NullPointerException: uri - LoaderManager / SQLiteManager / ListView

Question

在使用ListView和LoaderManager填充片段中的片段时，我会得到这个错误。如果我从适配器中删除SQLiteCursorLoader并直接填充，那么所有这些都能正常工作。如果我尝试一个自定义适配器，我会得到同样的错误。错误似乎是，mUri成员的CursorLoader从未填充并被保留为空，SQLiteCursorLoader从不调用设置mUri的CursorLoader上的构造函数。

public CursorLoader(Context context, Uri uri, String[] projection, String selection,
        String[] selectionArgs, String sortOrder) {
    super(context);
    mObserver = new ForceLoadContentObserver();
    mUri = uri;
    mProjection = projection;
    mSelection = selection;
    mSelectionArgs = selectionArgs;
    mSortOrder = sortOrder;
}

相反，它调用只传递上下文的构造函数的版本。

public CursorLoader(Context context) {
    super(context);
    mObserver = new ForceLoadContentObserver();
}

这是错误

03-08 15:00:36.906 2601-2683/com.overunitystudios.medbuddy E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
                                                                             Process: com.overunitystudios.medbuddy, PID: 2601
                                                                             java.lang.RuntimeException: An error occurred while executing doInBackground()
                                                                                 at android.os.AsyncTask$3.done(AsyncTask.java:309)
                                                                                 at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
                                                                                 at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
                                                                                 at java.util.concurrent.FutureTask.run(FutureTask.java:242)
                                                                                 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
                                                                                 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
                                                                                 at java.lang.Thread.run(Thread.java:818)
                                                                              Caused by: java.lang.NullPointerException: uri
                                                                                 at com.android.internal.util.Preconditions.checkNotNull(Preconditions.java:60)
                                                                                 at android.content.ContentResolver.query(ContentResolver.java:474)
                                                                                 at android.content.CursorLoader.loadInBackground(CursorLoader.java:64)
                                                                                 at android.content.CursorLoader.loadInBackground(CursorLoader.java:56)
                                                                                 at android.content.AsyncTaskLoader.onLoadInBackground(AsyncTaskLoader.java:312)
                                                                                 at android.content.AsyncTaskLoader$LoadTask.doInBackground(AsyncTaskLoader.java:69)
                                                                                 at android.content.AsyncTaskLoader$LoadTask.doInBackground(AsyncTaskLoader.java:66)
                                                                                 at android.os.AsyncTask$2.call(AsyncTask.java:295)
                                                                                 at java.util.concurrent.FutureTask.run(FutureTask.java:237)
                                                                                 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
                                                                                 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
                                                                                 at java.lang.Thread.run(Thread.java:818) 

下面是包含ListView的片段的代码

public class fragment_medications extends Fragment  implements LoaderManager.LoaderCallbacks<Cursor> {

    private database_helper_medications dbHelper;
    private SimpleCursorAdapter listAdapter;
    private SQLLiteCursorLoader loader=null;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        //return super.onCreateView(inflater, container, savedInstanceState);

        View view = inflater.inflate(R.layout.fragment_medications_layout, container,false);

        dbHelper = new database_helper_medications(getActivity());

        listAdapter = new SimpleCursorAdapter(getActivity(),R.layout.list_item_medications,null, new String[] {database_helper_medications.FIELD_MEDICATION}, new int[] {R.id.tvMedication},0);

        ListView lvMedications = (ListView) view.findViewById(R.id.lvMedications);
        lvMedications.setAdapter(listAdapter);

        return view;
    }

    @Override public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        getLoaderManager().initLoader(0, null, this);
    }

    //TODO: 03-08 00:16:48.563 3508-3516/com.overunitystudios.medbuddy W/SQLiteConnectionPool: A SQLiteConnection object for database '/data/user/0/com.overunitystudios.medbuddy/databases/medbuddy' was leaked!  Please fix your application to end transactions in progress properly and to close the database when it is no longer needed.

