Introduction

Transitioning from javax to jakarta namespaces is pivotal in the Java EE ecosystem, especially when maintaining or lacking control over javax libraries. The transformer-maven-plugin facilitates this migration. In this article, we'll generate a javax library from an XSD file, build a jar, and then create its jakarta version within a Maven project comprising two modules: one for javax, the other for jakarta.

Prerequisites

In this article, we'll be utilizing Java and Apache Maven. It's assumed that you have basic familiarity with these technologies. Additionally, we will employ the transformer-maven-plugin to aid in the migration process. Now, with the basics covered, let's proceed to set up our Maven project for migrating a javax library to a jakarta one.

Creating the Maven Project and Javax Module

1. Create a New Maven Project

    mvn archetype:generate -DgroupId=com.example -DartifactId=javax-to-jakarta -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false
   

2. Convert to a Multi-module Project

   Edit the pom.xml in the javax-to-jakarta directory to include the pom packaging type and Java 8 source/target:

    <packaging>pom</packaging>
    <properties>
       <maven.compiler.source>8</maven.compiler.source>
       <maven.compiler.target>8</maven.compiler.target>
    </properties>
   

3. Create a Module for Javax

    mvn archetype:generate -DgroupId=com.example -DartifactId=javax-module -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false
   

4. Generate Javax Library from XSD

   Create a directory src/main/xsd within javax-module, and place the following person.xsd file in it:

    <?xml version="1.0" encoding="UTF-8" ?>
    <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
      <xs:element name="Person" type="PersonType"/>
      <xs:complexType name="PersonType">
        <xs:sequence>
          <xs:element name="FirstName" type="xs:string"/>
          <xs:element name="LastName" type="xs:string"/>
          <xs:element name="Age" type="xs:integer"/>
        </xs:sequence>
      </xs:complexType>
    </xs:schema>
   

   Update javax-module/pom.xml to include the jaxb2-maven-plugin and javax dependencies:

    <dependencies>
        <dependency>
            <groupId>javax.xml.bind</groupId>
            <artifactId>jaxb-api</artifactId>
            <version>2.3.1</version>
        </dependency>
    </dependencies>
    <build>
      <plugins>
        <plugin>
          <groupId>org.codehaus.mojo</groupId>
          <artifactId>jaxb2-maven-plugin</artifactId>
          <version>2.5.0</version>
          <executions>
            <execution>
              <goals>
                <goal>xjc</goal>
              </goals>
            </execution>
          </executions>
        </plugin>
      </plugins>
    </build>
   

5. Build and Package the Javax Library

    mvn package
   

Setting up the Jakarta Module

1. Create a Module for Jakarta

    mvn archetype:generate -DgroupId=com.example -DartifactId=jakarta-module -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false
   

2. Update Parent POM

   Add jakarta-module to the parent pom.xml:

    <modules>
        <module>javax-module</module>
        <module>jakarta-module</module>
    </modules>
   

3. Configure Transformer-Maven-Plugin in Jakarta Module

   Navigate to the jakarta-module directory and edit its pom.xml to include the transformer-maven-plugin and specify the dependency on the javax module:

    <dependencies>
        <dependency>
            <groupId>jakarta.xml.bind</groupId>
            <artifactId>jakarta.xml.bind-api</artifactId>
            <version>3.0.1</version>
        </dependency>
    </dependencies>
    <build>
            <plugins>
                <plugin>
                    <groupId>org.eclipse.transformer</groupId>
                    <artifactId>transformer-maven-plugin</artifactId>
                    <version>0.5.0</version>
                    <extensions>true</extensions>
                    <configuration>
                        <rules>
                            <jakartaDefaults>true</jakartaDefaults>
                        </rules>
                    </configuration>
                    <executions>
                        <execution>
                            <id>default-jar</id>
                            <goals>
                                <goal>jar</goal>
                            </goals>
                            <configuration>
                                <artifact>
                                    <groupId>com.example</groupId>
                                    <artifactId>javax-module</artifactId>
                                    <version>${project.version}</version>
                                </artifact>
                            </configuration>
                        </execution>
                    </executions>
                </plugin>
            </plugins>
        </build>
   

4. Execute the Migration

   From the root project directory (javax-to-jakarta), execute the following command to perform the migration:

    mvn clean package
   

Conclusion

In this article, we walked through a structured approach to migrate a javax library to a jakarta library using the transformer-maven-plugin within a Maven multi-module project. This methodology proves to be beneficial especially when there's a need to maintain a javax library or when control over the javax library is not available. Our setup delineated clear, easy-to-follow steps for migration, which is not only reproducible but also sets a foundation for testing and validation, ensuring the integrity of the libraries across the transition.

The transformer-maven-plugin offers a range of features beyond what was explored in this guide, making it a robust tool worth exploring for similar migration projects.

The complete code for the project discussed is available on GitHub, providing a practical reference to jump-start your migration endeavors from javax to jakarta.