DBHelper.getWirtableDatabse 抛出空异常
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【中文标题】DBHelper.getWirtableDatabse 抛出空异常【英文标题】：DBHelper.getWirtableDatabse throws null exception 【发布时间】：2017-09-02 02:22:53 【问题描述】：我正在尝试为用户编写自己的内容提供程序。当我尝试验证用户是否存在时，当 DBHelper 尝试创建数据库时，它会抛出一个空异常。
这里是内容提供者：
public class MyUserProvider extends ContentProvider
private UserDBHelper db;
private static final int USERS = 10;
private static final int USER_ID = 20;
private static final String AUTHORITY =
"net.ifo420.ritc.agenda.userprovider";
private static final String BASE_PATH = "users";
public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY
+ "/" + BASE_PATH);
public static final String CONTENT_TYPE =
ContentResolver.CURSOR_DIR_BASE_TYPE + "/users";
public static final String CONTENT_ITEM_TYPE =
ContentResolver.CURSOR_ITEM_BASE_TYPE + "/user";
private static final UriMatcher uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
static
uriMatcher.addURI(AUTHORITY, BASE_PATH, USERS);
uriMatcher.addURI(AUTHORITY, BASE_PATH + "/#", USER_ID);
public MyUserProvider()
@Override
public boolean onCreate()
db = new UserDBHelper(getContext());
return false;
@Override
public Cursor query(Uri uri, String[] projection, String selection,
String[] selectionArgs, String sortOrder)
SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();
checkColumns(projection);
queryBuilder.setTables(UserTable.TABLE);
int uriType = uriMatcher.match(uri);
switch (uriType)
case USERS:
break;
case USER_ID:
queryBuilder.appendWhere(UserTable.COLUMN_ID + " = " + uri.getLastPathSegment());
break;
default:
throw new IllegalArgumentException("Uknown uri " + uri);
SQLiteDatabase db1 = db.getReadableDatabase();
Cursor cursor = queryBuilder.query(db1, projection, selection, selectionArgs,
null, null, sortOrder);
cursor.setNotificationUri(getContext().getContentResolver(), uri);
return cursor;
@Override
public String getType(Uri uri)
return null;
@Override
public Uri insert(Uri uri, ContentValues values)
int uriType = uriMatcher.match(uri);
SQLiteDatabase database = db.getReadableDatabase();
long id = 0;
switch (uriType)
case USERS:
id = database.insert(UserTable.TABLE, null, values);
break;
default:
throw new IllegalArgumentException("Unknown URI: " + uri);
getContext().getContentResolver().notifyChange(uri, null);
return Uri.parse(BASE_PATH + "/" + id);
@Override
public int delete(Uri uri, String selection, String[] selectionArgs)
int uriType = uriMatcher.match(uri);
SQLiteDatabase sqLiteDatabase = db.getReadableDatabase();
int rowsDeleted = 0;
switch (uriType)
case USERS:
rowsDeleted = sqLiteDatabase.delete(UserTable.TABLE, selection,
selectionArgs);
break;
case USER_ID:
String id = uri.getLastPathSegment();
if(TextUtils.isEmpty(selection))
rowsDeleted = sqLiteDatabase.delete(UserTable.TABLE,
UserTable.COLUMN_ID + " = " + id, null);
else
rowsDeleted = sqLiteDatabase.delete(UserTable.TABLE,
UserTable.COLUMN_ID + " = " + id + " and " +
selection, selectionArgs);
break;
default:
throw new IllegalArgumentException("Unknown URI: " + uri);
getContext().getContentResolver().notifyChange(uri, null);
return rowsDeleted;
@Override
public int update(Uri uri, ContentValues values, String selection,
String[] selectionArgs)
int uriType = uriMatcher.match(uri);
SQLiteDatabase sqLiteDatabase = db.getReadableDatabase();
int rowsUpdated = 0;
switch (uriType)
case USERS:
rowsUpdated = sqLiteDatabase.update(UserTable.TABLE,
values,
selection,
selectionArgs);
break;
case USER_ID:
String id = uri.getLastPathSegment();
if (TextUtils.isEmpty(selection))
rowsUpdated = sqLiteDatabase.update(UserTable.TABLE,
values,
UserTable.COLUMN_ID + "=" + id,
null);
else
rowsUpdated = sqLiteDatabase.update(UserTable.TABLE,
values,
