As a new Android programmer, I followed various online examples to create my own class that extends SQLiteOpenHelper...
public class MySQLiteHelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION=1;
private static final String DATABASE_NAME="MyDB";
// Main Database Operations
public MySQLiteHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
#Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_TABLE_A);
db.execSQL(CREATE_TABLE_B);
}
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
...
public long addRecord(){
SQLiteDatabase db = this.getWritableDatabase();
....
etc
This works throughout my various activities with no issues. When I want to use it, I call...
MySQLiteHelper db = new MySQLiteHelper(this);
Then I call the various routines like...
db.addRecord();
Now I have created a secondary class that I need to use throughout the application. In that class, there are some routines that need to process data from the database. Problem is, I can't declare the MySQLiteHelper because of the this which errors out.
Doing some online search I think I understand that I need to get the Context from the application (or the calling activity??), but not sure how to go about doing that with this code.
As I mentioned, I am new to Android... so any code examples would be greatly appreciated.
EDIT
I need to clarify this secondary class as mentioned above. From the aforementioned online examples, the second class is used to "hold" the database record information. It looks something like this...
public class Acct {
private int _id;
private String _name;
private String _phone;
public Acct() {
}
public int get_id(){
return this._id;
}
public void set_id(int id) {
this._id=id;
}
public void set_name(String name){
this._name=name;
}
public String get_name(){
return this._name;
}
public void set_phone(String phone){
this._name=phone;
}
public String get_phone(){
return this._phone;
}
...
This is typically used with something like this in an activity...
MySQLiteHelper db = new MySQLiteHelper(this);
Acct acct = new Acct();
acct=db.getAccount(searchId);
myEditText.setText(acct.get_name());
...
Where my problem arises, I want to create a routine and code it IN the Acct class so it can be referenced such as...
acct.UpdateData();
This UpdateData routine is where we need to access the db and thus the MySQLiteHelper. It needs to be able, for each account, to go into the database, access some information from another table, do some processing, and store a summary back into this table for easier reference. As mentioned, there is no Context in the Acct class, so this is where I am getting confused.
And to make matters worse, because the Acct class is a 'holding' place for data from the DB, the online examples also use Acct IN the 'MySQLiteHelper' itself during the getAccount routine....
public Acct getAccount(int id){
SQLiteDatabase db = this.getReadableDatabase();
String SQL_STRING="SELECT * FROM "+ACCT_TABLE+" WHERE "+ACCT_FLD_ID+" = "+String.valueOf(id);
Cursor cursor =
db.rawQuery(SQL_STRING, null);
Acct acct = new Acct();
if (cursor!=null) {
cursor.moveToFirst();
acct.set_id(cursor.getInt((cursor.getColumnIndex(ACCT_FLD_ID))));
acct.set_name(cursor.getString(cursor.getColumnIndex(ACCT_FLD_NAME)));
acct.set_phone(cursor.getString(cursor.getColumnIndex(ACCT_FLD_PHONE)));
cursor.close();
} else {
acct = null;
}
db.close();
return acct;
}
I hope all this additional helped clarify what I am trying to do for the couple comments and answers posted so far. If you need more information, please ask. I'd like to get this to work, just still not sure how.
Your problem is you need a Context to call the constructor of MySQLiteHelper. You've been successful doing so in an Activity (which is a Context), but now you have some other class (which I will call "Foo") that isn't a Context and doesn't have one.
A quick solution is to make Foo take a Context in its constructor, and instantiate your MySQLiteHelper like so:
public class Foo {
private MyOpenHelper openHelper;
public Foo(Context context) {
openHelper = new MyOpenHelper(context.getApplicationContext());
}
}
If Foo is a singleton, you can do the same thing in whatever method obtains the instance (i.e. force the caller to provide a Context). Every application component either is a Context (Activity, Service) or has a Context (BroadcastReceiver gets one in onReceive(), ContentProvider has getContext()).
The use of getApplicationContext() here is worth noting: The Application object for your app is always a singleton--only one instance of it will exist for as long as your app is running (this is guaranteed by the OS). Activities can be destroyed, and creating a MySQLiteHelper with one can cause a memory leak. The application Context always exists and so it cannot be leaked.
Instead of using this, Try to use MainActivity.this or getApplicationContext().
Hope this help!
Related
I have the following code below.
I am creating a database in my application that uses SQLiteOpenHelper.
I have couple of concerns and would appreciate some consults.
Direct answers for these were not found on stack overflow as they might be subjective.
1 - I will be using this database from several activities. However I am not planning on making this a singleton to avoid leaks, but rather I will be getting the getWritableDatabase() and getReadableDatabase() inside each method. I plan on doing a db.close() inside each activity's onDestroy() .Is this advisable ? given my app has couple of activites and is not a huge app.
2 - I am not following and DAO model, nor I am using a different class for every table.
