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Running the Emulator from the Command Line

The J2ME emulator included with the J2ME Wireless Toolkit is a command-line tool, which means that it must be run from the command line unless you are using a visual development environment. Even if you are using a visual development environment, it's not a bad idea to learn how to use the command-line emulator because you are likely to become more acquainted with the different emulator properties by running directly from the command line. Keep in mind that most visual J2ME development environments employ the same command-line emulator; they just launch it automatically without you ever having to deal with the command line yourself.

The standard J2ME emulator ships as part of the J2ME Wireless Toolkit. In addition to this emulator, some device vendors make available their own emulators for use in testing MIDlets with their respective devices. Motorola includes such an emulator as part of their Motorola SDK for J2ME. The next two sections show you how to run MIDlets using the emulators in the J2ME Wireless Toolkit and the Motorola SDK for J2ME.

The J2ME Wireless Toolkit

Unlike many command-line Java tools, the J2ME emulator is not an executable native application that you can run directly in your native operating environment. The emulator is actually a Java application, which means that you must use the Java interpreter to run it. Also, keep in mind that J2ME is built on a set of special APIs that must somehow be available to the Java interpreter to successfully emulate a MIDlet. Associating the APIs with the emulator is as simple as altering the CLASSPATH environment variable before invoking the Java interpreter.

The run.bat Script

To help make the process of setting the class path and invoking the emulator a little easier, the J2ME Wireless Toolkit includes a special script file called run.bat that sets the CLASSPATH variable accordingly and launches the emulator in the Java interpreter. This script is located in the apps\example\bin directory within the J2ME Wireless Toolkit installation directory. Figure 3.1 shows the result of executing the run.bat script at the command-line.


The run.bat script can be named differently on platforms other than Windows. It is common for Windows scripts (batch files) to be named with a .bat file extension.

As Figure 3.1 shows, the run.bat script launches the J2ME emulator and displays a splash screen showing the Java logo. Clicking the upper-right button with the mouse removes the splash screen and makes available several MIDlets for testing. In a moment I'll get into the details of these MIDlets and how to interact with them using the emulator's phone interface. For now, I want to take a look inside of the run.bat script to see what's going on. Following is excerpted code for the run.bat script, which shows how the class path is altered and how the Java interpreter is used to invoke the emulator:

cd ..\..\..\

set CLASSPATH=lib\kvem.jar;lib\kenv.zip;lib\lime.jar;apps\example\bin\example.jar

java -Dkvem.home=. com.sun.kvem.midp.Main DefaultGrayPhone -descriptor apps\

cd apps\example\bin

Figure 3.1 The run.bat script launches the J2ME emulator, which shows an image of a phone with a java splash screen.

The directory changes in the script code are necessary to establish the J2ME Wireless Toolkit installation directory as the root directory for emulation. More important is the modified CLASSPATH variable, which is altered to include the paths of relevant J2ME libraries that are required for emulation. The kvem.jar and kenv.zip files contain classes for the emulator KVM (virtual machine), whereas the lime.jar file contains other J2ME support classes. The example.jar file contains the executable code for the various sample MIDlets that are included in the J2ME Wireless Toolkit (more on these later in this section).

With the class path properly set, the remaining step is to launch the emulator using the Java interpreter. The –D option to the interpreter is used to set the kvem.home system property, which is in turn used by the emulator to identify the root directory of the J2ME Wireless Toolkit. This is the only system property that must be set for the emulator, although there are other emulator properties. The actual emulator class file specified to the Java interpreter is com.sun.kvem.midp.Main. The remaining arguments on the command-line are passed along to the emulator. The first emulator argument is the name of the device to emulate, which in this case is DefaultGrayPhone. The next argument must be set to –descriptor, followed by the name of the application descriptor file (JAD file). The JAD file is responsible for conveying to the emulator how the application is organized.

I mentioned that the kvem.home system property is the only required property of the emulator, but that there are several other optional properties. Table 3.2 lists all the properties supported by the J2ME emulator.