    @Override
    public void onDestroy() {
        super.onDestroy();
        if (dbHelper!=null) dbHelper.close();
    }

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        loader=new SQLLiteCursorLoader(getActivity(), dbHelper, dbHelper.DATABASE_SELECT_ALL, null);
        return(loader);
    }


    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        this.loader=(SQLLiteCursorLoader)loader;
        listAdapter.changeCursor(data);
    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {
        listAdapter.changeCursor(null);
    }

}

数据库助手

public class database_helper_medications extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "medbuddy";
    private static final String TABLE_NAME = "medications";
    private static final int DATABASE_VERSION = 1;

    public final static String FIELD_ID="_id"; //Changed table to use a column named "_id" instead of "id" as CursorAdapter must have an _id column in the table it is using.
    public final static String FIELD_MEDICATION="name";

    private SQLiteDatabase database;

    // Database creation sql statement
    private static final String DATABASE_CREATE = "create table if not exists " + TABLE_NAME + " ( " + FIELD_ID + " integer primary key," + FIELD_MEDICATION + " text not null);";
    public static final String DATABASE_SELECT_ALL = "select " + FIELD_ID + "," + FIELD_MEDICATION + " from " + TABLE_NAME + " order by " + FIELD_MEDICATION;

    public database_helper_medications(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
        database = this.getWritableDatabase();
    }

    // Method is called during creation of the database
    @Override
    public void onCreate(SQLiteDatabase database) {
        database.execSQL(DATABASE_CREATE);
    }

    // Method is called during an upgrade of the database,
    @Override
    public void onUpgrade(SQLiteDatabase database,int oldVersion,int newVersion){
        database.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME + "");
        onCreate(database);
    }

    public long createSingleRecord(Integer id, String medication){
        ContentValues values = new ContentValues();
        values.put(FIELD_ID, id);
        values.put(FIELD_MEDICATION, medication);
        return database.insert(TABLE_NAME, null, values);
    }

    public long createSingleRecord(String medication){
        ContentValues values = new ContentValues();
        //values.put(FIELD_ID, id); //We dont need to do this as it's marked autoincrement.
        values.put(FIELD_MEDICATION, medication);
        return database.insert(TABLE_NAME, null, values);
    }


    public Cursor selectAllRecords() {
        String[] cols = new String[] {FIELD_ID, FIELD_MEDICATION};
        Cursor mCursor = database.query(true, TABLE_NAME, cols,null, null, null, null, null, null);
        if (mCursor != null)
            mCursor.moveToFirst();

        return mCursor; // iterate to get each value.
    }

    public void DumpToLog()
    {
        Log.i("DumpToLog", DatabaseUtils.dumpCursorToString(selectAllRecords()));
    }
}

这里是SQLLiteCursorLoader (我重命名为流行的SQLiteCursorLoader)

public class SQLLiteCursorLoader extends CursorLoader
{

    SQLiteOpenHelper db=null;
    String rawQuery=null;
    String[] args=null;
    /**
     * Creates a fully-specified SQLiteCursorLoader. See
     * {@link SQLiteDatabase#rawQuery(SQLiteDatabase, String, String[])
     * SQLiteDatabase.rawQuery()} for documentation on the
     * meaning of the parameters. These will be passed as-is
     * to that call.
     */
    public SQLLiteCursorLoader(Context context, SQLiteOpenHelper db, String rawQuery, String[] args)
    {
        super(context);
        this.db=db;
        this.rawQuery=rawQuery;
        this.args=args;
    }

    /**
     * Runs on a worker thread and performs the actual
     * database query to retrieve the Cursor.
     */
    //@Override
    protected Cursor buildCursor() {
        return(db.getReadableDatabase().rawQuery(rawQuery, args));
    }

    /**
     * Writes a semi-user-readable roster of contents to
     * supplied output.
     */
    @Override
    public void dump(String prefix, FileDescriptor fd,PrintWriter writer, String[] args)
    {
        super.dump(prefix, fd, writer, args);
        writer.print(prefix);
        writer.print("rawQuery=");
        writer.println(rawQuery);
        writer.print(prefix);
        writer.print("args=");
        writer.println(Arrays.toString(args));
    }

    public void insert(String table, String nullColumnHack, ContentValues values)
    {
        buildInsertTask(this).execute(db, table, nullColumnHack, values);
    }

    public void update(String table, ContentValues values,String whereClause, String[] whereArgs)
    {
        buildUpdateTask(this).execute(db, table, values, whereClause,whereArgs);
    }

    public void replace(String table, String nullColumnHack,ContentValues values)
    {
        buildReplaceTask(this).execute(db, table, nullColumnHack, values);
    }

    public void delete(String table, String whereClause,String[] whereArgs)
    {
        buildDeleteTask(this).execute(db, table, whereClause, whereArgs);
    }

    public void execSQL(String sql, Object[] bindArgs)
    {
        buildExecSQLTask(this).execute(db, sql, bindArgs);
    }

    protected ContentChangingTask buildInsertTask(SQLLiteCursorLoader loader)
    {
        return(new InsertTask(loader));
    }

    protected ContentChangingTask buildUpdateTask(SQLLiteCursorLoader loader)
    {
        return(new UpdateTask(loader));
    }

    protected ContentChangingTask buildReplaceTask(SQLLiteCursorLoader loader)
    {
        return(new ReplaceTask(loader));
    }

    protected ContentChangingTask buildDeleteTask(SQLLiteCursorLoader loader)
    {
        return(new DeleteTask(loader));
    }

    protected ContentChangingTask buildExecSQLTask(SQLLiteCursorLoader loader)
    {
        return(new ExecSQLTask(loader));
    }

    protected static class InsertTask extends ContentChangingTask
    {
        InsertTask(SQLLiteCursorLoader loader)
        {
            super(loader);
        }

