Evaluate and Migrate from RestTemplate to RestClient/WebClient/Apache HttpClient

[katharinabassler]

What?
Evaluate and migrate the current usage of the soon to be deprecated RestTemplate to a modern HTTP client solution. The main candidates are:

• Spring’s RestClient (new in Spring 6, recommended for imperative code)
• Spring’s WebClient (reactive, non-blocking, recommended for reactive applications)
• Apache HttpClient (mature, widely used, flexible for custom use cases)

Why?

• Deprecation: RestTemplate will be deprecated or removed in future Spring versions. Migrating now avoids technical debt and ensures compatibility with upcoming Spring releases.
• Modern Features: RestClient and WebClient offer fluent APIs, better error handling, and improved integration with the Spring ecosystem. WebClient is especially suited for reactive applications (already used in the GUI).
• Flexibility: Apache HttpClient provides a robust, well-tested alternative for projects requiring advanced customization or non-Spring environments (already used in some API modules).
• Performance: All modern alternatives are more efficient than RestTemplate.

Scope:

• Evaluation: Assess the current usage of RestTemplate in the codebase and determine the best replacement for each use case .
• Migration: Replace RestTemplate with the chosen client(s) across all relevant modules.
• Testing: Update and validate related tests.
• Documentation: Document the new approach and provide guidelines for future HTTP client usage.