1. Make a Java class and package it into a JAR:

IzPack can execute Java classes provided by third party developers.  

This can be used to extend IzPack's functionality in a number of ways

Some examples:

• to control the writing of configuration files during installation or
• creating a custom log.  

IzPack can display output from these classes via a ProcessPanel, or you could create a custom (possibly hidden) panel that calls your Java code.  

This following example shows how to use a custom Java class with a ProcessPanel.

An external Java class must have a run() method with some parameters.

Note: This is not (NOT!) the run() method from the java.lang.Runnable interface. Nor is this codified as an interface in the IzPack javadoc.  Instead, this would seem to be a magic method signature that is otherwise undocumented.

a. Contents of a minimal file that matches the requirements:

package org.callimachusproject;
import com.izforge.izpack.panels.process.AbstractUIProcessHandler;

public class HelloWorld {
  public void run(AbstractUIProcessHandler handler, String[] args) {
    handler.logOutput("Hello, World!", false);
  }
}

This class uses the com.izforge.izpack.panels.process.AbstractUIProcessHandler class to log output into a calling ProcessPanel. See the IzPack javadoc for details.

b.  Compile your Java class

You will obviously need to include the standalone-compiler.jar on your classpath, in order to compile your class:

$ javac -cp "/path/to/izpack-standalone-compiler.jar:." org/callimachusproject/HelloWorld.java

If you are using maven, include the dependency izpack-panel to get the AbstractUIProcessHandler class.

c. Package your Java class into a separate JAR

IzPack can only reference JAR files not individual class files from a ProcessPanel.

$ jar cvf hello.jar org/callimachusproject/HelloWorld.class
added manifest
adding: org/callimachusproject/HelloWorld.class(in = 470) (out= 316)(deflated 32%)
$ cp hello.jar /path/to/installer/

NB: You should eventually use Maven to automate the compilation, packaging and moving of your JAR file.

2. Set up the install.xml file

a. Add <resources> entry.

In the <resources> section of your install.xml, reference an external file called "ProcessPanel.Spec.xml".

The ProcessPanel.Spec.xml file holds the XML configuration for the external Java class you want to execute.

<installation>
   ...

  <resources>
    <res id="ProcessPanel.Spec.xml" src="installer/ProcessPanel.Spec.xml"/>
    ...
  </resources>

  ...
</installation>

b.  Reference your JAR

At the top level of the install.xml file, add a reference to your JAR file containing the class.

This is partially documented at:
http://izpack.org/documentation/installation-files.html#the-jar-merging-element-jar

NB: The path to the JAR is the path at *compile time*.

<installation>

  <resources>
    <res id="ProcessPanel.Spec.xml" src="installer/ProcessPanel.Spec.xml"/>
  </resources>

  ...

  <jar src="path/to/hello.jar" stage="install"/>

  ...

</installation>

Adding the jar element inear the <resources> definition is a good idea.

c.  Use a ProcessPanel to execute your Java class

In the install.xml file in the panels section, use a ProcessPanel to execute your Java class.

<installation>

  <resources>
    <res id="ProcessPanel.Spec.xml" src="installer/ProcessPanel.Spec.xml"/>
  </resources>

  ...

  <jar src="path/to/hello.jar" stage="install"/>

  ...

  ...
  <panels>
    ...
    <panel classname="ProcessPanel"/>
  </panels>
  ....
</installation>

3. Create the ProcessPanel.Spec.xml file

This is partially documented in  https://izpack.atlassian.net/wiki/display/IZPACK/Process+Panel  

<processing>
  <logfiledir>$INSTALL_PATH</logfiledir>
  <job name="setup">
    <executeclass name="org.callimachusproject.HelloWorld">
		<arg>${someVariable}</arg>
	</executeclass>
  </job>
  <onFail previous="true" next="false" />
  <onSuccess previous="false" next="true" />
</processing>

IzPack javadoc

The IzPack javadoc is available in the distribution (although apparently not online). There is an RPM for Linux users.