Spring Boot 3 baseline: Java 17+, Jakarta EE 9+ (javax.* → jakarta.*).

Spring Boot 3 raised the floor in two ways: Java 17 is the minimum, and the entire enterprise namespace moved from javax.* to jakarta.*. The second is the single biggest breaking change and the one that bites every upgrade.

The Jakarta namespace migration

When Java EE moved to the Eclipse Foundation as Jakarta EE, Oracle's trademark meant the javax.* packages could no longer evolve. Jakarta EE 9 therefore performed a mechanical rename of every package — no API changes, just javax. → jakarta.. Spring Framework 6 / Boot 3 target Jakarta EE 9+, so your imports must change:

// Boot 2 (javax)                  // Boot 3 (jakarta)
import javax.persistence.Entity;   import jakarta.persistence.Entity;
import javax.servlet.Filter;       import jakarta.servlet.Filter;
import javax.validation.Valid;     import jakarta.validation.Valid;
import javax.transaction.Transactional; // -> jakarta.transaction.Transactional

This touches JPA entities, servlet filters, Bean Validation, JMS, and more. Because it's purely a rename, tools like the Eclipse Transformer or OpenRewrite (org.openrewrite.java.migrate.jakarta) can automate the bulk of it. The painful part is transitive: a third-party library still compiled against javax.* won't work, so you must upgrade dependencies (Hibernate 6, Tomcat 10, etc.) in lockstep — you can't run a mixed namespace.

The Java 17 baseline

Boot 3 is compiled against Java 17 and won't run on 11. Practically this means:

• You can adopt records for DTOs, sealed types for closed hierarchies, and pattern matching in your own code.
• The JVM brings better GC defaults and the foundation for virtual threads (Java 21, supported in Boot 3.2+).

The reason this matters in practice: most "Boot 3 won't start" failures aren't your code — they're a stray dependency dragging in javax.servlet classes that the container no longer provides.