Hexist

I’ve found myself in the position of wanting to develop a portable mobile application. Now, I’m not a java programmer, nor do I use windows (I’m running 64-bit Gentoo Linux), and I prefer to use Vim + Makefiles over using some IDE, so I had a few challenges to overcome in building my first Hello World MIDlet. In the end though, it isn’t too bad to get going, so if you are in a similar situation, here are some quick notes on how I got from zero to MIDlet.

1. First and foremost, you’ll need to have a full blown Java Development Kit (JDK) installed. I’m using Sun’s JDK 1.6.0.13, but I don’t think it is too important. (Both 32 bit and 64 bit versions seem to work fine.)
2. If you are running a 64 bit version of Linux, you’ll need to have a 32 bit java runtime environment (JRE) installed somewhere to run the WTK device emulator. In gentoo, this means doing an ‘emerge emul-linux-x86-java’ (in gentoo, your installation will be in /opt/emul-linux-x86-java-1.X.Y.Z/).
3. Next, you’ll need to download and install the Java Wireless Toolkit, I’m using version 2.5.2 (3.0 is out, but it is only for windows..). It is available here: http://java.sun.com/products/sjwtoolkit/download.html. You should end up with a file named something like “sun_java_wireless_toolkit-2.5.2_01-linuxi486.bin.sh”.  Run this shell script to install the WTK, and again, if you are using 64 bit linux, don’t forget to give the path to your 32 bit version of java when the installer asks for it.
4. Now we can start working on our midlet. Create a file called HelloWorld.java containing the following code

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   import javax.microedition.lcdui.Alert;
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   import javax.microedition.lcdui.Display;
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   import javax.microedition.midlet.MIDlet;
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   import java.util.Date;
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   public class HelloWorld extends MIDlet {
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       Alert timeAlert;
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       public HelloWorld() {
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           timeAlert = new Alert("Hello!");
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           timeAlert.setString("Hello World! The time is now " + new Date().toString());
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       }
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       public void startApp() { 
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           Display.getDisplay(this).setCurrent(timeAlert);
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       }
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       public void pauseApp() { }
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       public void destroyApp(boolean unconditional) { }
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   }

5. Now, create a file called Manifest, containing the following lines
   

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   MIDlet-Name: HelloWorld
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   MIDlet-Vendor: YourName
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   MIDlet-Version: 1.0
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   MIDlet-Icon: /icon.png
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   MIDlet-Info-URL: http://hexist.com/?p=29
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   MicroEdition-Configuration: CLDC-1.0
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   MicroEdition-Profile: MIDP-2.0
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   MIDlet-1: HelloWorld, /icon.png, HelloWorld

6. Save the following image as icon.png
7. Create a Makefile and paste the below into it. Modify the JDK_BASE and J2ME_BASE variables appropriately depending on where you installed your JDK and your J2ME/WTK packages.
   

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   ### Modify these appropriately for your system!
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   JDK_BASE=/opt/sun-jdk-1.6.0.13
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   J2ME_BASE=$(HOME)/WTK2.5.2
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   ### For the tutorial, you shouldn't have to modify anything below this line
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   SRC=HelloWorld.java
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   JAVAC=$(JDK_BASE)/bin/javac
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   JAVA_CFLAGS=-bootclasspath "$(J2ME_BASE)/lib/midpapi20.jar:$(J2ME_BASE)/lib/cldcapi10.jar"  \
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               -target 1.3                                                                     \
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               -source 1.3                                                                     \
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               -d compiled                                                                     \
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               -classpath compiled                                                             \
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               -sourcepath .                                                                   \
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               -g
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   PREVERIFY=$(J2ME_BASE)/bin/preverify
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   PREVERIFY_CLASSPATH=$(J2ME_BASE)/lib/midpapi20.jar:$(J2ME_BASE)/lib/cldcapi10.jar
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   all: HelloWorld.jar HelloWorld.jad
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   run: HelloWorld.jad
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    $(J2ME_BASE)/bin/emulator -Xdescriptor:HelloWorld.jad
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   HelloWorld.jar: verified compiled Manifest $(SRC:.java=.class)
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    jar cfm HelloWorld.jar Manifest icon.png -C verified $(SRC:.java=.class)
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   compiled:
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    mkdir -p compiled
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   verified:
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    mkdir -p verified
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   %.class: %.java
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    $(JAVAC) $(JAVA_CFLAGS) $<
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    $(PREVERIFY) -classpath $(PREVERIFY_CLASSPATH):compiled -d verified HelloWorld
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   %.jad: %.jar
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    unzip -aa -j -p $< "META-INF/MANIFEST.MF" > $@
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    echo "MIDlet-Jar-URL: $<" >> $@
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    echo "MIDlet-Jar-Size: " `stat -c%s $<` >> $@
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   clean:
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    rm -Rf compiled verified
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    rm -f *.jar *.jad
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   .PHONY: clean all run

Now, you should be able to run ‘make’ to build your JAR and JAD files, and ‘make run’ to run the WTK phone emulator. For those that just want to see the raw compile steps for everything that needs to be done to go from .java to a jad n jar, here they are:

mkdir -p verified
mkdir -p compiled
/opt/sun-jdk-1.6.0.13/bin/javac -bootclasspath "$HOME/WTK2.5.2/lib/midpapi20.jar:$HOME/WTK2.5.2/lib/cldcapi10.jar" -target 1.3 -source 1.3 -d compiled -classpath compiled -sourcepath . -g HelloWorld.java
$HOME/WTK2.5.2/bin/preverify -classpath $HOME/WTK2.5.2/lib/midpapi20.jar:$HOME/WTK2.5.2/lib/cldcapi10.jar:compiled -d verified HelloWorld
jar cfm HelloWorld.jar Manifest icon.png -C verified HelloWorld.class
unzip -aa -j -p HelloWorld.jar "META-INF/MANIFEST.MF" > HelloWorld.jad
echo "MIDlet-Jar-URL: HelloWorld.jar" >> HelloWorld.jad
echo "MIDlet-Jar-Size: " `stat -c%s HelloWorld.jar` >> HelloWorld.jad

Then a quick ‘make run’, which just executes

$HOME/WTK2.5.2/bin/emulator -Xdescriptor:HelloWorld.jad

HelloWorld MIDlet

and if everything went well, up will pop our device emulator ready to launch our MIDlet. Click ‘launch’, and you should see a screen that looks something like the image on the right.

To get the files installed on an actual phone, the easiest way (in my opinion) is to simply throw the .jad and .jar files up on a web page somewhere and to browse to the .jad file. Alternatively, most phones have USB hookups or some other way of installing apps without connecting them to the internet, so depending on your phone that might be easier for you.

Tar ball of all these files: HelloWorld.tar.bz2