我在同一个数据库上工作吗？ getReadableDatabase() 抛出空指针异常

Question

我是安卓编程新手！我有一个 MainActivity，其中 onCreate 看起来像这样：

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

    DBHandler db = new DBHandler(this);
    db.onUpgrade(db.getWritableDatabase(), 1, 2); // I have to figure out how to make the app create the database on install and not on app launch

    //Here all tables are filled up. It's hardcoded as the data is not intented to be changed by the user
    String ATABLENAME = "aTableName";
    db.addKeyPoint(new AKeyPoint(1, "sometext", 167, "areference"), ATABLENAME);
    db.addKeyPoint(new AKeyPoint(2, "someothertext", 0, "anotherreference"), ATABLENAME);
}

我还有一个 InitActivity，它会在按下按钮时启动。在此我有一个实现 LoaderManager.LoaderCallbacks 的 ListFragment：

public static class KeyFragment extends ListFragment implements LoaderManager.LoaderCallbacks<Cursor>{
    private SimpleCursorAdapter adapter;    

    @Override
    public void onActivityCreated(Bundle savedInstanceState){
        super.onActivityCreated(savedInstanceState);


        // the desired columns to be bound
        String[] columns = new String[] { "question1","answer1" };
        // the XML defined views which the data will be bound to
        int[] to = new int[] { R.id.question, R.id.answer };
        adapter = new SimpleCursorAdapter(getActivity(),R.layout.key_list_entry, null, columns, to, 0);       
        this.setListAdapter(adapter);

        LoaderManager lm = this.getLoaderManager();
        lm.initLoader(1, null, this);

    }

    public Loader<Cursor> onCreateLoader(int id, Bundle args) {

        final SimpleCursorLoader scloader = new SimpleCursorLoader(getActivity()) {
            @Override
            public Cursor loadInBackground() {
                final DBHandler db = new DBHandler(getActivity());
                Cursor c = db.getKeyPointCursor(1, "Crataegus");
                db.close();
                return c;
            }
        };
        return scloader;
    }

    public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) { 
        adapter.swapCursor(cursor);
    }

    public void onLoaderReset(Loader<Cursor> loader) {
        adapter.swapCursor(null);       
    }    
}

}

我将 SimpleCursorLoader 与 LoaderManager 一起使用。可以在这里找到：https://github.com/browep/AndroidCursorLoaderTutorial/blob/master/src/com/github/browep/cursorloader/SimpleCursorLoader.java。 在我的 onCreateLoader 中，loadInBackGround 以下列方式检索 Cursor

Cursor c = db.getKeyPointCursor(anInt, "aTableName");

并返回它。

在我的数据库处理程序 (db) 中，getKeyPointCursor 如下所示：

public Cursor getKeyPointCursor(int keyPointID, String tableName){
    Log.v("bae", "Now in getKeyPointCursor, attempting to get readable database");
    SQLiteDatabase db = this.getReadableDatabase();
    Log.v("bae", "Success... attempting to obtain a cursor");
    Cursor cursor = db.query(tableName, null, KEY_POINTID + "=?",
            new String[] { String.valueOf(keyPointID) }, null, null, null, null);
    Log.v("bae", "Success... closing db and returning cursor");
    db.close();
    return cursor;
}

它打印“现在在 getKeyPointCursor，试图获取可读数据库”，然后抛出 NullPointerException：

09-19 19:27:39.920: W/dalvikvm(3055): threadid=11: thread exiting with uncaught exception (group=0x40c7c1f8)
09-19 19:27:39.925: E/AndroidRuntime(3055): FATAL EXCEPTION: ModernAsyncTask #1
09-19 19:27:39.925: E/AndroidRuntime(3055): java.lang.RuntimeException: An error occured while executing doInBackground()
09-19 19:27:39.925: E/AndroidRuntime(3055):     at android.support.v4.content.ModernAsyncTask$3.done(ModernAsyncTask.java:137)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.util.concurrent.FutureTask.run(FutureTask.java:137)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.lang.Thread.run(Thread.java:856)
09-19 19:27:39.925: E/AndroidRuntime(3055): Caused by: java.lang.NullPointerException
09-19 19:27:39.925: E/AndroidRuntime(3055):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:157)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:231)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at jem.danskflora.DBHandler.getKeyPointCursor(DBHandler.java:235)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at jem.danskflora.InitKeyActivity$KeyFragment$1.loadInBackground(InitKeyActivity.java:79)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at jem.danskflora.SimpleCursorLoader.loadInBackground(SimpleCursorLoader.java:1)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at android.support.v4.content.AsyncTaskLoader.onLoadInBackground(AsyncTaskLoader.java:240)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at android.support.v4.content.AsyncTaskLoader$LoadTask.doInBackground(AsyncTaskLoader.java:51)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at android.support.v4.content.AsyncTaskLoader$LoadTask.doInBackground(AsyncTaskLoader.java:40)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at android.support.v4.content.ModernAsyncTask$2.call(ModernAsyncTask.java:123)
09-19 19:27:39.925: E/AndroidRuntime(3055):     at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
09-19 19:27:39.925: E/AndroidRuntime(3055):     ... 4 more

我怀疑我没有在同一个数据库上工作，或者我可能将错误的上下文传递给我的数据库处理程序？无论如何，我迫切需要帮助来解决这个问题！

Answer 1

我会说该异常的原因是这一行：

private DBHandler db = new DBHandler(getActivity());

此时getActivity() 很可能返回null，因为Fragment 尚未附加，并且此Context 引用用于getReadableDatabase() 和getWritableDatabase() 方法。尝试从onActivityCreated 中的KeyFragment 初始化db 引用，Activity 引用将在那里可用。

另外，避免这种情况：

db.onUpgrade(db.getWritableDatabase(), 1, 2);