A major strength of the MicroEJ environment is that it allows applications to be developed and tested in a Simulator rather than on the target device, which might not yet be built. To make this possible for devices that have a display or controls operated by the user (such as a touch screen or buttons), the Simulator must connect to a “mock” of the control panel (the “Front Panel”) of the device. This mock is called the mockFP. The mockFP generates a graphical representation of the required front panel, and is displayed in a window on the user’s development machine when the application is executed in the Simulator. The mockFP is the equivalent of the three embedded modules (Display, Inputs and LED) of the MicroEJ Platform (see MicroUI).

The Front Panel mock enhances the development environment by allowing User Interface  applications to be designed and tested on the computer rather than on the target device (which may not yet be built). The mock interacts with the user’s computer in two ways:

  • output: LEDs, graphical displays
  • input: buttons, joystick, touch, haptic sensors

Functional Description

  1. Creates a new Front Panel project.
  2. Creates an image of the required front panel. This could be a photograph or a drawing.
  3. Defines the contents and layout of the front panel by editing an XML file (called an fp file). Full details about the structure and contents of fp files can be found in chapter Front Panel.
  4. Creates images to animate the operation of the controls (for example button down image).
  5. Creates Listeners that generate the same MicroUI input events as the hardware.
  6. Creates a Display Extension that configures the simulated display to match the real display.
  7. Previews the front panel to check the layout of controls and the events they create, etc.
  8. Exports the Front Panel project into a MicroEJ Platform project.

The Front Panel Project

Creating a Front Panel Project

A Front Panel project is created using the New Front Panel Project wizard. Select:

New > Project... > MicroEJ > Front Panel Project

The wizard will appear:

New Front Panel Project Wizard

New Front Panel Project Wizard

Enter the name for the new project.

Project Contents

Project Contents

Project Contents

A Front Panel project has the following structure and contents:

  • The src folder is provided for the definition of Listeners and DisplayExtensions. It is initially empty. The creation of Listeners and DisplayExtensions will be explained later.
  • The JRE System Library is referenced, because a Front Panel project needs to support the writing of Java for the Listeners and DisplayExtensions.
  • The mockFPWidgets.jar contains the code for the front panel simulation, the widgets it supports and the types needed to implement Listeners and DisplayExtensions.
  • The definitions folder holds the file or files that define the contents and layout of the front panel, with a .fp extension (the fp file or files), plus some supporting files. A newly created project will have a single fp file with the same name as the project, as shown above. The contents of fp files are detailed later in this document.
  • The widgets.desc file contains descriptions of the widgets supplied with the Front Panel Designer. It is used by the Front Panel Designer tool and must not be edited.
  • The resources folder holds images used to create the mockFP. It is initially empty.

FP File

File Contents

The mock engine takes an XML file (the fp file) as input. It describes the panel using mock-widgets: They simulate the drivers, sensors and actuators of the real device. The mock engine generates the graphical representation of the real device, and is displayed in a window on the user’s development machine when the application is executed in the Simulator.

The following example file describes a typical board with one LCD, a touch panel, three buttons, a joystick and four LEDs:

<?xml version="1.0"?>
    xsi:schemaLocation=" .fp1.0.xsd">

    <description file="widgets.desc"/>

    <device name="MyBoard" skin="myboard.png">
            <pixelatedDisplay id="0" x="162" y="114" width="320" height="240" initialColor="0x000000"/>
            <pointer id="0" x="162" y="114" width="320" height="240" touch="true" listenerClass="com.is2t.microej.fp.PointerListenerImpl"/>

            <led2states id="0" x="277" y="374" ledOff="led0_0.png" ledOn="led0_1.png" overlay="false"/>
            <led2states id="1" x="265" y="374" ledOff="led1_0.png" ledOn="led1_1.png" overlay="false"/>
            <led2states id="2" x="254" y="374" ledOff="led2_0.png" ledOn="led2_1.png" overlay="false"/>
            <led2states id="3" x="242" y="372" ledOff="led3_0.png" ledOn="led3_1.png" overlay="false"/>

            <repeatPush id="0" x="250" y="395" skin="Button1_0.png" pushedSkin="Button1_1.png" repeatPeriod="250" listenerClass="com.is2t.microej.fp.ButtonListener"/>
            <repeatPush id="1" x="322" y="395" skin="Button1_0.png" pushedSkin="Button1_1.png" repeatPeriod="250" listenerClass="com.is2t.microej.fp.ButtonListener"/>
            <repeatPush id="2" x="456" y="395" skin="Button1_0.png" pushedSkin="Button1_1.png" repeatPeriod="250" listenerClass="com.is2t.microej.fp.ButtonListener"/>
            <joystick id="0" x="368" y="375" skin="Joy0.png" mask="JoyMask.png" enterSkin="Joy1.png" upSkin="Joy_UP.png" downSkin="Joy_DOWN.png" leftSkin="Joy_LEFT.png" rightSkin="Joy_RIGHT.png" listenerClass="com.is2t.microej.fp.JoystickListenerImpl"/>

The description element must appear exactly as shown. It refers to the widgets.desc file mentioned above.

The device skin must refer to a png file in the resources folder. This image is used to render the background of the front panel. The widgets are drawn on top of this background.

The body element contains the elements that define the widgets that make up the front panel. The name of the widget element defines the type of widget. The set of valid types is determined by the Front Panel Designer. Every widget element defines an id, which must be unique for widgets of this type, and the x and y coordinates of the position of the widget within the front panel (0,0 is top left). There may be other attributes depending on the type of the widget.