        @Override
        protected Void doInBackground(Object... params) {
            SQLiteOpenHelper db=(SQLiteOpenHelper)params[0];
            String table=(String)params[1];
            String nullColumnHack=(String)params[2];
            ContentValues values=(ContentValues)params[3];

            db.getWritableDatabase().insert(table, nullColumnHack, values);

            return(null);
        }
    }

    protected static class UpdateTask extends ContentChangingTask
    {
        UpdateTask(SQLLiteCursorLoader loader)
        {
            super(loader);
        }

        @Override
        protected Void doInBackground(Object... params) {
            SQLiteOpenHelper db=(SQLiteOpenHelper)params[0];
            String table=(String)params[1];
            ContentValues values=(ContentValues)params[2];
            String where=(String)params[3];
            String[] whereParams=(String[])params[4];

            db.getWritableDatabase().update(table, values, where, whereParams);

            return(null);
        }
    }

    protected static class ReplaceTask extends ContentChangingTask
    {
        ReplaceTask(SQLLiteCursorLoader loader)
        {
            super(loader);
        }

        @Override
        protected Void doInBackground(Object... params) {
            SQLiteOpenHelper db=(SQLiteOpenHelper)params[0];
            String table=(String)params[1];
            String nullColumnHack=(String)params[2];
            ContentValues values=(ContentValues)params[3];

            db.getWritableDatabase().replace(table, nullColumnHack, values);

            return(null);
        }
    }

    protected static class DeleteTask extends ContentChangingTask
    {
        DeleteTask(SQLLiteCursorLoader loader)
        {
            super(loader);
        }

        @Override
        protected Void doInBackground(Object... params) {
            SQLiteOpenHelper db=(SQLiteOpenHelper)params[0];
            String table=(String)params[1];
            String where=(String)params[2];
            String[] whereParams=(String[])params[3];

            db.getWritableDatabase().delete(table, where, whereParams);

            return(null);
        }
    }

    protected static class ExecSQLTask extends ContentChangingTask
    {
        ExecSQLTask(SQLLiteCursorLoader loader)
        {
            super(loader);
        }

        @Override
        protected Void doInBackground(Object... params) {
            SQLiteOpenHelper db=(SQLiteOpenHelper)params[0];
            String sql=(String)params[1];
            Object[] bindParams=(Object[])params[2];

            db.getWritableDatabase().execSQL(sql, bindParams);

            return(null);
        }
    }
}

我将不包括布局XML，正如我前面说过的，如果我直接从适配器填充并且不使用加载程序，它就能工作。

Answer 1

从CursorLoader API，

公共CursorLoader (上下文) 在API 11中添加创建一个空的、未指定的CursorLoader。您必须遵循setSelection(String)，，然后调用setUri(Uri)、等来指定要执行的查询。

因此，在自定义SQLLiteCursorLoader类中，必须在super(context);之后显式调用构造函数中的super(context);。

更新

看看你在下面的评论中提到的github项目，我发现了一个不同之处。

你的

public class SQLLiteCursorLoader extends CursorLoader 

而github的

public class SQLiteCursorLoader extends AbstractCursorLoader

你猜怎么着，CursorLoader有一个方法(链接)

/* Runs on a worker thread */
@Override
public Cursor loadInBackground() {
    Cursor cursor = getContext().getContentResolver().query(mUri, mProjection, mSelection,
            mSelectionArgs, mSortOrder);
    if (cursor != null) {
        // Ensure the cursor window is filled
        cursor.getCount();
        registerContentObserver(cursor, mObserver);
    }
    return cursor;
}

它使用mUri，这就是它不能为null的原因，而AbstractCursorLoader (链接)中的等效方法是：

@Override
public Cursor loadInBackground() {
    Cursor cursor=buildCursor();

    if (cursor!=null) {
      // Ensure the cursor window is filled
      cursor.getCount();
    }

    return(cursor);
}

因此，如果您更正SQLLiteCursorLoader类中的继承(使其扩展AbstractCursorLoader)，您最终将解决问题。