This topic explains how to add a custom listener that enables a virtual asset to receive and respond to messages over a transport that is not supported by default. Sections include:

About Custom Listeners

Each virtual asset’s configuration panel contains a Transports> Custom tab, which can be customized to display a message listener implementation to meet your group’s specific needs. 

Once the custom message listener implementation has been plugged in to Virtualize, the virtualization framework can correlate and generate response messages. The actual delivery and reception of messages over the necessary protocol is handled by the message listener implementation. 

Note that Parasoft Marketplace (accessible at or your organization’s CTP) provides examples for TCP/IP, FTP, and other fixed-length message types such as Equifax.

Interfaces to Implement for Custom Listeners

After setting up your environment as described in General Procedure of Adding an Extension, implement the following interfaces (described in the Extensibility API documentation):

ICustomMessageListener Implementation

This is a required class. The isReady() method should return true if the configuration has been properly configured, and false if the required settings have not been set. The startup() method is invoked when the listener should start listening. It takes:


This interface needs to be implemented so that an instance of it can be accepted by the IMessageHandler.handleMessage() implementation. Alternatively, if your message content format is best represented with a string, you can use the default implementation DefaultCustomMessage<T>. The parameterized type can be used for the property (or header) object types keyed with a string.

Defining parasoft-extension.xml for a Custom Listener

After you have implemented the necessary classes, define parasoft-extension.xml (introduced in General Procedure of Adding an Extension) as follows:


<?xml version="1.0" encoding="UTF-8"?>
<extension xmlns="urn:com/parasoft/extensibility-framework/extension"
       name='The name of your message listener, appears in the transports menu' 
       description='A more detailed description'>
  <class>com.mycompany.MyMessageListener</class> <!-- implements ICustomMessageListener-->
  <form xmlns="urn:com/parasoft/extensibility-framework/gui">
       <!-- This describes the fields you wish to appear in your transport GUI --> 
       <section label="field group 1">
          <field id="key 1" label="field 1"/>
          <field id="key 2" label="field 2"/> 
          <section label="field group 2">
          <field id="key 3" label="field 3" />
      <section label="field group 2">
          <field label="field 1" />


The field ids under the section elements are used to retrieve values from the ICustomMessageListenerConfiguration instance that is passed in to the various API methods, and that allows for the values provided by the user to be passed in to your implementation. They are also used to persist the user-provided values to the responder deployment descriptor file when it is saved. The field labels appear in the GUI that is constructed based on this XML. The section layout does not have a programmatic impact; it merely helps organize the various GUI fields into sections or categories for easy access and usability by the end user.

Verifying the New Listener

After building the project as described in General Procedure of Adding an Extension, restart Virtualize and verify that the custom listener name (as specified in parasoft-extension.xml) appears in the Transports> Custom tab of the Virtual Asset configuration panel (opened by double-clicking a virtual asset’s Virtualize Server node).

If only one custom listener is available, this tab will be dedicated to configuration for that custom format:



If multiple custom listeners are available, you can select the one you want to use from the Select Implementation box:



A few custom message listener implementations are available on the Parasoft Marketplace (accessible at or from your organization’s CTP).  See the documentation provided in those zip files for details on how to install and use these extensions.