Question

我只是编写一个SQLite驱动的Android应用程序，但是当我调用DatabaseHelper类时，我不断收到NullPointerException。似乎导致错误的代码如下：

public Cursor GetAllRows() {
        try {
            return db.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_PHRASE},
                    null, null, null, null, null);
        } catch (SQLException e) {
            Log.e("Exception on query", e.toString());
            return null;
        }
}

我已经翻过代码，看到没有错误，虽然我通常会错过这些简单的东西！

有人能看错吗？如果您认为错误存在于此之外，我可以发布更多代码，但我相当确定这是导致错误的块...

更新：DB适配器的完整源代码。（这是基于记事本示例，如果我没记错的话）。

package com.trapp.tts;

import android.content.ContentValues; import android.content.Context; import android.database.Cursor; import android.database.SQLException; import android.database.sqlite.SQLiteDatabase; import android.database.sqlite.SQLiteOpenHelper; import android.util.Log;

公共类DbAdapter {

public static final String KEY_PHRASE = "phrase";

public static final String KEY_ROWID = "_id";

private static final String TAG = "DbAdapter";
private DatabaseHelper mDbHelper;
private SQLiteDatabase mDb;

/**
 * Database creation sql statement
 */
private static final String DATABASE_CREATE =
        "create table  phrases (_id integer primary key autoincrement, "
                + "phrase text not null);";

private static final String DATABASE_NAME = "db";
private static final String DATABASE_TABLE = " phrases";
private static final int DATABASE_VERSION = 1;

private final Context mCtx;

private static class DatabaseHelper extends SQLiteOpenHelper {

    DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

        db.execSQL(DATABASE_CREATE);

        ContentValues cv=new ContentValues();

        cv.put(KEY_PHRASE, "1");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "2");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "3");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "4");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "5");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "6");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "7");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "8");
        db.insert("phrases", KEY_PHRASE, cv);

        cv.put(KEY_PHRASE, "9");
        db.insert("phrases", KEY_PHRASE, cv);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                + newVersion + ", which will destroy all old data");
        db.execSQL("DROP TABLE IF EXISTS notes");
        onCreate(db);
    }
}

/**
 * Constructor - takes the context to allow the database to be
 * opened/created
 * 
 * @param ctx the Context within which to work
 */
public DbAdapter(Context ctx) {
    this.mCtx = ctx;
}

/**
 * Open the notes database. If it cannot be opened, try to create a new
 * instance of the database. If it cannot be created, throw an exception to
 * signal the failure
 * 
 * @return this (self reference, allowing this to be chained in an
 *         initialization call)
 * @throws SQLException if the database could be neither opened or created
 */
public DbAdapter open() throws SQLException {
    mDbHelper = new DatabaseHelper(mCtx);
    mDb = mDbHelper.getWritableDatabase();
    return this;
}

public void close() {
    mDbHelper.close();
}


/**
 * Create a new note using the title and body provided. If the note is
 * successfully created return the new rowId for that note, otherwise return
 * a -1 to indicate failure.
 * 
 * @param title the title of the note
 * @param body the body of the note
 * @return rowId or -1 if failed
 */
public long createPhrase(String title, String body) {
    ContentValues initialValues = new ContentValues();
    initialValues.put(KEY_PHRASE, title);


    return mDb.insert(DATABASE_TABLE, null, initialValues);
}

/**
 * Delete the note with the given rowId
 * 
 * @param rowId id of note to delete
 * @return true if deleted, false otherwise
 */
public boolean deletePhrase(long rowId) {

    return mDb.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}

/**
 * Return a Cursor over the list of all notes in the database
 * 
 * @return Cursor over all notes
 */
public Cursor fetchAllPhrases() {

    return mDb.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_PHRASE},
                    null, null, null, null, null);
}

/**
 * Return a Cursor positioned at the note that matches the given rowId
 * 
 * @param rowId id of note to retrieve
 * @return Cursor positioned to matching note, if found
 * @throws SQLException if note could not be found/retrieved
 */
public Cursor fetchPhrase(long rowId) throws SQLException {

    Cursor mCursor =

            mDb.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
                    KEY_PHRASE}, KEY_ROWID + "=" + rowId, null,
                    null, null, null, null);
    if (mCursor != null) {
        mCursor.moveToFirst();
    }
    return mCursor;

}

/**
 * Update the note using the details provided. The note to be updated is
 * specified using the rowId, and it is altered to use the title and body
 * values passed in
 * 
 * @param rowId id of note to update
 * @param title value to set note title to
 * @param body value to set note body to
 * @return true if the note was successfully updated, false otherwise
 */
public boolean updatePhrase(long rowId, String title, String body) {
    ContentValues args = new ContentValues();
    args.put(KEY_PHRASE, title);


    return mDb.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
}

}

致电代码：

private void fillData() {
    mCursor = dbHelper.fetchAllPhrases();
    startManagingCursor(mCursor);
    ListAdapter adapter = new SimpleCursorAdapter(
            this, 
            android.R.layout.simple_list_item_1,
            mCursor,
            new String[] {"phrase"},
            new int[] {}
            );
    setListAdapter(adapter);


}

似乎在调用.fetchAllPhrases（）

时崩溃了

mCursor = dbHelper.fetchAllPhrases（）;

Answer 1

你可以从你得到的错误中提供完整的堆栈跟踪吗？

db是否已初始化？

您是否可能从调用此代码的代码中获取空引用异常？（即。假设非空的调用者将从此返回吗？）

Answer 2

1）你没有打开数据库，你可能忘记在某个地方打电话了

public DbAdapter(Context ctx) {
    this.mCtx = ctx;
    open();

}

2）你没有映射值（你应该使用相同的类适配器）

SimpleCursorAdapter adapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_1, mCursor, new String[] {"phrase"}, new int[] {android.R.id.text1});

3）你应该检查错误

        cv.put(KEY_PHRASE, "1");
     if ( db.insert("phrases", KEY_PHRASE, cv) == -1 ) {
      Log.e(TAG, "error inserting");
     }

Android数据库崩溃NullPointerException

2 个答案: