persistence

Mobile Application Development Persistence

Institute for Mobile and Distributed Systems 1Ch. Denzler

J2ME Persistence

Record Management System & File SystemChristoph DenzlerUniversity of Applied StudiesInstitute for Mobile and Distributed Systems

J2ME Persistence IMVS, Ch. Denzler 2

Lernziele

Sie verstehen wie mit MIDP Daten persistent

gespeichert werden knnen knnen die persistente Datenhaltung fr ein

MIDlet selbst planen und implementieren knnen den persistenten Speicher nutzen um

den beschrnkten Hauptspeicher zu entlasten




Inhalt

CRUD Operationen Enumerationen Filter Comparatoren

Types of memory (seen from J2ME perspective)


SmartPhone Memory

1024 kB

RAM10 RecordStores

File System (optional JSR 75)

MemoryCard(s)

File System (optional JSR 75)




Record Management System (RMS)

Record Store accessed by name (unique per MIDlet suite) persistent across multiple invocations of a MIDlet only accessible by the MIDlets of the MIDlet suite

which created the store contains collection of records Operations are

atomic synchronized processed serially

no SQL, no relational database management system


Record Management System (RMS)

Records identified by a record identifier (int) plain array of bytes: byte[] when accessing it is the MIDlets responsibility to

ensure the following: No thread may read a record that is concurrently changed or

deleted by another thread No thread may delete a record that is concurrently read or

changed by another thread No thread may change a record that is concurrently

changed, read or deleted by another threat synchronize with RecordStore object!



Example


RecordStoreCollectionEnumeration

null

1

2

12

24

781

nextRecord()

nextRecord()

Record Number

byte[]


Manipulating Record Stores

Managing RecordStore RecordStore openRecordStore(String name, boolean

createIfNecessary) String[] listRecordStores() void deleteRecordStore(String name) closeRecordStore

Handling Records int addRecord(byte[] data, int offset, int len) void setRecord(int id, byte[] data, int offset, int

len) void deleteRecord(int id) byte[] getRecord(int id) int getNumRecords() int getNextRecordID()

sta

tic



Code example (writing a record)

public int write(RecordStore rs)throws IOException, RecordStoreException { ByteArrayOutputStream baos =

new ByteArrayOutputStream();DataOutputStream dos = new DataOutputStream(baos);writeImage(dos, pict);dos.writeUTF(text); dos.writeLong(date.getTime());// extract byte arraybyte[] data= baos.toByteArray();dos.close();// write record and return new record numberreturn rs.addRecord(data,0,data.length);

}J2ME Persistence IMVS, Ch. Denzler 9

Code example (cont.)

void writeImage(DataOutputStream dos, Image img) {int w = img.getWidth(); // image sizeint h = img.getHeight(); int[] rgbi = new int[w*h];img.getRGB(rgbi,0,w,0,0,w,h); //img as int-array

dos.writeInt(w);dos.writeInt(h);for(int i = 0; i < rgbi.length; ++i) {dos.writeInt(rgbi[i]); //write pixels image

}}




Code example (reading record)

public void set(byte[] record){ByteArrayInputStream bais =

new ByteArrayInputStream(record);DataInputStream dis = new DataInputStream(bais);try {

pict = readImage(dis);text = dis.readUTF();long millis = dis.readLong();date = new Date(millis);dis.close();

} catch(IOException e){ date = null;

}}


Code example (cont.)

private Image readImage(DataInputStream dis)throws IOException {int w = dis.readInt();int h = dis.readInt();int[] rgbi = new int[w*h];

for(int i = 0; i < rgbi.length; ++i) {rgbi[i] = dis.readInt();

}return Image.createRGBImage(rgbi, w, h, true);

}




Enumeration 1

Similar to Iterator in Collection Framework Create enumeration withRecordEnumeration enumerateRecords(

RecordFilter filter, // may be nullRecordComparator comparator, // may be nullboolean keepUpdated);

use keepUpdated with caution! It sychronizeschanges in the records with the enumeration often better: use rebuild() on enumeration delete or insert invalidates enumeration update may put record out of sort order


Enumeration 2

Reuse objects instead of creating new ones:RecordEnumeration re =rs.enumerateRecords(null, null, false);

D d = new D(); //empty objectwhile(re.hasNextElement()) {byte[] record = re.nextRecord();d.set(record); // set fields of object

} do not:while(re.hasNextElement())D d = new D(re.nextRecord());




Comparator

Used to determine the order of an enumeration Caution: performance consuming! int compare(byte[] rec1, byte[] rec2)

returns -1 if rec1 < rec2: rec1 PRECEDES rec2 1 if rec1 > rec2: rec1 FOLLOWS rec2 0 if rec1 = rec2: rec1 is EQUIVALENT to rec2 in terms of sort order.


Filter

Used to skip records during enumeration Only records matching filter criterion will be

visited during enumeration boolean matches(byte[] candidate)

Returns true if the candidate matches the implemented criterion.




RecordListener

Be informed about changes in a record store

void recordAdded(RecordStore store, int recordId)void recordChanged(RecordStore store, int recordId)void recordDeleted(RecordStore store, int recordId)


File System

The existence of a file system is optional. The file system API is part of JSR75 File systems can be mounted and unmounted

(memory cards!) File systems support directories and files




Support for File System available?

Before use, check if file system is available:

String version = System.getProperty( "microedition.io.file.FileConnection.version");

version either contains the version of the API or null if JSR75 is not supported on the device


File System Roots

get roots of file systemEnumeration roots =FileSystemRegistry.listRoots();

while(roots.hasMoreElements()) {form.append((String)roots.nextElement() + "\n");

}




List Directory Content

String root = ... // get name of file systemFileConnection fc = null;try {fc = (FileConnection)Connector.open("file:///" + root);Enumeration files = fc.list();while(files.hasMoreElements()) {

form.append((String)files.nextElement() + "\n");}

} catch(Exception e){form.append(root + " - no content to show! " +

"reason: " + e.toString() + "\n");}


Read text file

FileConnection fi = null;InputStream is = null;String dir = ... //define directory name hereString file = ... //define file name heretry {fc = (FileConnection)Connector.open("file:///" + dir + file,

Connector.READ); if (fc.exists()) {

is = fc.openInputStream();StringBuffer sb = new StringBuffer();int i;while ((i = is.read()) != -1) sb.append((char)i); // do something meaningful with sb

} } catch(Exception e) { /* handle exception */ } 22J2ME Persistence IMVS, Ch. Denzler




Resources

JSR75:http://java.sun.com/javame/technology/msa/jsr75.jsp

Getting Started with the FileConnection APIshttp://developers.sun.com/mobility/apis/articles/fileconnection/index.html

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