The graphical user interface uses the Front Panel mock (see Front Panel Mock) and some extensions (widgets) to simulate the user interactions. It is the equivalent of the three embedded modules (Display, Input and LED) of the MicroEJ Platform (see MicroUI).

The Front Panel 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


This chapter completes the notions described in Front Panel Mock chapter.

Module Dependencies

The Front Panel project is a regular MicroEJ Module project. Its module.ivy file should look like this example:

<ivy-module version="2.0" xmlns:ea="" xmlns:ej="" ej:version="2.0.0">
<info organisation="com.mycompany" module="examplePanel" status="integration" revision="1.0.0"/>

   <configurations defaultconfmapping="default->default;provided->provided">
      <conf name="default" visibility="public" description="Runtime dependencies to other artifacts"/>
      <conf name="provided" visibility="public" description="Compile-time dependencies to APIs provided by the platform"/>

      <dependency org="ej.tool.frontpanel" name="widget" rev="1.0.0"/>

It depends at least on the Front Panel framework. This framework contains the Front Panel core classes. The dependencies can be reduced to:

   <dependency org="ej.tool.frontpanel" name="framework" rev="1.1.0"/>

To be compatible with Display module’s Graphics Engine, the project must depend on an extension of Front Panel framework. This extension provides some interfaces and classes the Front Panel is using to target simulated display and input devices. The extension does not provide any widgets. It is the equivalent of the embedded Low Level API. It fetches by transitivity the Front Panel framework, so the Front Panel framework dependency does not need to be specified explicitly:

   <dependency org="com.microej.pack.ui" name="ui-pack" rev="13.0.0">
      <artifact name="frontpanel" type="jar"/>


This extension is built for each UI pack version. By consequence a Front Panel project is made for a platform built with the same UI pack. When the UI pack mismatch, some errors may occur during the Front Panel project export step, during the platform build and/or during the application runtime.

The Front Panel extension does not provide any widgets. Some compatible widgets are available in a third library. The life cycle of this library is different than the UI pack’s one. New widgets can be added to simulate new kind of displays, input devices, etc. This extension fetches by transitivity the Front Panel extension, so this extension dependency does not need to be specified explicitly:

   <dependency org="ej.tool.frontpanel" name="widget" rev="2.0.0"/>


The minimal version 2.0.0 is required to be compatible with UI pack 13.0.0 and higher. By default, when creating a new Front Panel project, the widget dependency version is 1.0.0.

Widget Display

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 filter (see Widget). This filter defines the pixels inside and outside the real display area. The filter 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 input device widgets (button, joystick, touch, etc.) require a listener to know how to react on input events (press, release, move, etc.). The aim of this listener is to generate an event compatible with MicroUI Event Generator. Thereby, a button press action can become a MicroUI Buttons press event or a Command event or anything else.

A MicroUI Event Generator is known by its name. This name is fixed during the MicroUI static initialization (see Static Initialization). To generate an event to a specific event generator, the widget has to use the event generator name as identifier.

A Front Panel widget can:

  • Force the behavior of an input action: the associated MicroUI Event Generator type is hardcoded (Buttons, Pointer, etc.), the event is hardcoded (for instance: widget button press action may be hardcoded on event generator Buttons and on the event pressed). Only the event generator name (identifier) should be editable by the Front Panel extension project.
  • Propose a default behavior of an input action: contrary to first point, the Front Panel extension project is able to change the default behavior. For instance a joystick can simulate a MicroUI Pointer.
  • Do nothing: the widget requires the Front Panel extension project to give a listener. This listener will receive all widgets action (press, release, etc.) and will have to react on it. The action should be converted on a MicroUI Event Generator event or might be dropped.

This choice of behavior is widget dependant. Please refer to the widget documentation to have more information about the chosen behavior.

Heap Simulation

Graphics Engine is using two dedicated heaps: for the images (see Memory ) and the external fonts (see External Resources). Front Panel partly simulates the heaps usage.

  • Images heap: Front Panel simulates the heap usage when the application is creating a BufferedImage, when it loads and decodes an image (PNG, BMP, etc.) which is not a raw resource and when it converts an image in MicroEJ format in another MicroEJ format. However it does not simulate the external image copy in heap (see External Resource).
  • External fonts heap: Front Panel does not simulate this heap (see External Resources). There is no rendering limitation when application is using a font which is located outside CPU addresses ranges.

Image Decoders

Front Panel uses its own internal image decoders when the associated modules have been selected (see internal image decoders). Front Panel can add some additional decoders like the C-side for the embedded platform (see external image 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 or 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. See Installation to install the module.

The properties file can additional properties:

  • hardwareImageDecoders.list [optional, default value is “” (empty)]: Defines the available list of additional image decoders provided by the hardware (see Image Decoders). 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.