Android的SQLite的getReadableDatabase [英] Android Sqlite getReadableDatabase
问题描述
任何机构可以帮我请。我在做的时间表应用程序，所以使用数据库。
Can any body help me please. I'm doing timetable application, so working with database.
我的程序崩溃的位置：
DatabaseHandler dbHelper = new DatabaseHandler(this.getApplicationContext());
SQLiteDatabase db = dbHelper.getReadableDatabase(); //<---------------
Cursor cur=null;
cur = dbHelper.fetchAll(db, "SELECT * FROM timetable");
db.close();
我的数据库处理器类是在这里：
My DatabaseHandler class is here:
public DatabaseHandler (Context context) {
super(context,DATABASE_NAME, null,DATABASE_VERSION);
}
//Creating Table
@Override
public void onCreate(SQLiteDatabase db){
String CREATE_TABLE = "CREATE TABLE" + TABLE_NAME + "("
+ KEY_ID + "INTEGER PRIMARY KEY," + KEY_SUBJECT_NAME
+ "TEXT," + KEY_SUBJECT_CODE + "TEXT," + KEY_SUBJECT_VENUE
+ "TEXT," + KEY_TIME + "INTEGER," + KEY_DAY + "INTEGER" + ")";
db.execSQL(CREATE_TABLE);
}
//Upgrading Database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
// Drop older table if existed
db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
// Create tables again
onCreate(db);
}
/**
* All CRUD(Create, Read, Update, Delete) Operations
*/
//Insert data
public void insertValues(int day, int time, String name, String code, String venue) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(KEY_DAY, day);
values.put(KEY_TIME, time);
values.put(KEY_SUBJECT_NAME, name);
values.put(KEY_SUBJECT_CODE, code);
values.put(KEY_SUBJECT_VENUE, venue);
db.insert(TABLE_NAME, null, values);
//closing the database connection
db.close();
}
public Cursor fetchAll(SQLiteDatabase db, String query){
Cursor cursor = db.rawQuery(query, null);
db.close();
return cursor;
}
}
我会很乐意如果有人可以帮助我，因为这是我的毕业设计：）
I would be more than happy If someone could help me, cause this is my Final Year Project :)
推荐答案
最有可能你的上下文为空。尽量只使用这个而不是 getApplicationContext（）
Most likely your Context is null. Try to use just this instead of getApplicationContext()
dbHelper = new DatabaseHandler(this);
注意：我建议这样做，而这样的：
Note: I recommend to do it rather like this:
public Cursor fetchAll(SQLiteDatabase db, String query) {
Cursor cursor;
db = this.getReadableDatabase();
if (db != null) {
cursor = db.rawQuery(query, null); // also rather use query method, ist more saf
}
return cursor;
}
如果你有方法，返回光标，你不应该叫 db.close（）。如果您在活动调用它，你的光标将抛出错误是空的。
If you have method that returns Cursor, you shouldn't call db.close(). If you call it, your Cursor in Activity will thrown error that "is empty".
要松开光标等，我推荐使用的onPause（）或的onDestroy（ ） 活动的方法，但也这取决于法的语境。
To release Cursors etc, i recommend to use onPause() or onDestroy() methods of Activity but also it depends on Context of method.
因此，如果用于从数据库中添加，更新或删除数据，因此再在你的方法你的方法，我建议你使用尝试，终于块或的try-catch-最后，现在你应该叫 db.close（）有你的解决方案清洁无面条code
So if your method is used for adding, updating or removing data from database so then in your method i recommend to you use try-finally block or try-catch-finally and now you should call db.close() to have you solution clean without spaghetti code.
于是小例子：
public boolean addRecord(String name, String pass, String value) {
SQLiteDatabase db;
try {
db = this.getWritableDatabase();
ContentValues data = new ContentValues();
data.put(KEY_NAME, name);
data.put(KEY_PASS, pass);
data.put(KEY_VALUE, value);
long result = db.insertOrThrow(TABLE_NAME, <nullColumnHack>, data);
return result > 0;
catch (SQLException ex) {
Log.w("SQLException", ex.fillInStackTrace());
return false;
}
finally {
if (db != null) {
db.close();
}
}
}
在这里你是光标 数据库并他们返回例如字符串，的close（）方法上的光标并数据库有干净的解决方案。
Methods where you work with cursors, database and they return for example String, List<T> or other data-type so also here you should call close() method on cursors and database to have clean solution.
但我怎么会带来大约从数据库，不叫的close（）方法返回在 DB
But how i meant about method that returns fetched data from database, do not call close() method on db.
希望它帮助同事。
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