        protected Response<JSONObject> parseNetworkResponse(NetworkResponse response) {
            try {
                String jsonString =
                        new String(response.data, HttpHeaderParser.parseCharset(response.headers));
                return Response.success(new JSONObject(jsonString),
                        HttpHeaderParser.parseCacheHeaders(response));
            } catch (UnsupportedEncodingException e) {
                return Response.error(new ParseError(e));
            } catch (JSONException je) {
                return Response.error(new ParseError(je));
            }
        }
    }

}

我的ApplicationControllerJAVA文件如下：

package com.caduceususa.app.myApp;

import android.app.Application;
import android.text.TextUtils;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.VolleyLog;
import com.android.volley.toolbox.Volley;

public class ApplicationController extends Application {

    /**
     * Log or request TAG
     */
    public static final String TAG = "VolleyPatterns";

    /**
     * Global request queue for Volley
     */
    private RequestQueue mRequestQueue;

    /**
     * A singleton instance of the application class for easy access in other places
     */
    private static ApplicationController sInstance;

    @Override
    public void onCreate() {
        super.onCreate();

        // initialize the singleton
        sInstance = this;
    }

    /**
     * @return ApplicationController singleton instance
     */
    public static synchronized ApplicationController getInstance() {
        return sInstance;
    }

    /**
     * @return The Volley Request queue, the queue will be created if it is null
     */
    public RequestQueue getRequestQueue() {
        // lazy initialize the request queue, the queue instance will be
        // created when it is accessed for the first time
        if (mRequestQueue == null) {
            mRequestQueue = Volley.newRequestQueue(getApplicationContext());
        }

        return mRequestQueue;
    }

    /**
     * Adds the specified request to the global queue, if tag is specified
     * then it is used else Default TAG is used.
     *
     * @param req
     * @param tag
     */
    public <T> void addToRequestQueue(Request<T> req, String tag) {
        // set the default tag if tag is empty
        req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);

        VolleyLog.d("Adding request to queue: %s", req.getUrl());

        getRequestQueue().add(req);
    }

    /**
     * Adds the specified request to the global queue using the Default TAG.
     *
     * @param req
     * @param tag
     */
    public <T> void addToRequestQueue(Request<T> req) {
        // set the default tag if tag is empty
        req.setTag(TAG);

        getRequestQueue().add(req);
    }

    /**
     * Cancels all pending requests by the specified TAG, it is important
     * to specify a TAG so that the pending/ongoing requests can be cancelled.
     *
     * @param tag
     */
    public void cancelPendingRequests(Object tag) {
        if (mRequestQueue != null) {
            mRequestQueue.cancelAll(tag);
        }
    }
}

这是一个错误助手：

package com.caduceususa.app.myApp;

import android.content.Context;

import com.android.volley.AuthFailureError;
import com.android.volley.NetworkError;
import com.android.volley.NetworkResponse;
import com.android.volley.NoConnectionError;
import com.android.volley.ServerError;
import com.android.volley.TimeoutError;
import com.android.volley.VolleyError;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.util.HashMap;
import java.util.Map;

public class VolleyErrorHelper {
    /**
     * Returns appropriate message which is to be displayed to the user
     * against the specified error object.
     *
     * @param error
     * @param context
     * @return
     */
    public static String getMessage(Object error, Context context) {
        if (error instanceof TimeoutError) {
            return context.getResources().getString(R.string.generic_server_down);
        }
        else if (isServerProblem(error)) {
            return handleServerError(error, context);
        }
        else if (isNetworkProblem(error)) {
            return context.getResources().getString(R.string.no_internet);
        }
        return context.getResources().getString(R.string.generic_error);
    }

    /**
     * Determines whether the error is related to network
     * @param error
     * @return
     */
    private static boolean isNetworkProblem(Object error) {
        return (error instanceof NetworkError) || (error instanceof NoConnectionError);
    }
    /**
     * Determines whether the error is related to server
     * @param error
     * @return
     */
    private static boolean isServerProblem(Object error) {
        return (error instanceof ServerError) || (error instanceof AuthFailureError);
    }
    /**
     * Handles the server error, tries to determine whether to show a stock message or to
     * show a message retrieved from the server.
     *
     * @param err
     * @param context
     * @return
     */
    private static String handleServerError(Object err, Context context) {
        VolleyError error = (VolleyError) err;

        NetworkResponse response = error.networkResponse;

        if (response != null) {
            switch (response.statusCode) {
                case 404:
                case 422:
                case 401:
                    try {
                        // server might return error like this { "error": "Some error occured" }
                        // Use "Gson" to parse the result
                        HashMap<String, String> result = new Gson().fromJson(new String(response.data),
                                new TypeToken<Map<String, String>>() {
                                }.getType());

                        if (result != null && result.containsKey("error")) {
                            return result.get("error");
                        }

                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    // invalid request
                    return error.getMessage();

                default:
                    return context.getResources().getString(R.string.generic_server_down);
            }
        }
        return context.getResources().getString(R.string.generic_error);
    }
}

最后这是我的 AndroidManifest.xml 文件：

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

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".ConsumeWS">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

我真的不知道从哪里开始调试，因为我已经看到了多个我尝试过的建议解决方案，我想让我把代码放在那里，也许有人可以帮助我。

Answer 1

在清单标签中添加 ApplicationController class，因为它缺少 android:name 属性。因为它没有被引用，所以你会得到 NPE。

免责声明：我是该博客的作者 post 从那里引用了这段代码（排球使用部分）

在 Android Studio 中使用 Volley 从 Android 应用程序对 JSON 数据对象进行 asmx Web 服务调用时出现 NullPointerException

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