The way I see it, I don't need to. Do I ?
3 - (A question rather than consult)
In the code below, I am not creating a database of the form
private SQLiteDatabase database;
So all the references to the database (from my activities) are being done via the methods in the same subclassed SQLiteOpenHelper, therefore I am referencing the physically created database directly via getWritableDatabase and getReadableDatabase.
Do I need to create an instance of SQLiteDatabase and use it ? Even inside the subclass of SQLiteOpenHelper ?
Below is the code.
public class DbHelper extends SQLiteOpenHelper
{
private static final String DATABASE_NAME = "myDbName";
private static final String DATABASE_TABLE = "myTable";
private static final int DATABASE_VERSION = 1;
private Context ctx;
public DbHelper(Context context)
{
super(context, DATABASE_NAME, null, DATABASE_VERSION);
this.ctx = context;
}
#Override
public void onCreate(SQLiteDatabase db)
{
db.execSQL("CREATE TABLE myTable(_id INTEGER PRIMARY KEY, title TEXT);");
}
#Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
{
//nothing here now. maybe later.
}
public void insertTitle(String title)
{
ContentValues titleCV = new ContentValues();
titleCV .put("title", title);
getWritableDatabase().insert(DATABASE_TABLE, null, titleCV );
}
public void getTitles()
{
Cursor result = getReadableDatabase().rawQuery("SELECT _id, title FROM myTable", null);
while (result.moveToNext())
{
int id = result.getInt(0);
String titleGotten= result.getString(1);
}
result.close();
}
Q1
If you have a scenario within your app that have two parallel threads accessing the database, use a single instance of the SQLiteOpenHelper (singleton or member in the Application or whatever). If not you don't need to.
about calling db.close(), if it is in the onDestroy(), then it's fine.
Q2
a DAO is an abstraction layer to ease maintaining and scaling your project. If you are not going to scale or maintain your code (upcoming releases or something), then I suppose you don't need one.
Q3
You don't need to create an instance of SQLiteDatabse. when you call getReadableDatabase() or getWritableDatabase(), SQLiteOpenHelper creates and maintains an instance. The same instance is used the next time you call getReadable\WritableDatabase().
let me know if you still have questions.
step 1: make a staic instace of SqliteOpenHelper
step 2: you never close conexion to database, sqlite manage itself the sequencial access to write or read :)
private static ControladorBBDD instancia;
my class: public class ControladorBBDD extends SQLiteOpenHelper {
default :
private ControladorBBDD(Context ctx_p) throws Exception {
super(ctx_p, DB_NAME, null, DATABASE_VERSION);
try {
ControladorBBDD.ctx = ctx_p;
DB_PATH = ctx.getDatabasePath(DB_NAME).getAbsolutePath();
String myPath = DB_PATH;// + DB_NAME;
this.createDataBase();
db = SQLiteDatabase.openDatabase(myPath, null,
SQLiteDatabase.OPEN_READWRITE);
} catch (SQLiteException ex) {
Conexiones.escribirLog(Log.getStackTraceString(ex),
ctx.getString(R.string.versionReal));
db.close();
}
}
and my way to implement a conexion to database:
public static synchronized ControladorBBDD getBBDD(Context ctx_p)
throws Exception {
if (instancia == null) {
instancia = new ControladorBBDD(ctx_p);
}
return instancia;
}
and to call it from activities:
dblectura = ControladorBBDD.getBBDD(getApplicationContext());
where private ControladorBBDD dblectura;
i hope that it helps, important thing is that you use applicationContext, no Activity context ;))
well if i were u i would create a class and the dbhelper as a subclass then i would use a open and a close function for main class and also the insert
whenever i want to use database i do it like this
mainclass mc=new mainclass(this);
mc.open();
mc.insert();
mc.close();
So I've come up with some idea and I'm wondering if it is realizable.
Let's say I've multiple tables(database models) each of them is represented by some class.I don't wont to use singleton pattern with the open helper so I've created some simple class to provide single instance of the database.My idea is that as long as all tables hold reference to SQLiteDatabase(returned by the open helper) they will all work with the same DB instance and probably won't be needed to synchronized the work with the database since the open helper do this.When the last table finish it's work the GC will collect the open helper (since the last reference will be weak reference) -> finalize() is called and I close the db during this method to prevent any warning from OS. My question is: Is this could work?Will it close automatically the DB and will it leak or throw some exception?
