Spring Boot 4 is out, and everyone's asking me the same thing: is it worth the jump from 3.x right now? I broke down what actually changed under the hood versus what's just version noise. My honest take on timing this upgrade, no hype included
Let's talk about what Spring Boot 4 actually brings to the table, what's different under the hood, and honestly, whether you should bother upgrading right now or just wait it out a little longer. The headline change: this isn't a minor bump Spring Boot 4 lines up with Spring Framework 7, and that's the part people gloss over. It's not "3.5 but with a new number." Framework 7 changed enough plumbing that Boot 4 had to follow along, which means this upgrade behaves more like the 2.x to 3.x jump (remember javax to jakarta? that mess?) than a routine minor version bump. The good news: it's nowhere near as painful as that migration was. No package renaming nightmare this time. But there are real behavioral changes, not just dependency version bumps, and that's where teams get bitten if they just YOLO the version number in build.gradle and hope for the best. A few things stood out to me right away: Baseline Java moved. Boot 4 wants Java 17 at minimum, same as Boot 3 technically required, but in practice most teams building on Boot 4 are jumping straight to newer JDKs (I went with 26 on Code Training Lab ) to get virtual threads and the newer GC improvements without any compatibility shims. Null-safety annotations got serious. Spring's @Nullable and @NonNull handling is tighter now, and if you use IntelliJ or a static analysis tool, you'll start seeing warnings you didn't see before. Some of them are legit bugs you never noticed. HTTP client changes. RestClient is basically the default recommendation now over RestTemplate , which was already deprecated-in-spirit in 3.x but Boot 4 pushes it harder. Observability defaults shifted. Micrometer integration got cleaner, and some of the auto-configuration around tracing changed enough that if you had custom ObservationHandler beans, you need to double check they still fire the way you expect. None of these are catastrophic on their own. Stacked together, though, they add up to a "budget a real sprint for this" kind of upgrade, not a "bump the version and run the tests" kind of upgrade. What's actually different in code Let me show you a couple of things that tripped me up, because reading about them is different from hitting them in a real build. Configuration properties binding got stricter. On Boot 3, this kind of loose binding worked without complaint: @ConfigurationProperties(prefix = "app.job-fit") public record JobFitProperties( String provider, int timeoutSeconds ) {} On Boot 4, if app.job-fit.timeout-seconds is missing from your YAML and there's no default, you get a much noisier failure at startup instead of a silent zero. I actually prefer this. It surfaced two misconfigured properties in a staging profile that had been silently defaulting to zero for who knows how long. Nobody had noticed because the fallback logic downstream just... worked around it. Not great, but also, that's exactly the kind of thing that should fail loud. The other one worth mentioning: @RestControllerAdvice exception handling changed how it orders matching when you have multiple advice classes. On a Job Fit style feature (a candidate uploads a resume against a specific job posting and gets a fit score back), I had two advice classes, one generic and one specific to validation errors. Under Boot 3 the specific one always won. Under Boot 4, I had to add explicit @Order annotations because the resolution order isn't guaranteed the same way anymore. @Order(1) @RestControllerAdvice public class ValidationExceptionHandler { @ExceptionHandler(MethodArgumentNotValidException.class) public ResponseEntity<ErrorResponse> handleValidation(MethodArgumentNotValidException ex) { return ResponseEntity.badRequest().body(ErrorResponse.from(ex)); } } Took me maybe forty minutes to track down why validation errors were suddenly returning a generic 500 instead of a clean 400. Not a huge deal once you know about it. Annoying to discover blind. AOT and native images: finally feels less experimental Spring's Ahead-of-Time processing was introduced back in Boot 3 alongside GraalVM native image support, but it always felt like a beta feature bolted onto the side of the framework. You'd hit weird reflection issues, third-party libraries that didn't play nice with the native image compiler, and error messages that assumed you already knew what was wrong. Boot 4 tightens this up quite a bit. The AOT processing is more predictable, and the hint files (the reflect-config.json and friends) get generated more reliably for common Spring Data and Spring Security setups. I haven't pushed my Java/Spring Boot 3.5.6 backend at ITJobOpportunities toward native images yet (there's no pressing need, our cold-start times aren't a business problem), but for anyone running serverless-style deployments where cold start actually matters, this alone might be worth the migration. Honestly, if you're not deploying to something like AWS Lambda or a scale-to-zero container platform, native image support isn't a compelling reason to move. It's a nice-to-have, not a must-have, for most of the backend work I do. Jakarta EE 11 alignment Boot 4 tracks Jakarta EE 11, which brings updated versions of Servlet, Persistence (JPA), and Bean Validation specs. Practically, this means: Hibernate got bumped to a version that plays nicer with Java records as entities (still limited, but better). Bean Validation 3.1 adds a few new constraint annotations I actually found useful, @Range being one of them, since I used to hand-roll that with @Min and @Max pairs everywhere. Servlet spec changes are mostly invisible unless you're doing raw filter chain stuff, which most of us aren't in a Spring Boot app anyway. None of this is flashy. It's the kind of change you don't notice until a library you depend on requires it, and then you're stuck upgrading anyway. Which, by the way, is basically what happened with one of the PDF generation dependencies in a resume-improvement style pipeline I built. It required a newer Jakarta Validation API, full stop, no workaround. Should you upgrade? Here's my honest framework I don't think there's a universal answer here, and I get annoyed when people pretend there is. What I actually think about: Is your app in active, heavy feature development? If yes, and you've got the bandwidth, do it now while the codebase is still fresh in everyone's head. Waiting six months means the person who upgrades it later has to relearn everything you already know today. Are you running regulated or high-stakes production workloads with tight change windows? Then wait for Boot 4.2. Let the ecosystem (Spring Security, Spring Cloud, third-party starters) catch up first. I've seen too many teams jump on a major version day one and then spend three weeks fighting a starter library that hasn't published a compatible release yet. Do you rely heavily on a specific starter or library that hasn't announced Boot 4 support? Check first. Seriously, just go check. Some of the smaller community starters (I'm looking at you, a handful of the Kafka schema registry helpers) took months to catch up after Boot 3 shipped, and I'd bet the same pattern repeats here. Is native image deployment actually on your roadmap? If yes, the AOT improvements alone might justify moving sooner rather than later. For instance, Code Training Lab, our newer assessment platform, doesn't carry that same legacy weight. It's a smaller codebase, fewer integrations, and it was a good low-risk place to try Boot 4 for real instead of just reading about it. That's honestly my favorite way to evaluate a major framework version: find the smallest real project you own and just do it there first. A quick note on migration mechanics If you do decide to move, Spring's migration guide (check the official Spring Boot GitHub wiki for the 4.0 migration notes) covers the mechanical stuff well enough. But here's what I'd actually do, in order: Bump your Gradle or Maven Spring Boot plugin version first...