我确实有一个让我头疼的问题。 我通过自定义内容提供程序将我的城市的一些图像存储在sqlite数据库中。 但是，当我运行我的应用程序时，我得到一个空光标。

Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int android.database.Cursor.getCount()' on a null object reference

at theo.testing.androidcustomloaders.fragments.MainActivityFragment.onActivityCreated(MainActivityFragment.java:74)

at android.support.v4.app.Fragment.performActivityCreated(Fragment.java:2089)

at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1133)

at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1290)

at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:801)

at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1677)

at android.support.v4.app.FragmentController.execPendingActions(FragmentController.java:388)

at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:604)

at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:178)

at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1220)

at android.app.Activity.performStart(Activity.java:5992)

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2261)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2358)

at android.app.ActivityThread.access$800(ActivityThread.java:144)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:135)

at android.app.ActivityThread.main(ActivityThread.java:5219)

at java.lang.reflect.Method.invoke(Native Method)

at java.lang.reflect.Method.invoke(Method.java:372)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)

这意味着信息未正确存储，或者我的提供商的Uri出现了问题。 所以。

我的城市合同

public class MyCityContract {

public static final String CONTENT_AUTHORITY = "theo.testing.customloaders";

public static final Uri BASE_CONTENT_URI = Uri.parse("content://" + CONTENT_AUTHORITY);

public static final class MyCityEntry implements BaseColumns{

//table name

public static final String TABLE_MY_CITY = "my_city";

//columns

public static final String _ID = "_id";

public static final String COLUMN_NAME = "name";

public static final String COLUMN_ICON = "icon";

// create content uri

public static final Uri CONTENT_URI = BASE_CONTENT_URI.buildUpon()

.appendPath(TABLE_MY_CITY).build();

// create cursor of base type directory for multiple entries

public static final String CONTENT_DIR_TYPE =

ContentResolver.CURSOR_DIR_BASE_TYPE + "/" + CONTENT_AUTHORITY + "/" + TABLE_MY_CITY;

// create cursor of base type item for single entry

public static final String CONTENT_ITEM_TYPE =

ContentResolver.CURSOR_ITEM_BASE_TYPE +"/" + CONTENT_AUTHORITY + "/" + TABLE_MY_CITY;

// for building URIs on insertion

public static Uri buildFlavorsUri(long id){

return ContentUris.withAppendedId(CONTENT_URI, id);

}

}

}

MyCityDbHelper

public class MyCityDbHelper extends SQLiteOpenHelper{

public static final String LOG_TAG = MyCityDbHelper.class.getSimpleName();

//name & version

public static final String DATABASE_NAME = "city.db";

public static final int DATABASE_VERSION = 4;

// Create the database

public MyCityDbHelper(Context context) {

super(context, DATABASE_NAME,null,DATABASE_VERSION);

}

@Override

public void onCreate(SQLiteDatabase sqLiteDatabase) {

final String SQL_CREATE_MY_CITY_TABLE = "CREATE TABLE " +

MyCityContract.MyCityEntry.TABLE_MY_CITY + "(" + MyCityContract.MyCityEntry._ID +

" INTEGER PRIMARY KEY AUTOINCREMENT, " +

MyCityContract.MyCityEntry.COLUMN_NAME + " TEXT NOT NULL, " +

MyCityContract.MyCityEntry.COLUMN_ICON + " INTEGER NOT NULL);";

sqLiteDatabase.execSQL(SQL_CREATE_MY_CITY_TABLE);

}

@Override

public void onUpgrade(SQLiteDatabase sqLiteDatabase, int oldVersion, int newVersion) {

Log.w(LOG_TAG, "Upgrading database from version " + oldVersion + " to " +

newVersion + ". OLD DATA WILL BE DESTROYED");