Here is my class:
public class DatabaseHelper {
private static WeakReference<SomeCustomOpenHelper> sDBOpenHelper;
private void notifyDBCreate(SQLiteDatabase db) {
for (DBTable table : mTables) {
table.onDBCreate(db);
}
}
private void notifyDBUpgrade(SQLiteDatabase db) {
for (DBTable table : mTables) {
table.onDBUpgrade(db);
}
}
public SQLiteDatabase getDatabase(boolean readOnly) {
SomeCustomOpenHelper dbHelper = sDBOpenHelper.get();
if (dbHelper == null) {
dbHelper = new SomeCustomOpenHelper(context, name, factory, version, new DatabaseEventsCallback());
sDBOpenHelper = new WeakReference<SomeCustomOpenHelper>(dbHelper);
}
if (readOnly) {
return dbHelper.getReadableDatabase();
} else {
return dbHelper.getWritableDatabase();
}
}
private class DatabaseEventsCallback implements IDatabaseEventsCallback {
#Override
public void onCreate(SQLiteDatabase db) {
notifyDBCreate(db);
}
#Override
public void onUpgrade(SQLiteDatabase db) {
notifyDBUpgrade(db);
}
}
interface IDatabaseEventsCallback {
void onCreate(SQLiteDatabase db);
void onUpgrade(SQLiteDatabase db);
}
private static class SomeCustomOpenHelper extends SQLiteOpenHelper {
private IDatabaseEventsCallback mCB;
public SomeCustomOpenHelper(Context context, String name, CursorFactory factory, int version, IDatabaseEventsCallback cb) {
super(context, name, factory, version);
mCB = cb;
}
#Override
public void onCreate(SQLiteDatabase db) {
mCB.onCreate(db);
}
#Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
mCB.onUpgrade(db);
}
#Override
protected void finalize() throws Throwable {
this.close();
super.finalize();
}
}
}
Did not really know the answer neither, but got interested and looked it up.
The answer is written out properly here;
http://blog.foxxtrot.net/2009/01/a-sqliteopenhelper-is-not-a-sqlitetablehelper.html
But basically the core of the info is;
I created three SQLiteOpenHelper classes, one for each table, even though they all referenced only a single database file.
Here is where everything fell apart. Android maintains Versions for databases based on the package it’s associated with, the name of the database, and the version number you provide. The package and name go into decided what the path on the device will be, while the version is stored (somewhere) on the device so that it knows when it needs to call an OpenHelper’s onUpgrade event handler. It turns out that if, in the SQLiteOpenHelper Constructor, it determines that the database already exists, it won’t call your onCreate or onUpgrade methods at all, even if the particular class which is making the call has never been called before.
I've been through the same issue when I was working on a project. I also went crazy on the doubt if the static instance was using enough memory and causing a considerable memory leak.
I'm not sure if creating a weak reference would guarantee that database instance would be collected. However a possible workaround could be : Assigning a null value to static database instance once your all database transaction is done and you've close the database. This might ensure that the database instance no more allocates any memory.
Let me know if this works or if there is a better work-around.
You can do so. As you say the locking should be happening on the SQLite and I've never heard issues around that so you should be fine with this.
The only restriction you have is that all the tables will have to go into the same database since Android for now just allows you to have one file.
Closing the database is a different thing, that's why it is actually interesting to use the singleton pattern (you avoid closing + opening all the time).
Nonetheless with your approach you just need to make sure to close the db whenever you are done with it. As far as I'm concerned this is not automatically done.
Additionally Lars Vogel has written extremely useful and detailed articles around DB access in Android. You might want to have a look there. http://www.vogella.com/articles/AndroidSQLite/article.html
you can use one open helper for all the table .i am using the single instance in my app also like this .
public static synchronized DatabaseHelper getInstance(Context ctx)
{
if (dbhelper == null) {
dbhelper = new DatabaseHelper(ctx);
}
return dbhelper ;
}
My question is: Is this could work?Will it close automatically the DB
and will it leak or throw some exception?
NO it will not close automatically database , when your application will demand for DATABASE object and OS found some of your database instant are alive then Android framework try to connect that object reference (which is probably weak reference )
and i have to say , I don't recommend opening and closing a DATABASE on-demand or temporarily . It is always nice to open the DB up early and keep it open for the duration of your whole activity and close it when the activity gets finished or suspended .
I'm using the SQLiteOpenHelper (shown below) in all my apps quiet happily. Lots of upgrades to existing databases ended successful in many year.
This time I need to upgrade the database in one of my apps and this step will last for some time. So I need to put these upgrade statements in their own thread.
What's the best place to do so?
Any help is highly appreciated.
public class MySQLiteOpenHelper extends SQLiteOpenHelper {
protected static final Object lock = new Object();
private static final int DATABASE_NAME = "mydatabase.db";
private static final int DATABASE_VERSION = 3;
private Context context;
private SQLiteDatabase database;
public MySQLiteOpenHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
this.context = context;
}
#Override
public void onCreate(SQLiteDatabase database) {
synchronized (lock) {
this.database = database;
// Create database statements
}
}
#Override
public void onUpgrade(SQLiteDatabase database, int oldVersion, int newVersion) {
synchronized (lock) {
this.database = database;
switch (newVersion) {
case DATABASE_VERSION:
switch (oldVersion) {
case 1:
upgradeFrom1To2();
case 2:
upgradeFrom2To3();
}
break;
}
}
}
private void upgradeFrom1To2() {
// Upgrade database statements
}
private void upgradeFrom2To3() {
// Upgrade database statements
}
}
I'd put the code in an IntentService, which is an easy way to implement a background thread for a long-running operation, especially one that is saving data. Use broadcast Intent to send status from the IntentService to other components, and BroadcastReceiver to receive these Intent, if you need to.
