Bruce Scharlau, University of Aberdeen, 2010 Java ME Record Management System Mobile Computing.

Bruce Scharlau, University of Aberdeen, 2010

Java ME Record Management System

Mobile Computing


Mobiles + (Tunnels | Hills) = Lost connection


Only commercial databases for Java ME handsets

No widely available databases for Java ME on CLDC

There are some open source packages if you’re using CDC – see information page


A RecordStore is NOT a database

A RecordStore is an array of bytes

with MIDP 2.0 a RecordStore can be shared across MIDlet suites

RecordStore names must be unique


Set the MIDlet-Data-Size attribute when using RMS

Any MIDlet suite that uses RMS should specify the minimum number of bytes of data storage it requires, by setting the MIDlet-Data-Size attribute, in both the JAR manifest and the application descriptor.

If the MIDlet-Data-Size attribute is missing, the device assumes the MIDlet suite requires no space for data storage.

If the MIDlet-Data-Size attribute is missing, the device assumes the MIDlet suite requires no space for data storage.


RecordStore if LILOhttp://w

ww

.geh.org//fm/lw

hprints//m197701610011.jpg

I’m telling you, it’s last in, last

out

No transactions then. Hmm

http://www.geh.org//fm/lwhprints//m197701610011.jpg


If multiple apps share an RMS you need to write the logic for this

RMS operations are thread-safe, but threads must still coordinate the reading and writing of data to the same record store.

This is especially important if your RecordStore is shared in a MIDlet suite.


RecordStore uses ID not index

1 Tom 6789

2 Jane 3680

3 Henry 1472

1 Tom 6789

2 Jane 3680

3 Henry 1472

4 Mary 8547

1 Tom 6789

2 Jane 3680

3 Henry 1472

4 Mary 8547

1 Tom 6789

2 Jane 3680

3 Henry 3456

4 Mary 8547

5 Sarah 4354

1 2

3 4

Start with three Remove Jane, add Mary

Remove Tom Change data, add row


recordID = 1 for first record


Yes, but when it’s gone it’s

gone

Auto-increment,

nice.And it always

starts at 1



RecordStore Methods• All are static so can be called without instance• Can acquire a list of all record stores with

String[ ] listRecordStores()• Create/open record store with RecordStore openRecordStore(myStore, true) boolean to create or not if doesn’t exist

• Delete via void deleteRecordStore(myStore)


Adding Items

myRecords.addRecord(myBytes, 0, myBytes.length);

byteArray of data to be written to the store

Offset from which to start writing the data

How much of byteArray should be written


Modifying a Record

Same as writing to a record, but must also pass in the recordID as the first parameter

myRecords.addRecord(recordID, myBytes, 0, myBytes.length);


Reading a recordEither:

– A) byte [ ] recordData = myRecords.getRecord(1)

– B) byte[ ] myBuffer = new byte[BIG_ENOUGH];int readRecord = myRecords.getRecord(1, myBuffer, 0);• Instead of creating a new byte array to hold

the result, this version uses one from the application


Meta Information

• long lastModified() timestamp of store

• String getName() of the store

• int getNextRecordID() to be added to the store

• int getRecordSize(int myRecord) for the size of the specified record

• int getSize() of the complete store


Custom Objects

• Problem – how to serialise objects and their attributes?

• Create class and then write variables to bytes and then store this object to the array

• Reverse the process to recreate the object


Custom Object - class

class AuctionItems {

String descriptionAI;

String summaryAI;

String startPriceAI;

long startDateAI;

}

Create class to hold variables for custom object, which can then be passed around as required


Write Custom objectprivate byte[] getBytes() throws IOException{ByteArrayOutputStream baos = new ByteArrayOutputStream();

DataOutputStream dos = new DataOutputStream(baos);

dos.writeUTF(descriptionAI);dos.writeUTF(summaryAI);dos.writeUTF(startPriceAI);dos.writeLong(startDateAI);dos.close();baos.close();return baos.toByteArray();}

* Variables from Form* Could also write out Int, Long, Short, and others

* Variables from Form* Could also write out Int, Long, Short, and others


Read Custom Objectpublic AuctionJ2meItems(byte[] data) throws

RecordStoreNotOpenException, InvalidRecordIDException,

RecordStoreException, IOException {

ByteArrayInputStream bais = new ByteArrayInputStream(data);

DataInputStream dis = new DataInputStream(bais);

this.descriptionAI = dis.readUTF();

this.summaryAI = dis.readUTF();

this.startPriceAI = dis.readUTF();

this.startDateAI = dis.readLong();

}A read from the store is the constructor for object and gets used like this:byte[] inputData = auctionItems.getRecord(item);AuctionItems ai = new AuctionItems(inputData);

‘item’ is an int‘item’ is an int


Could write your own implementation to make RMS work like JDBC

To make working with RMS like using JDBC, and Java objects, need to write structure to handle this

http://developers.sun.com/mobility/midp/articles/databasemapextend/

http://developers.sun.com/mobility/midp/articles/databasemapextend/


Use either brute force or enumeration to traverse a RecordStore

Because record IDs are fixed, they are not continuous. A deleted record ID is not filled by an incoming record, so iteration doesn’t work.

RecordStore is NOT a database as shown by this operation


Fetch records one by one in a simple way...

RecordStore rs = ... // an open record store int lastID = rs.getNextRecordID(); int numRecords = rs.getNumRecords(); int count = 0; for( int id = 1; id < lastID && count < numRecords; +

+id ){ try { byte[] data = rs.getRecord( id ); ... // process the data ++count;

} catch( InvalidRecordIDException e ){ // just ignore and move to the next record

} catch( RecordStoreException e ){ // a more general error that should be handled // somehow break;

} } ...


