Java EE to Spring Boot: complete technical guide

Migrating a Java EE application to Spring Boot is one of the most common projects we deliver at Nearvik. Here's our complete approach, tested on over 20 migration projects.

Why migrate from Java EE to Spring Boot?

Java EE (now Jakarta EE) remains a solid framework, but the ecosystem has shifted to Spring Boot for several reasons: simplified deployment (no app server needed), larger and more active ecosystem, better integration with cloud tools and containers, and a more dynamic community.

The Strangler Fig approach

We never rewrite everything at once. The Strangler Fig method (named after the strangler fig tree) consists of building the new application around the old one, then progressively migrating features.

Step 1: Set up a reverse proxy (NGINX or Spring Cloud Gateway) in front of the existing application. All requests go through the proxy, which routes to the monolith.

Step 2: For each module to migrate, create a Spring Boot microservice. The proxy routes requests to the new service instead of the monolith. The monolith doesn't change.

Step 3: When all critical modules are migrated, the monolith can be decommissioned.

Pitfalls to avoid

Don't underestimate persistence layer migration (JPA/Hibernate can behave differently between Java EE and Spring Boot). Plan for javax to jakarta namespace migration. Test security (Spring Security vs Java EE Security). And don't forget performance testing after migration.

AI's role in migration

Claude Code is a massive accelerator for mechanical refactoring: EJB annotation conversion to Spring, namespace migration, unit test generation for legacy code, and automatic documentation of existing code.

Conclusion

A well-executed Java EE to Spring Boot migration transforms your application: faster to deploy, easier to maintain, cloud and AI-ready. The Strangler Fig approach minimizes risks and delivers value from the first weeks.