Table 3.2 Attributes for the Device Profiles Supported in the J2ME Wireless Toolkit




Default Value





























The http.proxyHost and http.proxyPort properties are used to set the host and port of the HTTP proxy to be used during emulation. The remaining properties are used to turn on and off tracing information for garbage collection, method calls, class loads, and exceptions. All the tracing properties are set to false by default, which means you must explicitly turn them on to view tracing information during emulation.

The Sample MIDlets

Referring to the emulator session that was invoked by the run.bat script file, you probably noticed several MIDlets listed in the phone's display (see Figure 3.2).

All these MIDlets are included as examples in the J2ME Wireless Toolkit, and are available for testing. Following are the sample MIDlets that ship with the standard J2ME Wireless Toolkit:

  • Sokoban—A game where you slide blocks into slots

  • Tickets—A concert ticket bidding system

  • Colors—A demonstration of using colors

  • Stock—A client/server stock ticker

  • Tiles—A tile puzzle game

  • Many Balls—A demonstration of threaded animation

  • Sampler—A demonstration of several graphics features

  • Properties—A demonstration of obtaining information about the device

  • Http Test—A demonstration of establishing an HTTP connection

  • Pong—A pong game

  • Star Cruiser—A game where you navigate a ship through a star field

  • Space Invaders—A space invaders game

  • UIDemo—A demonstration of various GUI controls

All of the MIDlets except for UIDemo are available for testing using the aforementioned run.bat script file. UIDemo is itself a MIDlet suite, and includes its own run.bat script file that is located in the apps\UIDemo\bin directory. Testing the UIDemo MIDlet suite is a great way to become familiarized with the different GUI components that are available for J2ME development. You learn all about these components and how to develop MIDlets using them on Day 8, "Making the Most of MIDlet GUIs."

Running Without a Script

Thus far you've seen how to use a script to invoke the emulator and execute a suite of MIDlets specified in the examples.jad file that is included in the J2ME Wireless Toolkit. Although this is handy for getting started with the emulator quickly, it isn't quite so convenient if you want to run other MIDlets or supply additional properties to the emulator. For this reason, it is important to understand how to invoke the emulator directly from the command line without the help of the standard run.bat script file. Of course, the run.bat script gives you virtually all the information you need to carry out this task. Following is the general form of invoking the emulator:

java -cp ToolkitRoot\lib\kvem.jar;ToolkitRoot\lib\kenv.zip;ToolkitRoot\lib\
-Dkvem.home=ToolkitRoot [-DProperty=Value] com.sun.kvem.midp.Main 
_DeviceName –descriptor DescriptorFile

I realize this looks rather messy, but bear with me because it's really not too bad when you understand what's going on. First of all, let's clarify what the placeholders mean:

  • ToolkitRoot—The root directory of the J2ME Wireless Toolkit (usually \j2mewtk)

  • JARFile—The JAR file containing the MIDlet(s) you are testing

  • Property/Value—A property/value pair for an emulator property (see Table 3.2 for available properties)

  • DeviceName—The name of the device profile to be used by the emulator

  • DescriptorFile—The relative or absolute path of the application descriptor (JAD) file

You might have noticed that this command-line form for invoking the emulator is very similar to the run.bat script you saw earlier, especially when you consider that the class path settings are now being carried out directly on the command line using the –cp option. The real trick is getting the placeholders filled in properly. If you're invoking the emulator from the J2ME root directory, you can just insert a period (.) for ToolkitRoot. Otherwise, you'll need to specify the root directory everywhere ToolkitRoot appears in the command. The JARFile specifies the JAR file containing the MIDlet classes that you are testing. The only property you must specify is kvem.home, which must be set to the J2ME root directory. The DeviceName can be set to any of the following standard device profiles: Pager, MinimumPhone, DefaultGrayPhone, or DefaultColorPhone. Finally, the JAD file for the MIDlet(s) must be specified as the last argument in the command.

