Apache Freemarker

For the Freemarker language output we are using an unmodified version of Apache Freemarker to generate output.

The boilerplate code for providing a PLC4X language module is located in the org.apache.plc4x.plugins:plc4x-code-generation-language-base-freemarker maven module, inside the FreemarkerLanguageOutput class.

This class configures a Freemarker context and provides standardized attributes inside this:

  • packageName: Java style package name which can be used to create some form of directory structure.

  • typeName: Simple string type name

  • type: ComplexTypeDefinition instance containing all the information for the type that code should be generated for.

  • helper: As some times it is pretty complicated to create all the output in Freemarker, the helper allows to provide code that is used by the template that help with generating output.

A Freemarker-based output module, has to provide a list of Template instances as well as provide a FreemarkerLanguageTemplateHelper instance.

What the FreemarkerLanguageOutput then does, is iterate over all complex types provided by the protocol module, and then iterates over all templates the current language defines.

The only convention used in this utility, is that the first line of output a template generates will be treated as the path relative to the base output directory.

It will automatically create all needed intermediate directories and generate the rest of the input to the file specified by the first line.

If this line is empty, the output is skipped for this instance.

Example Java output

package org.apache.plc4x.language.java;

import freemarker.template.*;
import org.apache.plc4x.plugins.codegenerator.protocol.freemarker.FreemarkerLanguageOutput;
import org.apache.plc4x.plugins.codegenerator.protocol.freemarker.FreemarkerLanguageTemplateHelper;
import org.apache.plc4x.plugins.codegenerator.types.definitions.ComplexTypeDefinition;

import java.io.*;
import java.util.*;

public class JavaLanguageOutput extends FreemarkerLanguageOutput {

    public String getName() {
        return "Java";

    protected List<Template> getTemplates(Configuration freemarkerConfiguration) throws IOException {
        return Arrays.asList(

    protected FreemarkerLanguageTemplateHelper getHelper(Map<String, ComplexTypeDefinition> types) {
        return new JavaLanguageTemplateHelper(types);


As you can see, this output generates up to two files for every complex type.

This is one file, providing the code for the POJO itself. The second one generates the IO-component, which contains all the code for parsing and serializing the corresponding POJO.

In other languages, for example C++ it’s possible to use a third one to generate Header files or for Python perhaps only one in total.

Here an example for a part of a template for generating Java POJOs:

${packageName?replace(".", "/")}/${typeName}.java
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *   http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
package ${packageName};

import org.apache.plc4x.java.utils.SizeAware;

public<#if type.abstract> abstract</#if> class ${typeName}<#if type.parentType??> extends ${type.parentType.name}</#if> implements SizeAware {

    ... SNIP ...


So as you can see, the first line will generate the file-path of the to be generated output. Which is followed