Occasionally, you might need to store a file in a database. The usual approach is to save
the file to disk and then update the record in the database that points to the
corresponding file name.
Android takes this protocol and automates it by defining a specific procedure for saving
and retrieving these files. Android uses a convention where a reference to the file name
is saved in a record with a reserved column name of _data.
When a record is inserted into that table, Android returns the URI to the caller. Once you
save the record using this mechanism, you also need to follow it up by saving the file in
that location. To do this, Android allows ContentResolver to take the Uri of the database
record and return a writable output stream. Behind the scenes, Android allocates an
internal file and stores the reference to that file name in the _data field.
If you were to extend the Notepad example to store an image for a given note, you could
create an additional column called _data and run an insert first to get a URI back. The
following code demonstrates this part of the protocol:
ContentValues values = new ContentValues();
values.put("title", "New note");
values.put("note","This is a new note");
//Use a content resolver to insert the record
ContentResolver contentResolver = activity.getContentResolver();
Uri newUri = contentResolver.insert(Notepad.Notes.CONTENT_URI, values);
Once you have the URI of the record, the following code asks the ContentResolver to
get a reference to the file output stream:
….
//Use the content resolver to get an output stream directly
//ContentResolver hides the access to the _data field where
//it stores the real file reference.
OutputStream outStream = activity.getContentResolver().openOutputStream(newUri);
someSourceBitmap.compress(Bitmap.CompressFormat.JPEG, 50, outStream);
outStream.close();
The code then uses that output stream to write to.