You can also use RecordEnumeration RecordStore rs = ... // an open record store RecordEnumeration enum = rs.enumerateRecords( null, null, false );

try { while( enum.hasNextElement() ){ int id = enum.nextRecordId(); byte[] data = rs.getRecord( id ); ... // do something here }

} catch( RecordStoreException e ){ // handle the error here

} ...


Sorting Objects

• Use RecordEnumerations to sort through records using compare() – Returns either: EQUIVALENT, FOLLOWS or

PRECEDES

• Can also use enumerateRecords() with its RecordFilter to sift through recordStore


Use RecordComparator to sort records for display

import javax.microedition.rms.*;import java.lang.*;

public class MyComparator implements RecordComparator { public int compare(byte rec1[], byte rec2[]) {

String recstr1 = new String(rec1);String recstr2 = new String(rec2);

int num = recstr1.compareTo(recstr2);if (num != 0) {

return ( num < 0 ? RecordComparator.PRECEDES : RecordComparator.FOLLOWS); }else {

return RecordComparator.EQUIVALENT; }

}}

http://www.microjava.com/articles/techtalk/rms2

http://www.microjava.com/articles/techtalk/rms2


Duke’s Auction MIDlet retrieves list of auction items

AuctionMIDlet

Servlet

Database

Send query to db

AuctionItems

ResultSet returned

Vector returned

AuctionJ2meItems

AuctionJ2meItems

Creates vector of these

Serialises to RMS


Call the servlet with byte

hc = (HttpConnection) Connector.open(url); baos = new ByteArrayOutputStream(); dos = new DataOutputStream(baos); dos = hc.openDataOutputStream();

dos.writeByte(OPERATION_LIST_ALL_ITEMS); dos.close();

Use DataInput/ OutputStreams and ByteArrayOutputStream to manage data


The servlet responds to the callprivate void j2me_selectItem(DataInputStream call, DataOutputStream result,

String quantity) throws IOException, SQLException {

AuctionItems auctionI = new AuctionItems();Vector<AuctionJ2meItems> items = new Vector<AuctionJ2meItems>();items = auctionI.getJ2meAuctionItems("all");

Iterator<AuctionJ2meItems> iter = items.iterator();while (iter.hasNext()) {

AuctionJ2meItems aItem = new AuctionJ2meItems();aItem = (AuctionJ2meItems) iter.next();result.writeUTF("next");result.writeUTF(aItem.descriptionAI);result.writeUTF(aItem.summaryAI);result.writeUTF(aItem.startPriceAI);result.writeLong(aItem.startDateAI);

}return;

}

Use AuctionItems and AuctionJ2meItems

Write stream to MIDlet


MIDlet captures stream and writes object to RMS

inputStream = hc.openDataInputStream();

int contentLength = (int) hc.getLength();

String k = "";

inputStream.skipBytes(4);

while (inputStream.available() > 0) {

k = inputStream.readUTF();

if (k.equals("next")) {

aItem = new AuctionJ2meItems(inputStream.readUTF(),

inputStream.readUTF(), inputStream.readUTF(),

inputStream.readLong());

ByteArrayOutputStream baos2 = new ByteArrayOutputStream();

DataOutputStream dos2 = new DataOutputStream(baos2);

aItem.serialize(dos2);

byte[] b = baos2.toByteArray();

try {

auctionItemStore.addRecord(b, 0, b.length);

} catch (Exception e) {

// more code left out here


Then we can read the RecordStore into a form with read(1);

private void read(int item) throws RecordStoreNotOpenException,

InvalidRecordIDException, RecordStoreException, IOException {

byte[] inputData = auctionItemStore.getRecord(item);

AuctionJ2meItems ai = new AuctionJ2meItems(inputData);

ai.print();

// set up form …

readForm.append(description);

readForm.append(summary);

readForm.append(startPrice);

readForm.append(startDate);

description.setString(ai.descriptionAI);

summary.setString(ai.summaryAI);

startPrice.setString(ai.startPriceAI);

startDate.setString(String.valueOf(new java.util.Date(ai.startDateAI)));

// add commnds to form …

readForm.setCommandListener(this);

display.setCurrent(readForm);

}

Notice that we convert startDataAI to Date from long

Use AuctionJ2meItem object


Navigate through records with previous and next commands

} else if (c == mPrevCommand) {try {

if (currentId == 1) {Alert alertPrev = new Alert("No previous records");alertPrev.setTimeout(Alert.FOREVER);alertPrev.addCommand(mReadAuctionCommand);alertPrev.setCommandListener(this);display.setCurrent(alertPrev);

} else {currentId--;read(currentId);

} catch (RecordStoreNotOpenException e) {// pl;us other exceptions …

} else if (c == mNextCommand) {try {

currentId++;read(currentId);…

Check for no record, and keep track of recordID


Using AuctionJ2meItem on server and handset causes issues

Closely couples the code between the two

Explore alternatives before coupling code

May be able to use interoperable option


The more classes in the app the more memory it consumes

Each class in the app will need to be loaded

All classes in MIDP are loaded dynamically at the start during class verification

Keeping classes small, and to a minimum is therefore important in mobile Java


Summary

• RMS is similar to database, with significant differences that you need to accommodate

• Map the bytes to objects for convenience

• Sometimes need to share code across platforms (server and Java ME client) to ease programming

Bruce Scharlau, University of Aberdeen, 2010 Java ME Record Management System Mobile Computing.

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