// Drop the table

sqLiteDatabase.execSQL("DROP TABLE IF EXISTS " + MyCityContract.MyCityEntry.TABLE_MY_CITY);

sqLiteDatabase.execSQL("DELETE FROM SQLITE_SEQUENCE WHERE NAME = '" +

MyCityContract.MyCityEntry.TABLE_MY_CITY + "'");

// re-create database

onCreate(sqLiteDatabase);

}

}

MyCityProvider

public class MyCityProvider extends ContentProvider {

private static final String LOG_TAG = MyCityProvider.class.getSimpleName();

private static final UriMatcher sUriMatcher = buildUriMatcher();

private MyCityDbHelper myCityDbHelper;

//Codes for UriMatcher

private static final int MY_CITY = 100;

private static final int MY_CITY_WITH_ID = 200;

private static UriMatcher buildUriMatcher(){

// Build a UriMatcher by adding a specific code to return based on a match

// It's common to use NO_MATCH as the code for this case.

final UriMatcher matcher = new UriMatcher(UriMatcher.NO_MATCH);

final String authority = MyCityContract.CONTENT_AUTHORITY;

//add code for each URI

matcher.addURI(authority,MyCityContract.MyCityEntry.TABLE_MY_CITY,MY_CITY);

matcher.addURI(authority,MyCityContract.MyCityEntry.TABLE_MY_CITY + "/#",MY_CITY_WITH_ID);

return matcher;

}

@Override

public boolean onCreate() {

myCityDbHelper = new MyCityDbHelper(getContext());

return true;

}

@Override

public String getType(Uri uri) {

final int match = sUriMatcher.match(uri);

switch (match){

case MY_CITY: {

return MyCityContract.MyCityEntry.CONTENT_DIR_TYPE;

}

case MY_CITY_WITH_ID:{

return MyCityContract.MyCityEntry.CONTENT_ITEM_TYPE;

}

default:{

throw new UnsupportedOperationException("Unknown uri: " + uri);

}

}

}

@Override

public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder){

Cursor retCursor;

switch(sUriMatcher.match(uri)){

// All Flavors selected

case MY_CITY:{

retCursor = myCityDbHelper.getReadableDatabase().query(

MyCityContract.MyCityEntry.TABLE_MY_CITY,

projection,

selection,

selectionArgs,

null,

null,

sortOrder);

return retCursor;

}

// Individual flavor based on Id selected

case MY_CITY_WITH_ID:{

retCursor = myCityDbHelper.getReadableDatabase().query(

MyCityContract.MyCityEntry.TABLE_MY_CITY,

projection,

MyCityContract.MyCityEntry._ID + " = ?",

new String[] {String.valueOf(ContentUris.parseId(uri))},

null,

null,

sortOrder);

return retCursor;

}

default:{

// By default, we assume a bad URI

throw new UnsupportedOperationException("Unknown uri: " + uri);

}

}

}

@Override

public Uri insert(Uri uri, ContentValues contentValues) {

final SQLiteDatabase db = myCityDbHelper.getWritableDatabase();

Uri returnUri;

switch (sUriMatcher.match(uri)){

case MY_CITY:

long _id = db.insert(MyCityContract.MyCityEntry.TABLE_MY_CITY,null,contentValues);

Log.d("id",String.valueOf(_id));

// insert unless it is already contained in the database

if(_id>0){

returnUri = MyCityContract.MyCityEntry.buildFlavorsUri(_id);

}else {

throw new android.database.SQLException("Failed to insert row into: " + uri);

}

break;

default: {

throw new UnsupportedOperationException("Unknown uri: " + uri );

}

}

getContext().getContentResolver().notifyChange(uri,null);

return returnUri;

}

@Override

public int delete(Uri uri, String selection, String[] selectionArgs) {

final SQLiteDatabase db = myCityDbHelper.getWritableDatabase();

final int match = sUriMatcher.match(uri);

int numDeleted;

switch(match){

case MY_CITY:

numDeleted = db.delete(

MyCityContract.MyCityEntry.TABLE_MY_CITY, selection, selectionArgs);

