Add support for YAML configuration file format

[YangSiJun528]

This PR adds an option to generate project configuration in YAML format. Currently, only .properties is supported. (#1109)

This PR has many changes, so I'm opening this PR early to get feedback before completing all implementation details. The core features work well, but I still need to add more comprehensive tests - currently only basic tests are implemented.

Main Changes

• Add ConfigurationFileFormat interface with factory and implementations for Properties and YAML
• Update ApplicationProperties to write YAML format
• Add ApplicationYamlPropertiesContributor
• Add @ConditionalOnConfigurationFileFormat and OnConfigurationFileFormatCondition to choose between Properties and YAML
• Add config file format support to MutableProjectDescription, InitializrMetadata, and ProjectRequest
• Update tests: since configuration file format is now configurable, remove application.properties validation or set format explicitly where needed.

Need Feedback On

• Is the name and package location of ConfigurationFileFormat appropriate? (io.spring.initializr.generator.configuration.format)
• Any feedback on the overall design?
• Are there any missing features or edge cases to consider?

Once I receive feedback on the approach, I will add comprehensive tests and polish the implementation before requesting final review.

---

Fixes gh-1109

[mhalbritter]

Thanks @YangSiJun528, I'm going to take a look. I'm in the middle of something right now, but i plan to look at it this week.

[mhalbritter]

This changes the metadata v2.0.0 format. I think we should add a new metadata version v2.3 which adds the configurationFileFormat field. You can take a look at InitializrMetadataV22JsonMapper and InitializrMetadataV21JsonMapper for that.

[YangSiJun528]

I reverted the changes to InitializrMetadataV2JsonMapper and added InitializrMetadataV23JsonMapper.

[mhalbritter]

Hi @YangSiJun528 ,

thanks for the PR. I've left some comments for your consideration.

I think your name ConfigurationFileFormat is a good one. Aside from the new metadata version the design looks sane, and I can't see any missing edge cases.

[YangSiJun528]

Applied feedback to address metadata format compatibility issues.

configuration file format addition changes the metadata structure, requiring format versioning:

• Revert InitializrMetadataV2JsonMapper to preserve v2.0 format
• Create InitializrMetadataV23JsonMapper for new v2.3 format support
• Add InitializrMetadataVersion.V2_3 enum value
• Add v2.3 endpoints in ProjectMetadataController

This maintains v2.0 compatibility while enabling configuration file format features in v2.3.

[YangSiJun528]

And I have added compliance tests to ApplicationPropertiesComplianceTests.

This PR should now be ready for review. @mhalbritter

[mhalbritter]

Thanks @YangSiJun528 !

I've polished a bit, for example the created file is now named application.yaml (i've added the a).

Users who upgrade to initializr 0.23.0 will have to update their application configuration, with something like this:

  configuration-file-formats:
    - name: Properties
      id: properties
      default: true
    - name: YAML
      id: yaml
      default: false

[YangSiJun528]

Thanks for making this change!

I initially used .yml in this PR because Spring Initializr does, and in my experience, .yml is more common in Spring projects. Since the Spring Boot docs mix .yaml and .yml, I assumed .yml would be preferred.

After looking into it, I learned .yaml is officially recommended by the YAML spec, so I agree using .yaml makes sense.

Just leaving a note in case anyone wonders why .yml wasn’t used.