NullPointerException當試圖執行SQL

當我試圖對數據庫運行一些SQL時，我得到一個NullPointerException。NullPointerException當試圖執行SQL

我有一個類叫CustomApp，其中extends Application。這個類創建一個數據幫助類，它控制數據庫的創建和打開。這個數據幫助類的實例被稱爲data - 這個類中的數據庫被稱爲db。

然後我有我自己的類，在那裏我存儲靜態的數據庫功能，方便接入 - 讓我只用一個單一的代碼行執行標準數據庫的東西。這是一個名爲Db的類，其中嵌入了一個名爲Functions的靜態類。因此，如果我想舉個例子，插入一個聯繫人，我只需調用Db.Functions.insertContact()函數。我遇到的問題是一個叫做'setupDb（）`的函數，如果這些表尚不存在，它就會運行表創建字符串。

我CustomApp類：

public class CustomApp extends Application 
{ 

public static CustomDataHelper data; 

public static CustomApp app; 

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

    app = this; 
    data = new CustomDataHelper(); 
} 
}

我CustomDataHelper類：

import android.content.Context; 
import android.database.sqlite.SQLiteDatabase; 
import android.database.sqlite.SQLiteOpenHelper; 

public class CustomDataHelper { 

private static final int DATABASE_VERSION = 1; 
public SQLiteDatabase db = null; 

public CustomDataHelper() 
{ 
    SQLiteOpenHelper o = new MyOpenHelper(CustomApp.app, "MyData.db"); 
    this.db = o.getWritableDatabase(); 
} 
// Rest of standard DataHelper class 

private class MyOpenHelper extends SQLiteOpenHelper 
{ 
    MyOpenHelper(Context context,String DatabaseName) 
    { 
     super(context, DatabaseName, null, DATABASE_VERSION); 
    } 

    @Override 
    public void onCreate(SQLiteDatabase db) 
    { 
    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
    { 
      // Standard data code here 
    } 

} 
}

相關代碼Db類：

import android.content.ContentValues; 
import android.database.Cursor; 
import android.util.Log; 
import java.util.ArrayList; 
import java.util.List; 
import java.util.UUID; 

public class Db 
{ 
public static Contact contact = null; 

static final String TAG = "DbFunctions";  
public static class Functions 
{  
    static final String TABLE_CONTACTS = "Contact";  
    static final String CONTACTS_ID = "id"; 
    static final String CONTACTS_FNAME = "firstName"; 
    static final String CONTACTS_LNAME = "lastName"; 
    static final String CONTACTS_MOBNUM = "mobileNumber"; 
    static final String CONTACTS_SQL = "create table if not exists " + TABLE_CONTACTS + 
      "(" + CONTACTS_ID + " text not null, " + CONTACTS_FNAME + " text null, " + CONTACTS_LNAME + " text null, " + 
      CONTACTS_MOBNUM + " text null)"; 

    static final String ALL_SQL = CONTACTS_SQL; 

    public static boolean setupDb() 
    { 
     try 
     { 
      CustomApp.data.db.execSQL(ALL_SQL); 
      return true; 
     } 
     catch (Exception ex) 
     { 
      Log.e(TAG, ex.getMessage()); 
      return false; 
     } 
    }  
} 
}

當我調用的代碼行：Db.Functions.setupDb();，我得到NullPointerException。

我的logcat：

07-09 16:24:26.596: E/AndroidRuntime(18660): FATAL EXCEPTION: main 
07-09 16:24:26.596: E/AndroidRuntime(18660): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testproj/com.example.testproj.MainActivity}: java.lang.NullPointerException: println needs a message 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.ActivityThread.access$700(ActivityThread.java:140) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1237) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.os.Looper.loop(Looper.java:137) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.ActivityThread.main(ActivityThread.java:4921) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at java.lang.reflect.Method.invokeNative(Native Method) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at java.lang.reflect.Method.invoke(Method.java:511) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at dalvik.system.NativeStart.main(Native Method) 
07-09 16:24:26.596: E/AndroidRuntime(18660): Caused by: java.lang.NullPointerException: println needs a message 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.util.Log.println_native(Native Method) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.util.Log.e(Log.java:297) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at com.example.testproj.Db$Functions.setupDb(Db.java:55) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at com.example.testproj.MainActivity.onCreate(MainActivity.java:16) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.Activity.performCreate(Activity.java:5206) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094) 
07-09 16:24:26.596: E/AndroidRuntime(18660): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074) 
07-09 16:24:26.596: E/AndroidRuntime(18660): ... 11 more

所以我用的斷點追查這行代碼是造成問題的，其CustomApp.data.db.execSQL(ALL_SQL);。現在我發現這是因爲data對象實際上是空的 - 但爲什麼呢？當創建CustomApp，它應該已經自動實例化對象datahelper「數據」 - 這，反過來，應該有自動實例化的數據庫對象。所以這讓我非常難過。任何幫助是極大的讚賞。

來源

2013-07-09 Mike Baxter

發表您的logcat –

@AleksG完整的堆棧跟蹤完成:) –

你已經註冊您的自定義應用程序類的'Manifest.xml'？ – nif