CloverDX Transformation Language: How to Extend CTL with Java Functions


In this article, I will walk you through the process of extending CTL (CloverDX Transformation Language) with your own custom functions implemented in Java. Think of this as adding your own Java functions to the standard CTL function library so that you can transparently use them wherever there’s a piece of CTL code—in Reformat transformations, Filter expressions, etc.

Why Java for Custom Functions (And Not CTL)?

Imagine you have some custom complex logic already implemented in Java packages, maybe some 3rd party library with potentially dozens of outside dependencies. Let’s say you want to share that functionality across applications, CloverDX included. There might be thousands of lines of code that you don’t want to be re-implementing in CTL.

Other reasons for using Java over pure CTL are performance and the expressive power of Java. While performance begins to matter only with large volumes of data, CTL has never been designed to fully replace general purpose languages. CTL makes working with data records easy, however for complex algorithms it lacks the expressiveness and eco-system of libraries that Java can give you.

Compare to Function Libraries in Plain CTL

In this article, we’re talking about native Java code working as CTL functions. That’s quite different from simply writing a shared function library in plain CTL. To do that, save a piece CTL code as a .ctl file somewhere in your project and then import it into other transformations using import construct like this:


import "trans/filterFunctions.ctl";

function integer transform() {
    $out.0.field1 = filterChars($in.0.field1); // filterChars() is declared in filterFunctions.ctl

    return ALL;

From then, the code and functions declared in the .ctl file will be available in the scope the library was imported in.

Example Scenario

As an example, we’ll be implementing two functions:

  • string getWords()
  • string getWords(integer count)

These functions will produce “Lorem Ipsum”—Latin-like bogus text for testing. For this purpose, we’ll use an external loremipsum Java library by Sven Jacobs (see, we already have external dependencies!).

Architecture & Extensibility of CloverDX

CloverDX is an open platform that can be extended by users in many ways. The core of CloverDX lies in its modular “engine”. This is the execution layer that takes care of running your data transformations and that sits at the center of CloverDX Designer (Runtime) and Server. CTL and all user extensions are treated as “engine plugins”. For our purpose we’ll need to extend the engine with our own plugin (Java package) containing our Lorem Ipsum functions.

CloverDX Designer presents another layer, working rather independently from the engine/Runtime. Designer is built on Eclipse and it comes with its own set of plugins that form the GUI. For Designer to recognize our new functions we’ll need to make it aware of our engine plugin by wrapping it with a separate “Designer plugin”. This will make the new functions available for context assist, correct syntax highlighting in editors, etc.

Brief Overview

Here’s what we’ll do:

  1. We’ll need our CloverDX Engine plugin first:
    • We’ll implement the desired CTL functions as a Java class that will extend org.jetel.ctl.extensions.TLFunctionLibrary.
    • Then, we’ll create a plugin.xml descriptor with ctlfunction extension definition.
    • And we’ll use org.jetel.ctl.extensions.TLFunctionAnnotation annotation to mark “entry point” methods that will serve as the implementation of our CTL functions (as you may have some additional supporting private methods that you don’t want to turn into CTL functions).
  2. Secondly, we’ll add a Designer plugin:
    • For our functions to work in Designer, we’ll need another plugin.xml descriptor—this time with engineplugin extension definition.
    • Finally, we’ll import the engine plugin into this new Designer plugin.

Once finished, the new functions will be available in Designer dialogs (e.g. Source tab in Transform Editor, Reformat code, etc.).

So, let’s get to it!

Step-by-step Guide to Custom CTL Functions

Initial Setup

  1. Download the external Lorem Ipsum Java library (
  2. Download, install and run CloverDX Designer (
  3. Install Eclipse Plug-in Development Environment (PDE):
    • Help ⇒ Install New Software…
    • Select your Eclipse release update site named after the codename of your version
      (e.g. “Indigo –”)
  4. Search for General Purpose Tools / Eclipse Plug-in Development Environment.
  5. Once installed, switch to Java perspective (upper-right corner).