UserTable.COLUMN_ID + "=" + id
+ " and "
+ selection,
selectionArgs);
break;
default:
throw new IllegalArgumentException("Unknown URI: " + uri);
getContext().getContentResolver().notifyChange(uri, null);
return rowsUpdated;
private void checkColumns(String[] projection)
String[] available = UserTable.COLUMN_USERNAME, UserTable.COLUMN_PASSWORD,
UserTable.COLUMN_ID;
if (projection != null)
HashSet<String> requestedColumns =
new HashSet<String>(Arrays.asList(projection));
HashSet<String> availableColumns =
new HashSet<String>(Arrays.asList(available));
if (!availableColumns.containsAll(requestedColumns))
throw new IllegalArgumentException("Unknown columns in projection");
这是我的 DBHelper
public class UserDBHelper extends SQLiteOpenHelper
private static final String DB_NAME = "agenda.db";
private static final int DB_VERSION = 1;
public UserDBHelper (Context context)
super(context, DB_NAME, null, DB_VERSION);
@Override
public void onCreate(SQLiteDatabase db)
UserTable.onCreate(db);
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
UserTable.onUpgrade(db, oldVersion, newVersion);
Here is the sample code I am using to verify if the user exists (sorry a bit is in french):
public void onClick(View view)
switch (view.getId())
case R.id.boutonLogin:
if (userExists())
Toast.makeText(this, "L'utilisateur existe.", Toast.LENGTH_SHORT).show();
else
Toast.makeText(this, "L'utilisateur n'existe pas.", Toast.LENGTH_SHORT).show();
private boolean userExists()
String username = nomUtilisateur.getText().toString();
String password = motPasse.getText().toString();
Cursor users = userProvider.query(MyUserProvider.CONTENT_URI,
null,
" WHERE username = " + nomUtilisateur.getText().toString() +
" AND password = " + motPasse.getText().toString(), null, null);
if (users.getCount() != 0)
return true;
else
return false;
这是 logcat 给我的：
java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase net.info420.ritc.agendacegep.UserDBHelper.getReadableDatabase()' on a null object reference
at net.info420.ritc.agendacegep.MyUserProvider.query(MyUserProvider.java:70)
at net.info420.ritc.agendacegep.LoginActivity.utilisateurExiste(LoginActivity.java:69)
at net.info420.ritc.agendacegep.LoginActivity.onClick(LoginActivity.java:57)
at android.view.View.performClick(View.java:5610)
at android.view.View$PerformClick.run(View.java:22260)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
【问题讨论】：
在onCreate中尝试return true
还是不行
【参考方案1】：
您不能直接实例化和调用 ContentProvider - 您必须通过内容解析器。否则，它不会经历提供者的生命周期，并且不会调用 onCreate。
【讨论】：
当我尝试获取游标返回的结果数时，它现在会引发异常。它说它是空的，当我尝试查看数据库时，它没有被创建。 好的，所以我一直在运行一些带有标签的测试，我注意到在我的应用程序开始时调用了 onCreate 方法，而不是在我想使用 getWritableDatabase() 时调用。而且query()方法在我调用的时候也不会被调用，所以游标为空是正常的，但是数据库仍然没有创建。 得到了我的答案，部分问题是你提到的。谢谢！【参考方案2】：嗯，我有自己的答案。是的，我无法实例化我的 ContentProvider，但不仅如此。我更改了代码并意识到我只是没有真正理解查询中的 selection 和 selectionArgs 参数。
所以它来自：
private boolean userExists()
String username = nomUtilisateur.getText().toString();
String password = motPasse.getText().toString();
Cursor users = userProvider.query(MyUserProvider.CONTENT_URI,
null,
" WHERE username = " + nomUtilisateur.getText().toString() +
" AND password = " + motPasse.getText().toString(), null, null);
if (users.getCount() != 0)
return true;
else
return false;
收件人：
private boolean userExists()
String username = nomUtilisateur.getText().toString();
String password = motPasse.getText().toString();
String[] selection = username, password;
Cursor users = getApplicationContext().getContentResolver().query
(Agenda.UserEntry.CONTENT_URI,
null,
"username = ? AND password = ?", selection, null);
if (users.getCount() != 0)
return true;
else
return false;
所以，光标为空，因为我没有进行正确的查询，这就是我的数据库没有创建的原因：因为我在代码的末尾不需要它。
【讨论】：
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