Question

我创建了一个简单的Android应用程序，它连接到远程sql数据库..问题是当我点击注册应用程序崩溃时。请帮帮我们

package com.example.akshay.webservices;

/**
 * Created by Akshay on 7/20/2015.
 */
import com.example.akshay.webservices.AppConfig;
import com.example.akshay.webservices.AppController;
import com.example.akshay.webservices.SQLiteHandler;
import com.example.akshay.webservices.SessionManager;

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

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

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.android.volley.Request.Method;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;

public class RegisterActivity extends Activity {
    private static final String TAG = RegisterActivity.class.getSimpleName();
    private Button btnRegister;
    private Button btnLinkToLogin;
    private EditText inputFullName;
    private EditText inputEmail;
    private EditText inputPassword;
    private ProgressDialog pDialog;
    private SessionManager session;
    private SQLiteHandler db;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register);

        inputFullName = (EditText) findViewById(R.id.name);
        inputEmail = (EditText) findViewById(R.id.email);
        inputPassword = (EditText) findViewById(R.id.password);
        btnRegister = (Button) findViewById(R.id.btnRegister);
        btnLinkToLogin = (Button) findViewById(R.id.btnLinkToLoginScreen);

        // Progress dialog
        pDialog = new ProgressDialog(this);
        pDialog.setCancelable(false);

        // Session manager
        session = new SessionManager(getApplicationContext());

        // SQLite database handler
        db = new SQLiteHandler(getApplicationContext());

        // Check if user is already logged in or not
        if (session.isLoggedIn()) {
            // User is already logged in. Take him to main activity
            Intent intent = new Intent(RegisterActivity.this,
                    MainActivity.class);
            startActivity(intent);
            finish();
        }

        // Register Button Click event
        btnRegister.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                String name = inputFullName.getText().toString();
                String email = inputEmail.getText().toString();
                String password = inputPassword.getText().toString();

                if (!name.isEmpty() && !email.isEmpty() && !password.isEmpty()) {
                    registerUser(name, email, password);
                } else {
                    Toast.makeText(getApplicationContext(),
                            "Please enter your details!", Toast.LENGTH_LONG)
                            .show();
                }
            }
        });

        // Link to Login Screen
        btnLinkToLogin.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {
                Intent i = new Intent(getApplicationContext(),
                        LoginActivity.class);
                startActivity(i);
                finish();
            }
        });

    }

    /**
     * Function to store user in MySQL database will post params(tag, name,
     * email, password) to register url
     * */
    private void registerUser(final String name, final String email,
                              final String password) {
        // Tag used to cancel the request
        String tag_string_req = "req_register";

        pDialog.setMessage("Registering ...");
        showDialog();

        StringRequest strReq = new StringRequest(Method.POST,
                AppConfig.URL_REGISTER, new Response.Listener<String>() {

            @Override
            public void onResponse(String response) {
                Log.d(TAG, "Register Response: " + response.toString());
                hideDialog();

                try {
                    JSONObject jObj = new JSONObject(response);
                    boolean error = jObj.getBoolean("error");
                    if (!error) {
                        // User successfully stored in MySQL
                        // Now store the user in sqlite
                        String uid = jObj.getString("uid");

                        JSONObject user = jObj.getJSONObject("user");
                        String name = user.getString("name");
                        String email = user.getString("email");
                        String created_at = user
                                .getString("created_at");

                        // Inserting row in users table
                        db.addUser(name, email, uid, created_at);

                        // Launch login activity
                        Intent intent = new Intent(
                                RegisterActivity.this,
                                LoginActivity.class);
                        startActivity(intent);
                        finish();
                    } else {

                        // Error occurred in registration. Get the error
                        // message
                        String errorMsg = jObj.getString("error_msg");
                        Toast.makeText(getApplicationContext(),
                                errorMsg, Toast.LENGTH_LONG).show();
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }

            }
        }, new Response.ErrorListener() {

            @Override
            public void onErrorResponse(VolleyError error) {
                Log.e(TAG, "Registration Error: " + error.getMessage());
                Toast.makeText(getApplicationContext(),
                        error.getMessage(), Toast.LENGTH_LONG).show();
                hideDialog();
            }
        }) {

            @Override
            protected Map<String, String> getParams() {
                // Posting params to register url
                Map<String, String> params = new HashMap<String, String>();
                params.put("tag", "register");
                params.put("name", name);
                params.put("email", email);
                params.put("password", password);

                return params;
            }

        };