Creating the Engine Plugin

  1. Start a new Java project (File ⇒ New ⇒ Other ⇒ Java/Java Project) and name it com.cloveretl.custom.loremipsum. Hit Next when done.
    Extending CTL with Java Functions
  2. Add CloverDX Engine to the project build path:
    • Switch to the Libraries tab, click Add Library ⇒ CloverDX Engine.
    • Click Next, then Finish.
    Extending CTL with Java Functions
  3. Now that you’ve created the Java project, create a new folder named lib in the project root directory (right-click the project in Package Explorer or in Navigator to create that folder)
  4. Put the “Lorem Ipsum” library into it (lib/loremipsum-1.0.jar)
    Extending CTL with Java Functions
  5. Add the library to Java classpath (Project ⇒ Properties ⇒ Java Build Path section ⇒ Libraries tab ⇒ Add JAR… ⇒ Navigate to the lib directory ⇒ Choose loremipsum-1.0.jar) and click OK.
    Extending CTL with Java Functions
  6. Create the main Java class. We’ll call it LoremIpsumCTLLibrary and it will extend org.jetel.ctl.extensions.TLFunctionLibrary.

    Let’s put the class into a package called com.cloveretl.customctl.

    TLFunctionLibrary comes from the CloverDX Library we added to the project earlier. However, you don’t need to worry about this for now, it is already a part of the code below).

    Extending CTL with Java Functions
    You don't need to set anything else here—you will paste the full source code of this class in the next step anyway (will work only if you kept the same name and package, otherwise you’ll have to do a bit of editing).
  7. Copy & paste the following code. You can modify this later to suit your needs but for now this one class implements everything we’ll need for our two getWords() CTL functions.
    package com.cloveretl.customctl;
    import org.jetel.ctl.Stack;
    import org.jetel.ctl.extensions.TLFunctionAnnotation;
    import org.jetel.ctl.extensions.TLFunctionCallContext;
    import org.jetel.ctl.extensions.TLFunctionLibrary;
    import org.jetel.ctl.extensions.TLFunctionPrototype;
    import de.svenjacobs.loremipsum.LoremIpsum;
    public class LoremIpsumCTLLibrary extends TLFunctionLibrary {
        LoremIpsum loremIpsum = new LoremIpsum();
        public TLFunctionPrototype getExecutable(String functionName)
                throws IllegalArgumentException {
            TLFunctionPrototype ret =
                    "getWords".equals(functionName) ? new GetWordsFunction() : null;
            return ret;
        public String getName() {
            return "LoremIpsumCTLLibrary";
        @TLFunctionAnnotation("Generates Lorem Ipsum text")
        public String getWords(TLFunctionCallContext context) {
            return this.loremIpsum.getWords();
        @TLFunctionAnnotation("Generates Lorem Ipsum text")
        public String getWords(TLFunctionCallContext context, Integer count) {
            if(count==null) {
                return this.getWords(context);
            return this.loremIpsum.getWords(count);
        class GetWordsFunction implements TLFunctionPrototype {
            public void execute(Stack stack, TLFunctionCallContext context) {
                if (context.getParams().length > 0) {
                    stack.push(getWords(context, stack.popInt()));
            public void init(TLFunctionCallContext context) { }
  8. As the last step of this “engine plugin” part, let’s create the plugin.xml descriptor that the engine needs to properly load our code.
  9. Create a new file called plugin.xml in the root of the engine plugin project:
    Extending CTL with Java Functions
  10. Put the following code into plugin.xml:
            <library path="bin/"/>
            <library path="lib/loremipsum-1.0.jar"/>
        <extension point-id="ctlfunction">
            <parameter id="libraryName" value="LoremIpsumCTLLibrary"/>
            <parameter id="className"
  11. Your engine plugin is now done!

Alright, you’re half way through! Now let’s get on to setting up the Designer plugin.