The file and tags specifications are available in chapter Front Panel.

Working with fp Files

To edit an fp file, open it using the Eclipse XML editor (right-click on the fp file, select Open With > XML Editor). This editor features syntax highlighting and checking, and content-assist based on the schema (XSD file) referenced in the fp file. This schema is a hidden file within the project’s definitions folder. An incremental builder checks the contents of the fp file each time it is saved and highlights problems in the Eclipse Problems view, and with markers on the fp file itself.

A preview of the front panel can be obtained by opening the Front Panel Preview (Window > Show View > Other... > MicroEJ > Front Panel Preview).

The preview updates each time the fp file is saved.

A typical working layout is shown below.

Working Layout Example

Working Layout Example

Within the XML editor, content-assist is obtained by pressing ctrl+space.  The editor will list all the elements valid at the cursor position, and insert a template for the selected element.

Skins and Filters

The widgets which simulate the input devices use images (or “skins”) to show their current states (pressed and released). The user can change the state of the widget by clicking anywhere on the skin: it is the active area. This active area is, by default, rectangular.

These skins can be associated with an additional image called a filter or mask. This image defines the widget’s active area. It is useful when the widget is not rectangular.

Active Area

Active Area

The filter image must have the same size as the skin image. The active area is delimited by the color 0xFF00FF (pink). Every pixel in the filter image which is not this color is considered not part of the active area.

Display Mask

By default, a display area is rectangular. Some displays can have another appearance (for instance: circular). The front panel is able to simulate that using a mask. This mask defines the pixels inside and outside the real display area. The mask image must have the same size than display rectangular area. A display pixel at a given position will be not rendered if the pixel at the same position in mask is fully transparent.

Inputs Extensions

The mock engine also requires several extensions to know how to react to input events. The extension names (package and classname) are specified in the fp file.

Widgets and Listeners

For widgets that generate events, a Listener class must be specified within the .fp file.

As an example, consider this snippet of an .fp file for defining a push button:

.fp File - Push Example
<push id="0" x="54" y="117"
    listenerClass="com.is2t.MyPushListener" />

The value of the listenerClass attribute is the fully qualified name of a class which has to implement the com.is2t.microej.frontpanel.input.listener.PushButtonListener interface. This class,  com.is2t.MyPushListener, is written by the developer, and implements the PushButtonListener interface.

The package com.is2t.microej.frontpanel.input.listener provides Listeners required by other widgets too:

  • push requires PushButtonListener
  • repeatPush requires RepeatButtonListener
  • joystick requires JoystickListener
  • pointer requires PointerListener

A listener implementation can target several widgets. Each action method receives the ID of the widget that has changed as a parameter.

Event Generator

The Listener needs to be written to generate the same events that the hardware will. In order to send an event, the implementation of a Listener needs to use the EventGenerator class provided. For example, a PushButtonListener can generate button events by calling static methods sendButtons[...]Event.


A Listener can generate events using any methods of the provided EventGenerator. In some cases, this may provide useful functionality.

Each EventGenerator method requires a unique ID of the MicroUI event generator it targets. This unique ID is available in the interface com.is2t.microej.microui.Constants which has been generated by the Static MicroUI Initializer tool.

The EventGenerator class targets six MicroUI event generators:

  • EventGenerator: sendEvent, sendEvents
  • CommandEventGenerator: sendCommandEvent
  • ButtonsEventGenerator: sendButtons[...]Event
  • PointerEventGenerator: sendPointer[...]Event
  • TouchEventGenerator: sendTouch[...]Event
  • StatesEventGenerator: sendState[...]Event

Image Decoders

Front Panel uses its own internal image decoders when the internal image decoders related modules have been selected (see Internal Decoders).

Front Panel can add some additional decoders like the C-side for the embedded platform (see External Decoders). However, the exhaustive list of additional decoders is limited (Front Panel is using the Java AWT ImageIO API). To add an additional decoder, specify the property hardwareImageDecoders.list in front panel configuration properties file (see Installation) with one or several property values:

Front Panel Additional Image Decoders
Type Property value
Graphics Interchange Format (GIF) gif
Joint Photographic Experts Group (JPEG) jpeg | jpg
Portable Network Graphics (PNG) png
Windows bitmap (BMP) bmp

The decoders list is comma (,) separated. Example:



  • MicroUI module (see MicroUI).
  • Display module (see Display): This module gives the characteristics of the graphical display that are useful for configuring the Front Panel.


Front Panel is an additional module for MicroUI library. When the MicroUI module is installed, install this module in order to be able to simulate UI drawings on the Simulator.

In the platform configuration file, check UI > Front Panel to install the Front Panel module. When checked, the properties file frontpanel > is required during platform creation to configure the module. This configuration step is used to identify and configure the front panel.

The properties file must / can contain the following properties:

  • [mandatory]: Defines the name of the front panel project (same workspace as the platform configuration project). If the project name does not exist, a new project will be created.
  • [optional, default value is “” (empty)]: Defines the front panel file (*.fp) to export (in case “” contains several fp files). If empty or unspecified, the first “.fp” file found will be exported.
  • hardwareImageDecoders.list [optional, default value is “” (empty)]: Defines the available list of additional image decoders provided by the hardware. Use comma (‘,’) to specify several decoders among this list: bmp, jpg, jpeg, gif, png. If empty or unspecified, no image decoder is added.


Launch a MicroUI application on the Simulator to run the Front Panel.