To help you cement in your mind the use of the emulator from the command line, here is an example of invoking the emulator from a single command using the pager profile and the J2ME example MIDlets:

java -cp.\lib\kvem.jar;.\lib\kenv.zip;.\lib\lime.jar;.\apps\example\bin\ _example.jar
-Dkvem.home=. com.sun.kvem.midp.Main Pager -descriptor apps\ _example\bin\example.jad

The result of issuing this command is shown in Figure 3.3, which reveals the flexible nature of device profiles and how much variance there can be among different devices.

Figure 3.3 By specifying pager as the device name when executing the emulator, a pager device is used as the basis for testing midlets.

If you are a command-line guru, you might already realize that this approach has a problem. The problem is that some command lines limit commands to 128 characters, which isn't enough for this hefty command. The workaround is to create your own script (batch) file to house the command. In practice, you will probably want to create scripts for your own MIDlets anyway because typing out such long commands can get cumbersome.

Controlling the Device

Because you can't make any calls or receive pages using the virtual devices represented by the J2ME emulator, it shouldn't be too surprising that you can't touch the screen to click the buttons. Instead, you can use the mouse to click any of the buttons displayed as part of a device in the emulator. Additionally, certain keys on your computer keyboard are mapped to device keys. The numeric keypad on the keyboard maps directly to the keypad on a phone device. Similarly, the letter keys and several command keys on the keyboard are all usable on the pager device. Arrow keys on the keyboard apply to all of the standard devices that are available for testing with the emulator.

The Motorola SDK for J2ME

Now that you know how to run the standard emulator included with the J2ME Wireless Toolkit, it's time to turn your attention to the Motorola SDK for J2ME and its very capable emulator. The drill is very similar to the standard J2ME emulator in that there is a script to help make things easy. The Motorola version of run.bat is called runEmul.bat, and its excerpted source code follows:

@if NOT "%1"=="" set MYCLASS=%1
@if   "%1"=="" set MYCLASS=com.mot.j2me.midlets.bounce.Bounce


set J2ME_RESOURCE_DIR=c:\MotoSDK\lib\resources

cd ..\bin

java -Djava.library.path=../lib -classpath ./Emulator.jar;./ConfigTool.jar 
_com.mot.tools.j2me.emulator.Emulator -classpath../demo/midlets;../lib
javax.microedition.midlet.AppManager %MYCLASS% -JSA 1 1

cd ..\scripts

One interesting thing about this code is that it accepts a command-line argument, which is referenced by %1 in the code. The idea is that you can specify the name of a MIDlet class file to runEmul.bat and it will run it using the emulator. The second line of the script shows the default behavior if you don't provide a class file—the Bounce example MIDlet is executed.

Instead of using the CLASSPATH environment variable, the Motorola emulator specifies dependent libraries directly in the command that invokes the interpreter. One environment variable that it does set, however, is J2ME_RESOURCE_DIR, which is the directory where any resources are stored for use with the MIDlets. Instead of using the CLASSPATH environment variable, the path of the standard J2ME libraries is stored in the java.library.path system property. Additionally, several support JAR files are specified using the –classpath option to the Java interpreter.

The emulator class itself is specified as com.mot.tools.j2me.emulator.Emulator. Everything in the command that appears after the emulator class is passed along to the emulator application as arguments. So, the first argument to the emulator is the path to the MIDlet(s) being executed. The second argument is the application manager that is responsible for running the MIDlets. The most significant argument is the %MYCLASS% argument, which contains the name of the actual MIDlet being executed. Finally, the –JSA 1 1 arguments at the end of the command are used internally by the emulator and are specified the same for all MIDlets.

Now that you understand how the runEmul.bat script works, let's take it for a test drive. If you change to the directory containing the script and simply enter runEmul at the command line, the Bounce example MIDlet will be executed, as shown in Figure 3.4.

To execute a different MIDlet, specify the full classname on the command line as the only argument to the runEmul script. Following is the command necessary to run the Scribble MIDlet using the Motorola emulator:

runEmul com.mot.j2me.midlets.scribble.Scribble

Figure 3.4 The runemul.bat script launches the motorola J2ME emulator and executes the bounce sample midlet by default.

