Question

I am using code in this article to copy a pre-populated db I have into a freshly created db thus overwriting it.

The overwrite is successful. I am able to see the correctly copied db in the right path data/data/my.package/databases/myDatabase. But this is ONLY if I look at the database before leaving getWritableDatabse().

If I leave getWritableDatabse() the db is invalidated: it still keeps its right size BUT it has only the android_metadata instead of my tables that are then all gone.

I didn’t manage to get access to the Android source code for Eclipse (followed all the instructions but it still doesn’t work) but using GrepCode I managed to narrow down the problem to lines 173-176 in method getWritableDatabase().

173 db.setVersion(mNewVersion);
174 db.setTransactionSuccessful();
175 } finally {
176   db.endTransaction();

Anyone has a clue why this piece of code might rewrite my db and empty it?

This is the relevant stack:

Thread [<1> main] (Suspended)   
StournamentDbAdapter$DatabaseHelper(SQLiteOpenHelper).getWritableDatabase() line: 180   
StournamentDbAdapter.open() line: 192   
ActivityTournamentsList.onCreate(Bundle) line: 57   
ActivityTournamentsList(Activity).performCreate(Bundle) line: 4465  
Instrumentation.callActivityOnCreate(Activity, Bundle) line: 1049   
ActivityThread.performLaunchActivity(ActivityThread$ActivityClientRecord, Intent) line: 1920    
ActivityThread.handleLaunchActivity(ActivityThread$ActivityClientRecord, Intent) line: 1981 
ActivityThread.access$600(ActivityThread, ActivityThread$ActivityClientRecord, Intent) line: 123    
ActivityThread$H.handleMessage(Message) line: 1147  
ActivityThread$H(Handler).dispatchMessage(Message) line: 99 
Looper.loop() line: 137 
ActivityThread.main(String[]) line: 4424    
Method.invokeNative(Object, Object[], Class, Class[], Class, int, boolean) line: not available [native method]  
Method.invoke(Object, Object...) line: 511  
ZygoteInit$MethodAndArgsCaller.run() line: 784  
ZygoteInit.main(String[]) line: 551 
NativeStart.main(String[]) line: not available [native method]  

I fist use mDb = mDbHelper.getWritableDatabase(); which calls to DatabaseHelper.onCreate()where the creation of the db actually happens. In DatabaseHelper.onCreate() I call only copyDatabase(); which looks like this:

private void copyDatabase() throws IOException
    {
        //Open your local db as the input stream
        InputStream myInput =  sContext.getAssets().open(StournamentConstants.Database.DATABASE_NAME);

        // Path to the just created empty db
        String outFileName = StournamentConstants.Database.DB_PATH + StournamentConstants.Database.DATABASE_NAME;

        //Open the empty db as the output stream
        OutputStream myOutput = new FileOutputStream(outFileName);

        //transfer bytes from the inputfile to the outputfile
        byte[] buffer = new byte[1024];
        int length;
        while ((length = myInput.read(buffer))>0)
        {
            myOutput.write(buffer, 0, length);
        }

        //Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();
    } 

All this goes fine and the db is correctly copied. Only when I climb out of the stack and return to getWritableDatabase() line: 180 that something goes wrong: the size of the db stays the same but it loses all tables besides the àndroid_metadata`.

---

Intermediate conclusion

I dug a bit more into the code (the above mentioned GrepCode site was VERY helpful) because I wanted to get to the bottom of this. After going into getWritableDatabase(), on line 165 it will call onCreate(), where I replace to database by using copyDatabase();. The issue is that before, on getWritableDatabase() line 162 db.beginTransaction(); is called. Why is that a problem? I will explain by showing the sequence of events:

1. call getWritableDatabase()
2. getWritableDatabase() calls db = SQLiteDatabase.create(null); hence creating a db
3. getWritableDatabase() calls db.beginTransaction();
4. getWritableDatabase() calls onCreate()
5. inside onCreate() I replace the database by calling copyDatabase(); which copies the db from asset to the db location. Not sure why the already created db doesn’t crash at that moment. Maybe because there is no lock on it?
6. on line 176 getWritableDatabase() calls db.endTransaction(); which is the crucial part: it is nice and well that I copied my db into the initially created db, BUT when db.endTransaction(); is called, the system commits all the changes to my newly copied db. But remember: I did no changes, I created no tables – I just copied the db to the correct location. So the commit commits NOTHING thus rewriting the db with NOTHING. For that I get an empty db.

I had the evil thought to:

1. Copy the db in onCreate()
2. Open the newly copied db and replace the db getWritableDatabase() uses (onCreate() gets the db as parameter from getWritableDatabase()) with it.

Something like that:

@Override
public void onCreate(SQLiteDatabase db) 
{
    try 
    {               
        copyDatabase();

        String dbPath = StournamentConstants.Database.DB_PATH + StournamentConstants.Database.DATABASE_NAME;
        SQLiteDatabase activeDb = SQLiteDatabase.openDatabase(dbPath, null, SQLiteDatabase.OPEN_READONLY);              

         db = activeDb;
    } 
       catch (IOException e) 
    {
       Log.i(getClass().getName() + ".onCreate ", e.getMessage());
   }   
}

The problem with that is that I get an android.database.sqlite.SQLiteDatabaseLockedException: database is locked so I can’t get the newly copied db. And besides that – I am not sure what sinister consequences lurk behind such a devious deed.

So I will do what @Barak suggests and move my copying into the constructor – way before the system tries to create the db. So when getWritableDatabase() is finally reached, the db is already there and the system will not attempt to touch it. I learned a lot today – am very pleased.

Conclusion: if you can read (code), you have a definite advantage :^)

Will keep you posted on how this worked for me.

---

Final conclusion

I solved the issue.

Few differences:

1. I DON’T do it inside DatabaseHelper‘s constructor. That would have taken too many code changes. Instead I am doing it inside open() of my private static class DatabaseHelper extends SQLiteOpenHelper class
2. I DON’T use getReadableDatabase() but getWritableDatabase(). The reason is simple: getReadableDatabase() will ALWAYS call getWritableDatabase() so I thought I save that one…

I also tried to be a wiseguy and instead of calling getWritableDatabase() (which has some overhead of code) simply CREATING the db file on my own. I failed. I think it is due to permission issues or something. This is what I tried inside the copyDatabase():

File f = new File(DB_PATH);
if (!f.exists()) 
{
    f.mkdir();
}

But it always fails. I later found out that OutputStream myOutput = new FileOutputStream(outFileName);actually should take care of the file creation.

Hope this helps anyone!

Answer 1

Well, the only difference I see is that I don’t do the database copying/creation in onCreate, that is empty. I do it all in the constructor, like this:

private static class DatabaseHelper extends SQLiteOpenHelper {
    DatabaseHelper(Context context, String dbname, int dbversion) {
        super(context, dbname, null, dbversion);
            if (checkDataBase(dbname)) {
                openDataBase(dbname);
            } else {
                try {
                    this.getReadableDatabase();
                    copyDataBase(dbname);
                    this.close();
                    openDataBase(dbname);
                } catch (IOException e) {
                     throw new Error("Error copying database");
                }
                Toast.makeText(context,"Initial " + dbname + " database has been created",
                    Toast.LENGTH_LONG).show();
            }
      }
    @Override
    public void onCreate(SQLiteDatabase db) {
    }
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    }
}