J2ME

1
J2ME

• Java for Mobile Environments
• Based on J2ME In A Nutshell, by Kim Topley,
  OReilly Associates Inc., 2002,and Tetris by
  Alexei Patinov.

2
What is J2ME?

• J2ME Java 2 Micro Edition
• J2ME defines two configurations, ranges of
  hardware for which it can generate software
• CLDC Connected Limited Device Configuration
• CLDC devices include cell phones, PDAs, other
  items with roughly up to 512Kb of RAM.
• CLDC devices have very limited processors for
  example, CLDC devices are not required to support
  double or float types.
• CDC Connected Device Configuration
• CDC devices fill the gap between PDAs and
  desktops typical CDC devices might include
  Psions, smart phones, and set-top web devices.
• Choice of Virtual machine is up to the hardware
  vendor.
• Many CLDC device vendors use Suns KVM, the
  Kilobyte Virtual Machine.

3
J2ME Java 2 Micro Edition

• Profiles
• J2ME complements configurations with profiles,
  software to match the hardware.
• Each configuration supports several profiles.
• The most common profile in mobile phones is MIDP,
  the Mobile Information Device Protocol, which
  adds networking, user interface, and persistent
  storage support to the CLDC configuration.
• J2SE re-use
• J2ME is designed to re-use J2SE code wherever
  possible, or at least to pretend to.
• Within the scope of features supported by a given
  configuration and set of profiles, wherever
  possible the J2ME implementation must not change
  interfaces or behavior from the J2SE original.

4
J2ME Java 2 Micro Edition

• CLDC hardware requirements
• 128 Kb permanent storage for the J2ME engine and
  classes
• 32 Kb of RAM for applications to run in
• MIDP hardware requirements
• A screen at least 96 pixels wide and 54 pixels
  tall
• Things the CLDC does not require
• Keyboard
• Mouse
• Floating-point support
• Persistent app data storage
• (This is supported by the MIDP profile.)
• Multithreading or multiprocessing
• (This is not required of the hardware its left
  to the JVM.)

5
J2ME Java 2 Micro Edition

• J2SE features missing from the CLDC
• Reflection (java.lang.reflect)
• Weak references (java.lang.ref)
• Object finalization (java.lang.Object has no
  finalize() method)
• J2SE features reduced in the CLDC
• Advanced thread handling
• Advanced error handling
• JNI (the Java Native Interface)
• J2SE behavior changed in the CLDC
• A number of security restrictions
• The class verification step
• (J2SE verifies bytecode on load. J2ME shifts the
  bytecode verification step to the vendors
  compiler.)

6
MIDP Software Development

• MIDP applications are called MIDlets.
• (Because everything in Java is a somethinglet.)
• MIDlets are groups of classes, one of which is
  derrived from the abstract class
  javax.microedition.midlet.MIDlet.
• MIDlets can be packaged in MIDlet suites to group
  one or more MIDlets into the same space.
• MIDlets sharing a suite can access each others
  persistent data and are run concurrently, giving
  an application-level interface to multithreading.
• MIDlets sharing a suite are installed and
  uninstalled together.
• Classes defined in a MIDlet suite are shared
  between all concurrent midlets only one instance
  of each class in the suite is loaded per suite.

7
MIDP Software Development

• MIDlet packaging and installation
• Deploying a MIDlet requires several more steps
  than deploying a conventional application.
• JAR files and JAD files
• The MIDlet suite being deployed is packaged,
  along with all of its supporting classes and data
  files, into a JAR file.
• The JAR also needs to include a manifest which
  will detail the contents of the JAR.
• The JAR must be paired with a JAD (Java
  Application Descriptor) file for installation.
• The .jad is almost identical to the JARs
  manifest.mf where the manifest is to allow
  installed software to be described to the user
  after loading, the .jad is used to describe the
  application to the user before its downloaded as
  part of the installation process.

