How to add an extra source directory for maven to compile and include in the build jar?

In Maven, the default source code directory is src/main/java. If you wish to add additional source code directories, you can achieve this by modifying the project's pom.xml file. Below are the specific steps and explanations for accomplishing this:

1. Modify the pom.xml File

To add additional source code directories to a Maven project, you need to update the build section of the pom.xml configuration. Specifically, use the build-helper-maven-plugin to incorporate new source code directories. This plugin enables you to introduce additional source paths within Maven's standard build lifecycle.

2. Add build-helper-maven-plugin Configuration

Insert the following configuration into the plugins section of your pom.xml:

xml
<project>
    ...
    <build>
        <plugins>
            <!-- Add build-helper-maven-plugin -->
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>build-helper-maven-plugin</artifactId>
                <version>3.2.0</version>
                <executions>
                    <execution>
                        <id>add-source</id>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>add-source</goal>
                        </goals>
                        <configuration>
                            <sources>
                                <source>src/extra/java</source> <!-- Specify additional source code directory -->
                            </sources>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            ...
        </plugins>
    </build>
    ...
</project>

3. Example Explanation

In this example, we added a source code directory named src/extra/java. When executing a Maven build, the build-helper-maven-plugin adds this directory as a source path during the generate-sources phase. Consequently, the Java files within this directory will be compiled and included in the final JAR file during both compilation and packaging.

4. Build the Project

After updating the pom.xml, build the project using the standard Maven command:

bash
mvn clean install

This command cleans prior build artifacts, recompiles all source code, and generates a new JAR file.

5. Verification

To confirm that the additional source code is correctly compiled and packaged, inspect the build output or extract the JAR file to verify the presence of the corresponding class files.

The steps above detail how to add additional source code directories to a Maven project and ensure the code is compiled and packaged. This approach is highly beneficial for managing multi-module code in large projects, resulting in a clearer and more modular project structure.