        // Adding request to request queue
        AppController.getInstance().addToRequestQueue(strReq, tag_string_req);
    }

    private void showDialog() {
        if (!pDialog.isShowing())
            pDialog.show();
    }

    private void hideDialog() {
        if (pDialog.isShowing())
            pDialog.dismiss();
    }
}

AppController.java

package com.example.akshay.webservices;

import android.app.Application;

/**
 * Created by Akshay on 7/19/2015.
 */
import android.app.Application;
import android.text.TextUtils;

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

public class AppController extends Application {

    public static final String TAG = AppController.class.getSimpleName();

    private RequestQueue mRequestQueue;

    private static AppController mInstance;

    @Override
    public void onCreate() {
        super.onCreate();
        mInstance = this;
    }

    public static synchronized AppController getInstance() {
        return mInstance;
    }

    public RequestQueue getRequestQueue() {
        if (mRequestQueue == null) {
            mRequestQueue = Volley.newRequestQueue(getApplicationContext());
        }

        return mRequestQueue;
    }

    public <T> void addToRequestQueue(Request<T> req, String tag) {
        req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
        getRequestQueue().add(req);
    }

    public <T> void addToRequestQueue(Request<T> req) {
        req.setTag(TAG);
        getRequestQueue().add(req);
    }

    public void cancelPendingRequests(Object tag) {
        if (mRequestQueue != null) {
            mRequestQueue.cancelAll(tag);
        }
    }
}

logcat的：

07-20 20:27:07.195    8233-8233/? E/Zygote﹕ MountEmulatedStorage()
07-20 20:27:07.195    8233-8233/? E/Zygote﹕ v2
07-20 20:27:07.195    8233-8233/? E/SELinux﹕ [DEBUG] get_category: variable seinfo: default sensitivity: NULL, cateogry: NULL
07-20 20:27:18.355    8233-8233/com.example.akshay.webservices E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.akshay.webservices, PID: 8233
    java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.akshay.webservices.AppController.addToRequestQueue(com.android.volley.Request, java.lang.String)' on a null object reference
            at com.example.akshay.webservices.RegisterActivity.registerUser(RegisterActivity.java:185)
            at com.example.akshay.webservices.RegisterActivity.access$300(RegisterActivity.java:32)
            at com.example.akshay.webservices.RegisterActivity$1.onClick(RegisterActivity.java:81)
            at android.view.View.performClick(View.java:5191)
            at android.view.View$PerformClick.run(View.java:20916)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:5972)
            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:1399)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

这是logcat，我无法理解发生了什么。请帮忙..

Answer 1

问题正在发生，导致AppController.getInstance（）返回null，你必须改变你的AppController.getInstance（）方法：

public static AppController getInstance() {
    if (mInstance == null) {
        mInstance = new AppController;
    }
    return mInstance;
}

还要确保您在清单中注册了应用类你应该在你的清单应用程序中有一个属性，如下所示：

<application
        android:name="package.AppController"  <--------
        android:allowBackup="true"
        android:debuggable="true"
        android:icon="@drawable/ic_launcher"
        android:label="xyz"
        android:screenOrientation="landscape"
        android:theme="@style/AppTheme">

此处有更多信息：http://developer.android.com/reference/android/app/Application.html

＆＃34;需要维护全局应用程序状态的人的基类。 您可以通过在AndroidManifest.xml标记中指定其名称来提供自己的实现，这将导致在创建应用程序/包的过程时为您实例化该类。＆＃34;

Answer 2

嘿，我有同样的问题，当我尝试这个解决方案时，它最终没有用，我找到了出路。

返回空指针的问题是因为方法getInstance（）和getRequestQueue（）没有返回任何关于上下文的内容。要解决这个问题，请按照以下步作为先决条件广告，公共类AppController下的以下全局变量扩展了应用程序私有上下文myContext。

第1步。创建一个定义上下文的构造函数

public AppController(Context context){
    myContext = context;
    mRequestQueue = getRequestQueue();
 }

步骤2.修改hyou getInstance（）方法以在上下文中工作，如下所示：

public static synchronized AppController getInstance(Context context) {

        if(mInstance==null){
        mInstance = new AppController(context);
         }
        return mInstance;
    }

第3步。最后修改getRequestQueue（）方法以在上下文中工作，如下所示

public RequestQueue getRequestQueue() {
        if (mRequestQueue == null) {
            mRequestQueue = Volley.newRequestQueue(myContext.getApplicationContext());
        }

        return mRequestQueue;
    }
Finally got the calling class and call the methods from AppController.java with respect to the context of the activity or broadcaster.

AppController.getInstance(this).addToRequestQueue(strReq, tag_string_req);

当尝试使用volley在远程数据库mysql上创建新记录时app崩溃

2 个答案: