Programming Tutorials

  The migration from Spring Boot 2.7 to Spring Boot 3.0+ is not a trivial version bump; it is a paradigm shift in the Java ecosystem. The most immediate and jarring obstacle developers face is the application crashing at startup with   java.lang.NoClassDefFoundError: javax/servlet/http/HttpServlet   or   java.lang.ClassNotFoundException: javax.persistence.Entity . This error signifies that your bytecode is referencing the legacy Java EE APIs ( javax.* ), but the runtime environment (Spring Boot 3 / Jakarta EE) only provides the modern Jakarta EE APIs ( jakarta.* ). The Root Cause: The "Big Bang" Namespace Shift To understand the fix, you must understand the architecture change. When Oracle transferred Java EE to the Eclipse Foundation to become Jakarta EE, they retained the trademark rights to the  javax  namespace. Consequently, starting with  Jakarta EE 9 , all specifications were legally required to move from the  javax.*  package namespac...

  The Crash You have successfully refactored your codebase to Spring Boot 3. You updated your imports, replaced  javax.persistence  with  jakarta.persistence  in your entities, and the build passes successfully. However, upon startup or during a specific API call, the application crashes with the following stack trace: java.lang.ClassNotFoundException: javax.servlet.Servlet at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641) at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188) at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520) ... This error halts migration efforts immediately. It indicates that while your application code is targeting the modern Jakarta EE APIs, a compiled dependency deep in your classpath is still attempting to load the deprecated Java EE classes. The Root Cause: The Great Namespace Shift Spring Boot 3 is built on Spring Framework 6,...