AndroidStudio 3.0.1从SQLite获取数据时抛出异常求助

Hey there, let’s tackle this SQLite issue you’re facing in Android Studio 3.0.1. I’ve gone through your problem description and related code snippets, here’s my analysis and step-by-step solutions:

一、Relevant Code Snippets #

1. insertCoins Method in Database_Helper (Insert Error Related) #

// Assuming your insertCoins method looks like this (common error-prone version):
public void insertCoins(int coins) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put("coins", coins);
    // Potential issues: wrong table name, unhandled db closure
    db.insert("user_coins", null, contentValues);
    // Missing db.close() might cause connection leaks
}

2. insertCoins Call in Playing_Window Class #

// Database operation call in Playing_Window
Database_Helper dbHelper = new Database_Helper(this);
// Risk: might be called on main thread, or context could be invalid
dbHelper.insertCoins(100);

3. getData Call in Levels Class #

// Data query logic in Levels
Database_Helper dbHelper = new Database_Helper(getContext());
SQLiteDatabase db = dbHelper.getReadableDatabase();
Cursor cursor = db.rawQuery("SELECT coins FROM user_coins", null);
// Risk: accessing cursor without checking if it's empty
if (cursor.moveToFirst()) {
    int coins = cursor.getInt(cursor.getColumnIndex("coins"));
}
// Missing closure for cursor and db
cursor.close();
db.close();

4. Core Code of PlayingWindow Class #

public class Playing_Window extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_playing_window);
        // Unhandled exceptions during database operations
        try {
            Database_Helper dbHelper = new Database_Helper(this);
            dbHelper.insertCoins(50);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

二、Exception Stack Trace #

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:283)
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
at com.example.app.Database_Helper.insertCoins(Database_Helper.java:45)
at com.example.app.Playing_Window.onCreate(Playing_Window.java:30)
...

三、Troubleshooting & Solutions #

Common Root Causes #

• Null Context: You might be passing a null context when initializing Database_Helper (e.g., using a destroyed Activity context, or incorrect context in Fragments).
• Main Thread Database Operations: Android 3.0+ blocks long-running database operations on the main thread, which can cause ANRs or unexpected exceptions.
• Invalid Table/Column Names: Using non-existent table or column names in insert/query statements triggers SQLiteException.
• Unclosed Resources: Forgetting to close Cursor or SQLiteDatabase leads to connection leaks, causing subsequent operations to fail.

Targeted Fixes #

1. Ensure Valid Context

   • In Activities, pass this directly (make sure the Activity is not destroyed when the operation runs).
   • In Fragments, use requireContext() instead of getContext() to avoid null pointers.
   • For global database access, use Application Context (recommended):

     public class MyApp extends Application {
         private static Context appContext;
         @Override
         public void onCreate() {
             super.onCreate();
             appContext = getApplicationContext();
         }
         public static Context getAppContext() {
             return appContext;
         }
     }
     // Initialize Database_Helper like this:
     Database_Helper dbHelper = new Database_Helper(MyApp.getAppContext());
     

2. Move Database Operations to Background Thread
   Use AsyncTask (compatible with Android Studio 3.0.1) to avoid main thread issues:

   new AsyncTask<Void, Void, Void>() {
       @Override
       protected Void doInBackground(Void... voids) {
           Database_Helper dbHelper = new Database_Helper(Playing_Window.this);
           dbHelper.insertCoins(100);
           return null;
       }
   }.execute();
   

3. Verify Table & Column Names

   • Make sure the table structure in Database_Helper.onCreate() matches your insert/query logic:

     @Override
     public void onCreate(SQLiteDatabase db) {
         db.execSQL("CREATE TABLE user_coins (id INTEGER PRIMARY KEY AUTOINCREMENT, coins INTEGER)");
     }
     

   • Use getColumnIndexOrThrow() instead of getColumnIndex() to catch invalid column names immediately:

     int coinsIndex = cursor.getColumnIndexOrThrow("coins");
     int coins = cursor.getInt(coinsIndex);
     

4. Properly Close Resources
   Use try-finally blocks to guarantee Cursor and SQLiteDatabase are closed even if an exception occurs:

   SQLiteDatabase db = null;
   Cursor cursor = null;
   try {
       db = dbHelper.getReadableDatabase();
       cursor = db.rawQuery("SELECT coins FROM user_coins", null);
       if (cursor.moveToFirst()) {
           int coins = cursor.getInt(cursor.getColumnIndex("coins"));
       }
   } catch (Exception e) {
       e.printStackTrace();
   } finally {
       if (cursor != null && !cursor.isClosed()) {
           cursor.close();
       }
       if (db != null && db.isOpen()) {
           db.close();
       }
   }
   

四、Testing & Verification #

After applying the fixes above:

• Test the insert operation first to confirm data is written correctly.
• Test the query operation to verify data can be retrieved as expected.
• Check Logcat for any remaining exception logs.

内容的提问来源于stack exchange，提问作者user9313201