Moving on To the Designer Plugin

  1. Staying in the same workspace, create a new Plug-in project (not Java!) using File ⇒ New ⇒ Plug-in Development ⇒ Plug-in Project.
    Extending CTL with Java Functions
  2. Name the project com.cloveretl.designer.loremipsum.
  3. Click Next
    Extending CTL with Java Functions
  4. On the second page of the project wizard:
    • Uncheck Generate an activator, a Java class that controls the plug-in's lifecycle.
    • Uncheck This plug-in will make contributions to the UI (just ignore what it says for now).
    Extending CTL with Java Functions
  5. After clicking Finish, choose No when Designer asks to switch to a Plugin perspective.
    Extending CTL with Java Functions
  6. Now we have our Designer plugin project ready. Let’s put some classes in.
  7. Create a “Class Loader Provider” Class
    • File ⇒ New ⇒ Class
    • Class name: CLProvider
    • Package is automatically prefilled to com.cloveretl.designer.loremipsum (you can leave that)
    • Superclass: com.cloveretl.gui.plugin.engine.AbstractClassLoaderProvider
    Extending CTL with Java Functions
  8. Click Finish.
    Don’t worry that the AbstractClassLoaderProvider cannot be resolved in the code yet, we’ll fix it later.
  9. Now double click META-INF/MANIFEST.MF (in the Explorer on the left).
    This will get you to Overview tab of the Plugin screen.
    Extending CTL with Java Functions
  10. Switch to the Dependencies tab (bottom tabs)
  11. Add com.cloveretl.gui plugin to the list of Required Plug-ins
    Extending CTL with Java Functions
  12. Switch to the Extensions tab
  13. Add com.cloveretl.gui.engine to Plugin extension
  14. On the same screen, set the following:
    • Directory: plugins
    • classLoaderProvider: com.cloveretl.designer.loremipsum.CLProvider
  15. Save all your changes (Ctrl-S) and review the previously created class ( The missing dependencies should be resolved now.
    Extending CTL with Java Functions
  16. Go back to the Designer plugin ("com.cloveretl.designer.loremipsum" in main tabs).
  17. Switch to plugin.xml tab (bottom tabs).
  18. Add the following lines to the enginePlugin section:
  19. Now we’ll import our engine plugin we created earlier into this Designer plugin.
  20. Create a new directory called plugins under the com.cloveretl.designer.loremipsum project folder. As best practice, put your engine plugins into folders named after the plugin IDs.
  21. Create a sub-directory called com.cloveretl.custom.loremipsum (see image below):
    Extending CTL with Java Functions
  22. Right-click plugins/com.cloveretl.custom.loremipsum folder and select Import... ⇒ General ⇒ File system.
    • Set From directory to com.cloveretl.custom.loremipsum project location (the engine plugin project).
    • Check plugin.xml (in the right panel).
    • Check bin.
    • Check lib.
    Extending CTL with Java Functions
  23. Click Finish and verify that files have been copied to the desired folder:
    Extending CTL with Java Functions
  24. Open plugin.xml in the com.cloveretl.designer.loremipsum project—the one in the main directory, not the one that we have just imported to plugins folder!.
    • Switch to Build tab (bottom tabs).
    • Make sure the following items are checked in the Binary Build section:
      • META-INF
      • plugin.xml
      • plugins
    Extending CTL with Java Functions
  25. Switch to Runtime tab and add the following folders to the classpath:
    • . (the Designer plugin folder)
    • plugins/com.cloveretl.custom.loremipsum/bin/
    • plugins/com.cloveretl.custom.loremipsum/lib/loremipsum-1.0.jar
    Extending CTL with Java Functions
  26. You are very close to victory now!
  27. Right click com.cloveretl.designer.loremipsum (the Designer plugin) project and select “Export…” from the context menu.
    • Select Plug-in development ⇒ Deployable plug-ins and fragments.
    • On the Destination tab select Install into host. Repository: (no need to change anything else)
    Extending CTL with Java Functions
  28. Click Finish.
  29. Congratulations! Now all your projects will be able to use your magnificent new CTL functions!

Now you can try your new functions anywhere in CloverDX, let's see it in Reformat's Source tab:

Extending CTL with Java Functions
Posted on September 01, 2017
Try CloverDX for 45-days  Full access to Tech Support as if you were a customer

Where to go next