// reset _ID

db.execSQL("DELETE FROM SQLITE_SEQUENCE WHERE NAME = '" +

MyCityContract.MyCityEntry.TABLE_MY_CITY + "'");

break;

case MY_CITY_WITH_ID:

numDeleted = db.delete(MyCityContract.MyCityEntry.TABLE_MY_CITY,

MyCityContract.MyCityEntry._ID + " = ?",

new String[]{String.valueOf(ContentUris.parseId(uri))});

// reset _ID

db.execSQL("DELETE FROM SQLITE_SEQUENCE WHERE NAME = '" +

MyCityContract.MyCityEntry.TABLE_MY_CITY + "'");

break;

default:

throw new UnsupportedOperationException("Unknown uri: " + uri);

}

return numDeleted;

}

@Override

public int bulkInsert(Uri uri, ContentValues[] values){

final SQLiteDatabase db = myCityDbHelper.getWritableDatabase();

final int match = sUriMatcher.match(uri);

switch(match){

case MY_CITY:

// allows for multiple transactions

db.beginTransaction();

// keep track of successful inserts

int numInserted = 0;

try{

for(ContentValues value : values){

if (value == null){

throw new IllegalArgumentException("Cannot have null content values");

}

long _id = -1;

try{

_id = db.insertOrThrow(MyCityContract.MyCityEntry.TABLE_MY_CITY,

null, value);

}catch(SQLiteConstraintException e) {

Log.w(LOG_TAG, "Attempting to insert " +

value.getAsString(

MyCityContract.MyCityEntry.COLUMN_NAME)

+ " but value is already in database.");

}

if (_id != -1){

numInserted++;

}

}

if(numInserted > 0){

// If no errors, declare a successful transaction.

// database will not populate if this is not called

db.setTransactionSuccessful();

}

} finally {

// all transactions occur at once

db.endTransaction();

}

if (numInserted > 0){

// if there was successful insertion, notify the content resolver that there

// was a change

getContext().getContentResolver().notifyChange(uri, null);

}

return numInserted;

default:

return super.bulkInsert(uri, values);

}

}

@Override

public int update(Uri uri, ContentValues contentValues, String selection, String[] selectionArgs){

final SQLiteDatabase db = myCityDbHelper.getWritableDatabase();

int numUpdated = 0;

if (contentValues == null){

throw new IllegalArgumentException("Cannot have null content values");

}

switch(sUriMatcher.match(uri)){

case MY_CITY:{

numUpdated = db.update(MyCityContract.MyCityEntry.TABLE_MY_CITY,

contentValues,

selection,

selectionArgs);

break;

}

case MY_CITY_WITH_ID: {

numUpdated = db.update(MyCityContract.MyCityEntry.TABLE_MY_CITY,

contentValues,

MyCityContract.MyCityEntry._ID + " = ?",

new String[] {String.valueOf(ContentUris.parseId(uri))});

break;

}

default:{

throw new UnsupportedOperationException("Unknown uri: " + uri);

}

}

if (numUpdated > 0){

getContext().getContentResolver().notifyChange(uri, null);

}

return numUpdated;

}

}

MainActivityFragment

public class MainActivityFragment extends Fragment implements LoaderManager.LoaderCallbacks{

private static final String LOG_TAG = MainActivityFragment.class.getSimpleName();

private MyCityAdpapter myCityAdpapter;

private static final int CURSOR_LOADER_ID = 0;

private GridView mGridView;