8
MIDP Software Development

• Sample .jad or manifest.mf
• MIDlet-1 Tetris,, TetrisMIDlet
• MIDlet-Description Tetris MIDlet demo
• MIDlet-Name Tetris
• MIDlet-Vendor Me
• MIDlet-Version 1.0.0
• MicroEdition-Configuration CLDC-1.0
• MicroEdition-Profile MIDP-1.0

9
MIDP Software Development

• Software youll need to install in order to
  develop MIDP applications
• The Java 2 SDK
• I use J2SDK version 1.4.1.01
• The J2ME CLDC
• Suns Wireless Toolkit 1.04
• At least one cell phone emulator
• I use the Nokia 7210 MIDP SDK 1.0 emulator
• Software to deploy to your phone
• I use the Nokia Developer Suite

10
MIDP Software Development

• Compiling and deploying a MIDlet
• 1) Java compiler
• javac.exe -target 1.1 -bootclasspath
  "Wireless toolkit path\lib\midpapi.zip" -d
  preverify .java
• 2) Preverify
• preverify.exe-classpath "Wireless toolkit
  path\lib\midpapi.zip" -cldc -d postverify
  .\preverify
• 3) JAR builder
• jar.exe cvfm Project.jar Project.jad -C
  postverify .
• 4) Deploy and test

11
MIDP Software Deployment

• Deploying MIDlets over the WWW
• After youve tested your MIDlet on your local
  emulator and on a locally-connected cell phone
  (perhaps connected over an IR port) youll want
  to ship it to the world at large.
• Youll need to configure your web server to
  support the JAD MIME type. The JAD MIME type is
• text/vnd.sun.j2me.app-descriptor
• The user can now click on an HTML link such as
• Click lta hrefsample.jadgtherelt/agt to...
• This will download the JAD file. An attribute in
  the JAD file is the MIDlet-Jar-URL if the phone
  user chooses to continue, the phone requests this
  URL from the server.
• The JAR file is then transmitted with MIME type
• application/java-archive

12
Anatomy of a MIDlet

• A MIDlet consists of at least one class
  extending javax.microedition.midlet.MIDlet, plus
  supporting classes.
• You must implement three abstract methods
• startApp()
• pauseApp()
• destroyApp(boolean unconditional)
• In startApp() youll create a user interface
  object--a Canvas or Screen.

13
Anatomy of a MIDlet

• J2ME provides two user interface approaches
• High-level UI Screen
• The Screen class and those derrived from it
  provide support for an assisted layout of
  high-level objects like buttons and text fields.
  This means that while its easier to control what
  goes where and how events are passed and so on,
  its harder to control precisely the contents of
  the screen.
• Derived classes Alert, Form, List and TextBox.
• Low-level UI Canvas
• The Canvas class gives the developer complete
  control over the contents of the screen. While
  this has the upshot of giving you advanced
  graphical abilities and precise screen control,
  it has the downside that you have to do
  everything yourself. Tetris was written on a
  Canvas.

14
Anatomy of a MIDlet

• Hello World, the MIDlet
• import javax.microedition.midlet.
• import javax.microedition.lcdui.
• public class HelloWorldMIDlet extends MIDlet
• public HelloWorldMIDlet()
  
• public void pauseApp()
  
• public void destroyApp(boolean unconditional)
  
• public void startApp()
• if (Display.getDisplay(this).getCurrent()
  null)
• HelloScreen helloScreen new
  HelloScreen(this, "Hello World.")
• Display.getDisplay(this).setCurrent(helloScr
  een)

15
Anatomy of a MIDlet

• Hello World screen class
• import javax.microedition.lcdui.
• class HelloScreen
• extends TextBox
• implements CommandListener
• private final HelloWorldMIDlet m_midlet
• private final Command m_exitCommand
• HelloScreen(HelloWorldMIDlet midlet, String
  string)
• super("HelloWorldMIDlet", string, 256,
  0)
• m_midlet midlet
• m_exitCommand new Command("Exit",
  Command.EXIT, 1)
• addCommand(m_exitCommand)
• setCommandListener(this)

16
Recommended Reading

• J2ME In A Nutshell, by Kim Topley, OReilly
  Associates Inc., 2002
• Dense yet informative details the guts of J2ME
  from below CLDC on up to forms and timers. Also
  has a handy reference to the new J2ME Java
  classes.
• http//java.sun.com
• The resource of all things Java.
• This is where you can download the J2SDK and
  Wireless Toolkit.
• http//forum.nokia.com
• Good site to download phone emulators and SDKs.
• Non-intrusive signup process required