Spring Session, JSESSIONID, and Server Detection | Generated by AI

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Let’s break this into two parts: how Spring Session/JSESSIONID works, and how to tell whether Spring is running on Tomcat or Netty.

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1. How Spring Session and JSESSIONID Work

• JSESSIONID is the standard HTTP session identifier used by servlet containers (Tomcat, Jetty, Undertow, etc.).
• When a client makes the first request that requires a session, the container (or Spring Session if you use it) generates a random session ID, stores session data on the server, and sends the ID back to the client in a Set-Cookie header.
• The client includes this JSESSIONID cookie on subsequent requests, allowing the server to find the correct session.

• By default, the session data is stored in the servlet container’s memory. For example:

  • Tomcat stores sessions in memory (unless you configure persistence).
  • If you use Spring Session, session storage can be externalized (Redis, JDBC, Hazelcast, etc.), while still using the JSESSIONID cookie to identify users.
• Spring Security and other Spring features rely on this session mechanism for authentication and state management.

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2. How to Know Whether Spring Uses Tomcat or Netty

Spring Boot can run with different embedded servers:

• Tomcat (default for Spring Boot starter-web)
• Jetty (if you swap dependencies)
• Undertow (alternative servlet container)
• Netty (used by Spring WebFlux, not servlet-based)

Ways to Check:

1. Look at your dependencies

   • If your pom.xml or build.gradle includes spring-boot-starter-web, it uses Tomcat by default.
   • If it includes spring-boot-starter-webflux, it uses Netty by default.

2. Check startup logs

   • When the app starts, Spring Boot logs the embedded server.

   • Example (Tomcat):

     Tomcat started on port(s): 8080 (http)
     

   • Example (Netty):

     Netty started on port(s): 8080
     

3. Check runtime classes

   • In code or via actuator, check what server classes are loaded.
   • Tomcat classes start with org.apache.catalina.
   • Netty classes start with io.netty.

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✅ Summary:

• JSESSIONID is just the cookie pointing to a server-side session, managed by Tomcat (default) or by Spring Session (Redis/DB).
• You can tell whether Spring uses Tomcat or Netty by checking dependencies, startup logs, or runtime classes.

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Do you want me to also explain how session handling differs between Spring MVC (Tomcat) and Spring WebFlux (Netty)? That’s where the distinction matters most.

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