I would really avoid doing this in AsyncTask within an Activity; there's too much risk that the operation would be killed. Anyway, AsyncTask is much more complicated than IntentService.
One note: IntentService doesn't persist anything, including its class fields. To protect yourself, you may want to store state in SharedPreferences.
On my app I make use of two datatabases.
This is the class that handles the database management and all the query that are made to it.
public class Database {
private DbHelper DBHelper;
private final Context Context;
private SQLiteDatabase MyDBone, MyDBtwo;
static Context ctx;
private static class DbHelper extends SQLiteOpenHelper {
public DbHelper(Context context, String dbName, int dbVersion) {
super(context, dbName, null, dbVersion);
}
#Override
public void onCreate(SQLiteDatabase db) {
// This is where the two databases are created
}
}
#Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVesion) {
// database upgrades are handled here
}
}
}
// database constructor
public Database(Context c) {
Context = c;
ctx = c;
}
// database open
public Database open() throws SQLException {
DBHelper = new DbHelper(Context, BD_NAME, BD_VERSION);
// I have here some if code to decide witch one of the bellow is used
if{
MyDBone = DBHelper.getWritableDatabase();
} else{
MyDBtwo = DBHelper.getWritableDatabase();
}
return this;
}
// database close
public void close() {
DBHelper.close();
}
public Cursor getData(........) {
// My querys are made here
}
}
My problem is that the databases are too big. In the onCreate method I'm getting the error: The code of method onCreate(SQLiteDatabase) is exceeding the 65535bytes limit. On the other side, my app is getting very big on size.
I would like to know what's the best way to address this issue since I can't change my databases.
Since my app must be run offline I can't make query's on a webserver.
I beleive that the best aproach would be to, on the first run of the app, download the databases from somewhere on the internet (drive, dropbox or other side) but since my programming skils are a little green I must pospone this to a must do in the future.
Is it possible, maintaining my Database class, prepack the apk with the databases and install them on the sdcard? On the other side this will increase the apk size (the total of the databases is 15 mb).
Please advise on the best way to address this issue.
Regards,
favolas
Exception:
CREATE TABLE android_metadata failed
Failed to setLocale() when constructing, closing the database
android.database.sqlite.SQLiteException: database is locked
My app works fine and has no db issues, except when onUpgrade() is called.
When onUpgrade is automatically called, it tries to use the CarManager class below to do data manipulation required for the upgrade. This fails because the db is locked.
Because this seems like it should be a normal thing to do, it seems that I must not be structuring the following code correctly (two classes follow, a helper and a table manager):
public class DbHelper extends SQLiteOpenHelper {
private Context context;
//Required constructor
public DbAdapter(Context context)
{
super(context, "my_db_name", null, NEWER_DB_VERSION);
this.context = context;
}
#Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
{
overrideDB = db;
CarManager.migrateDataForOnUpgrade(context);
}
}
public class CarManager {
DbHelper dbHelper;
public CarManager(Context context)
{
dbHelper = new DbHelper(context);
}
public void addCar(String make, String model)
{
ContentValues contentValues = new ContentValues();
contentValues.put("make", make);
contentValues.put("model", model);
SQLiteDatabase db = dbHelper.getWritableDatabase();
db.insert("car", null, contentValues);
db.close();
}
public static void migrateDataForOnUpgrade()
{
//Code here that migrates data when onUpgrade() is called
//Db lock happens here
}
}
Any ideas?
Do people set up table manager (ex: dao) differently than this?
edit: I talked to the google team # android developer hours, and they said onUpgrade3 was never meant to do anything like structural changes (alters). So yes, it seems like there are some hacks that must be used in many instances right now.
I use the following model by extending the Application class. I maintain a single static instance of my db helper which all other app components use...
public class MyApp extends Application {
protected static MyAppHelper appHelper = null;
protected static MyDbHelper dbHelper = null;
#Override
protected void onCreate() {
super.onCreate();
...
appHelper = new MyAppHelper(this);
dbHelper = MyAppHelper.createDbHelper();
dbHelper.getReadableDatabase(); // Trigger creation or upgrading of the database
...
}
}
From then on any class which needs to use the db helper simply does the following...
if (MyApp.dbHelper == null)
MyApp.appHelper.createDbHelper(...);
// Code here to use MyApp.dbHelper