The result of this command is shown in Figure 3.5 after I've done some doodling in the Scribble MIDlet.

Figure 3.5 The scribble midlet is executed in the motorola emulator by specifying the full classname as the only argument to runemul.bat.

Changing Devices

Although the default device shown in Figure 3.5 is generally reminiscent of a real mobile phone, it would be much more interesting to see an actual Motorola phone in the emulator. If you recall from earlier in the lesson, the Motorola SDK for J2ME supports several commercial Motorola devices, all of which are mobile phones. You can easily tweak the emulator to use one of these devices with a simple property addition to the command that invokes the emulator in the Java interpreter. The property is called deviceFile, and is used to specify a device file containing a custom device profile. Following is an example of how the main command in runEmul.bat changes to use the Motorola iDEN mobile phone device file:

java -Djava.library.path=../lib -classpath ./Emulator.jar;./ConfigTool.jar 
_com.mot.tools.j2me.emulator.Emulator -classpath../demo/midlets;../lib
-deviceFile resources/MotorolaiDENPlatform.props
javax.microedition.midlet.AppManager %MYCLASS% -JSA 1 1

Notice in the code that the device file is specified as resources/MotorolaiDENPlatform.props. This simple change to the emulator command results in the Motorola iDEN device profile being used instead of the generic phone profile. Keep in mind, however, that you've been using runEmul.bat to run MIDlets using the Motorola emulator, as opposed to invoking it directly with the Java interpreter by hand. Fortunately, the Motorola SDK for J2ME includes additional scripts for running MIDlets with each different device:

  • runEmul.bat—Runs a MIDlet using the generic phone.

  • runMotoiDEN.bat—Runs a MIDlet using the Motorola iDEN phone.

  • runMotoi1000.bat—Runs a MIDlet using the Motorola iDEN i1000 phone.

  • runStarTac.bat—Runs a MIDlet using the Motorola StarTac phone.

  • runMyDevice.bat—Runs a MIDlet using a custom device.

All of these scripts except for the last one are used to run the emulator using existing device profiles with which you probably have some degree of familiarity. The last script, runMyDevice.bat, is used to run the emulator with an entirely custom device. You learn how to create a custom device and use it with the Motorola emulator on Day 6.

Getting back to the device profiles for Motorola devices, following is an example of executing the PaddleBall example MIDlet in the emulator using the runMotoiDEN.bat script:

runMotoiDEN com.mot.j2me.midlets.paddleball.PaddleBall

Figure 3.6 shows the result of this command, which reveals a photo-realistic Motorola iDEN phone in the emulator.

Figure 3.6 The paddleball midlet is executed in the motorola emulator using the motorola iden phone.

The Sample MIDlets

Similar to the J2ME Wireless Toolkit, the Motorola SDK for J2ME includes several sample MIDlets that you can try out using the Motorola emulator. Following are the sample MIDlets that ship with the Motorola toolkit:

  • Bounce—An animation demonstration

  • PaddleBall—A paddle ball game

  • Scribble—A demonstration of drawing with basic graphics

  • FontDemo—A demonstration of using fonts

  • GraphicsDemo—A demonstration of several graphics features

  • RecordStoreDemo—A database demonstration

  • AlertTest—A demonstration of timed and modal alerts

  • ChoiceGroupTest—A demonstration of the choice group GUI component

  • DateFieldTest—A demonstration of the date field GUI component

  • FormTest—A demonstration of forms

  • GaugeTest—A demonstration of the gauge GUI component

  • KeyEventsTest—A demonstration of handling key events

  • TextBoxTest—A demonstration of the text box GUI component

  • TextFieldTest—A demonstration of the text field GUI component

  • TickerTest—A demonstration of the ticker GUI component

  • UDP Tutorial MIDlets—Two MIDlets that demonstrate UDP network communication

All of the MIDlets can be tested using any of the runXXX.bat Motorola script files.

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