Migrating .NET + Java to Next.js — Not What I Expected

The temptation was to rewrite everything. The reality was that the backends were stable, well-tested, and processed business logic that was too complex and too risky to rewrite under time pressure. The JSP frontend was the pain point — slow, not mobile-friendly, and requiring a browser that most employees had stopped using.

The pragmatic decision: replace only the frontend. Build a Next.js application that calls the existing .NET and Java services through a BFF (Backend for Frontend) layer. The BFF handled authentication, aggregated responses from multiple services, and provided a clean API for the new frontend.

The BFF layer was written in Next.js API routes — close to the frontend team, easy to iterate. Each API route called the appropriate backend service, handled errors consistently, and returned a shape optimised for the UI rather than for the backend's internal model.

The session migration was the hardest part. The old system used server-side sessions tied to the JSP container. Moving to JWT-based authentication required coordinating with the .NET team, which meant a few weeks of parallel running where both auth systems were active.