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 set of Template instances as well as provide a FreemarkerLanguageTemplateHelper instance.

In general, we distinguish between these types of templates:

  • Spec Templates (Global output generated once per driver in total)

  • Complex Type Templates (Generates output for a complex type)

  • Enum Templates (Generates output for enum types)

  • DataIO Templates (Generates output for reading and writing PlcValues, which are our PLC4X form of presenting input and output data to our users)

For each of these, the developer can provide a list of templates, which then can generate multiple files per type (Which is important for languages such as C where for every type we need to generate a Header file (.h) and an Implementation (.c))

What the FreemarkerLanguageOutput then does, is iterate over all types provided by the protocol module, and then iterate 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 type.

Example Java output

package org.apache.plc4x.language.java;

import com.google.googlejavaformat.java.Formatter;
import com.google.googlejavaformat.java.FormatterException;
import freemarker.template.Configuration;
import freemarker.template.Template;
import org.apache.commons.io.FileUtils;
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.TypeDefinition;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.File;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.util.*;

public class JavaLanguageOutput extends FreemarkerLanguageOutput {

    private static final Logger LOGGER = LoggerFactory.getLogger(JavaLanguageOutput.class);

    private final Formatter formatter = new Formatter();

    public String getName() {
        return "Java";

    public Set<String> supportedOptions() {
        return Collections.singleton("package");

    public List<String> supportedOutputFlavors() {
        return Arrays.asList("read-write", "read-only", "passive");

    protected List<Template> getSpecTemplates(Configuration freemarkerConfiguration) {
        return Collections.emptyList();

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

    protected List<Template> getEnumTypeTemplates(Configuration freemarkerConfiguration) throws IOException {
        return Collections.singletonList(

    protected List<Template> getDataIoTemplates(Configuration freemarkerConfiguration) throws IOException {
        return Collections.singletonList(

    protected FreemarkerLanguageTemplateHelper getHelper(TypeDefinition thisType, String protocolName, String flavorName, Map<String, TypeDefinition> types,
                                                         Map<String, String> options) {
        return new JavaLanguageTemplateHelper(thisType, protocolName, flavorName, types, options);

    protected void postProcessTemplateOutput(File outputFile) {
        try {
                    FileUtils.readFileToString(outputFile, StandardCharsets.UTF_8)
        } catch (IOException | FormatterException e) {
            LOGGER.error("Error formatting {}", outputFile, e);

The getName method returns Java, this is what needs to be defined in the plc4x-maven-plugin configuration in the language option in order to select this output format.

supportedOptions tells the plugin which option tags this code-generation output supports. In case of the Java output, this is only the package option, which defines the package name of the generated output.

With supportedOutputFlavors we tell the user, that in general we support the three options: read-write, read-only and passive as valid inputs for the outputFlavor config option of the code-generation plugin.

In this case Java doesn’t require any global files being generated for java, so we simply return an empty collection.

For complex types, we currently use two templates (however this will soon be reduced to one). So for every complex type in a protocol definition, the templates: templates/java/pojo-template.java.ftlh and templates/java/io-template.java.ftlh will be executed.

In case of enum types, only one template is being used.

Same as for data-io.

The next important method is the getHelper method, which returns an object, that is passed to the templates with the name helper. As mentioned before, a lot of operations would be too complex to implement in pure Freemarker code, so with these helpers every language can provide a helper utility for handling the complex operations.

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

${helper.packageName(protocolName, languageName, outputFlavor)?replace(".", "/")}/${type.name}.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
 *   https://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 ${helper.packageName(protocolName, languageName, outputFlavor)};

... imports ...

// Code generated by code-generation. DO NOT EDIT.

public<#if type.isDiscriminatedParentTypeDefinition()> abstract</#if> class ${type.name}<#if type.parentType??> extends ${type.parentType.name}</#if> implements Message {

    ... SNIP ...


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

As when creating more and more outputs for different languages, we have realized, that a lot of the code needed in the Helper utility repeats, we therefore introduced a so-called BaseFreemarkerLanguageTemplateHelper which contains a lot of stuff, that is important when generating new language output.