For data storage and retrieval, MIDP 2.0 features a Record Management System (RMS) that is modeled after a simple record oriented database. A record store is a collection of data that remains persistent across multiple invocations of a MIDlet.
MIDlets are allowed to create multiple record stores, as long as the record stores are given different names. The names are case sensitive, and have a maximum of 32 characters. MIDlets can access record stores relating to other MIDlets within the same MIDlet suite. Also, if explicit permission is given, MIDlets within other suites can also access the record store.
A record store
is always stored on the same drive as the MIDlet itself (phone memory
or memory card). The size of record store database is limited only
by free disk space. A RecordStoreFullException is thrown when free space runs out.
Each record in a record store consists of a single binary field of variable size. The responsibility for interpreting the contents of a record falls entirely on the MIDlet. RMS only provides the storage and a unique identifier.
The following methods do not return until all listeners have been called:
addRecord()
setRecord()
deleteRecord()
As implied above, a blocking listener causes blocking of thread that asked for the record modification. The implementation does not, however, block other threads accessing the record store (unless the accesses lead to same blocking condition of a listener.)
Basic RMS operations can be divided into three groups performance wise:
light: getRecord
medium: addRecord, setRecord and deleteRecord, closeRecordstore
heavy: openRecordstore
An operation in a heavier group can be about five to ten times more costly than in lighter group.
To optimize RMS usage, keep the record store open as long as possible. Avoid the pattern of opening the record store, getting a single record and closing the record store.
An application that lists Top-100 Movies in your favorite
video rental store. The application has two views: a main window listing
Top-100 movies and a Details view with information
on the cast, storyline and so on. The application uses RMS to store
movie information, each movie in its own RecordStore .
for each of 100 RecordStores
open RecordStore
get record (movie poster saved as PNG)
close RecordStore
show movie posters in a list
This means that whenever the user goes back from the Details view to the Top-100, the application opens same
the RecordStores again and again. Opening RecordStore is a heavy operation and there is a visible
pause in the application when the user returns to the main view.
The user experience of the application can be improved greatly
by simply closing RecordStores when application
exits. This means that returning to the Top-100 view is practically
instant, instead of having a delay of about half a second. The example application illustrates this issue.
For more information, see:
Storing application settings in Java ME (Nokia Developer Wiki)