MyCity[] mMyCity = {

new MyCity("Ancient Theatre - Larisa", R.drawable.larissa1),

new MyCity("Ancient Theatre - Larisa", R.drawable.larissa2),

new MyCity("Municipality park", R.drawable.larissa3),

new MyCity("Municipality park", R.drawable.larissa4),

new MyCity("Old trains",R.drawable.larissa5),

new MyCity("Old trains",R.drawable.larissa6),

new MyCity("Church",

R.drawable.larissa7),

new MyCity("Church",

R.drawable.larissa8),

new MyCity("Alcazar park",

R.drawable.larissa9),

new MyCity("Alcazar park",

R.drawable.larissa10),

new MyCity("AEL FC Arena",

R.drawable.larissa11),

new MyCity("AEL FC Arena",

R.drawable.larissa12),

new MyCity("Larissa Fair",

R.drawable.larissa13),

new MyCity("Larissa Fair",

R.drawable.larissa14),

new MyCity("Larissa Fair",

R.drawable.larissa15),

new MyCity("Larissa Fair",

R.drawable.larissa16)

};

public MainActivityFragment() {

// Required empty public constructor

}

@Override

public void onActivityCreated(@Nullable Bundle savedInstanceState) {

Cursor c =

getActivity().getContentResolver().query(MyCityContract.MyCityEntry.CONTENT_URI,

new String[]{MyCityContract.MyCityEntry._ID},

null,

null,

null);

if (c.getCount() == 0){

insertData();

}

// initialize loader

getLoaderManager().initLoader(CURSOR_LOADER_ID, null, this);

super.onActivityCreated(savedInstanceState);

}

@Override

public View onCreateView(LayoutInflater inflater, ViewGroup container,

Bundle savedInstanceState) {

// Inflate the layout for this fragment

// inflate fragment_main layout

final View rootView = inflater.inflate(R.layout.fragment_main_activity, container, false);

// initialize our FlavorAdapter

myCityAdpapter = new MyCityAdpapter(getActivity(), null, 0, CURSOR_LOADER_ID);

// initialize mGridView to the GridView in fragment_main.xml

mGridView = (GridView) rootView.findViewById(R.id.flavors_grid);

// set mGridView adapter to our CursorAdapter

mGridView.setAdapter(myCityAdpapter);

return rootView;

}

// insert data into database

public void insertData(){

ContentValues[] cityValuesArr = new ContentValues[mMyCity.length];

// Loop through static array of MyCity, add each to an instance of ContentValues

// in the array of ContentValues

for(int i = 0; i < mMyCity.length; i++){

cityValuesArr[i] = new ContentValues();

cityValuesArr[i].put(MyCityContract.MyCityEntry.COLUMN_ICON, mMyCity[i].image);

cityValuesArr[i].put(MyCityContract.MyCityEntry.COLUMN_NAME,

mMyCity[i].name);

}

// bulkInsert our ContentValues array

getActivity().getContentResolver().bulkInsert(MyCityContract.MyCityEntry.CONTENT_URI,

cityValuesArr);

}

@Override

public Loader onCreateLoader(int id, Bundle args){

return new CursorLoader(getActivity(),

MyCityContract.MyCityEntry.CONTENT_URI,

null,

null,

null,

null);

}

@Override

public void onLoadFinished(Loader loader, Cursor data) {

myCityAdpapter.swapCursor(data);

}

@Override

public void onLoaderReset(Loader loader){

myCityAdpapter.swapCursor(null);

}

}

我在片段的onCreateView(...)方法中读取了Cursor

@Override

public void onActivityCreated(@Nullable Bundle savedInstanceState) {

Cursor c =

getActivity().getContentResolver().query(MyCityContract.MyCityEntry.CONTENT_URI,

new String[]{MyCityContract.MyCityEntry._ID},

null,

null,

null);

if (c.getCount() == 0){

insertData();

}

// initialize loader

getLoaderManager().initLoader(CURSOR_LOADER_ID, null, this);

super.onActivityCreated(savedInstanceState);

}

这就是我引发null游标异常的地方。

有任何想法吗？

谢谢，

西奥

编辑

我改变了这个条件

if (c.getCount() == 0){

insertData();

}

至

if (c == null){

insertData();

}

我得到这个例外！

Caused by: java.lang.IllegalArgumentException: Unknown URL content://theo.testing.customloaders/my_city

因此提供程序存在错误。 唔...