[BUG][Java][Spring] Source folder should be added to Maven pom.xml if it is not src/main/java

Created by: jmaister

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
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Description

I am generating a project setting a different source folder than the default for a Maven project. The idea is to have the implementations in src/main/java and the generated sources in src/gen/java.

openapi-generator-cli generate -g spring -i ../openapi/tags.yml -o ../src/service-tags-java/ --enable-post-process-file --additional-properties=.......,sourceFolder=src/gen/java,......

The problem is that the gen folder is not added as a source folder in the pom.xml file. Therefore, the Maven build fails:

openapi-generator version

"@openapitools/openapi-generator-cli": "2.4.18"

OpenAPI declaration file content or url

No specific OpenAPI file, the problem would happen with any file.

Generation Details

Generate the project

openapi-generator-cli generate -g spring -i openapi/tags.yml -o service-tags-java/ --enable-post-process-file --additional-properties=apiPackage=com.example.tags.api,artifactDescription=tags_api,groupId=com.example,artifactId=service-tags,artifactVersion=1.0.0,basePackage=com.example.tags,dateLibrary=java8,delegatePattern=true,java8=true,library=spring-boot,sourceFolder=src/gen/java,invokerPackage=com.example.tags,modelPackage=com.example.tags.model,configPackage=com.example.tags.config

Steps to reproduce

Maven build after generating the code

cd service-tags-java/ mvn clean install

Related issues/PRs

Suggest a fix

The fix consists in adding an additional source folder to the project. There is an example in here: https://www.baeldung.com/maven-project-multiple-src-directories

In the pom.xml file, add the next plugin if the sourceFolder parameter is different from src/main/java

<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>build-helper-maven-plugin</artifactId>
    <version>3.0.0</version>
    <executions>
        <execution>
            <phase>generate-sources</phase>
            <goals>
                <goal>add-source</goal>
            </goals>
            <configuration>
                <sources>
                    <source>src/main/another-src</source>
                </sources>
            </configuration>
        </execution>
    </executions>
</plugin>