我是Android编程的新手！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，只需按一下按钮即可启动。在这里我有一个实现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。它可以在这里找到：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");
并将其返回。
在我的databasehandler（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“Now in 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
我怀疑我不是以某种方式在同一个数据库上工作，或者我将错误的上下文传递给我的databasehandler？无论如何，我迫切需要帮助来解决这个问题！
答案 0 :(得分：7)
我想说这个例外的原因是这一行：
private DBHandler db = new DBHandler(getActivity());
此时getActivity()很可能会返回null，因为Fragment尚未附加，Context和getReadableDatabase()引用用于getWritableDatabase()和db {1}}方法。尝试初始化KeyFragment onActivityCreated中的Activity引用，db.onUpgrade(db.getWritableDatabase(), 1, 2);
引用将在那里提供。
另外，请避免这样：
{{1}}