Introduction

• A standardized Java API that allows web servers to execute Java code for HTTP requests.
• When web server receives request:
  • servlet processes the request
  • servlet sends back HTTP response (HTML / JSON / etc.)
• Spring is built on top of Servlets.
  • so it is foundation of Spring MVC/ Spring Boot
  • Tomcat use thread to execute the DispatcherServlet, which then delegates to Spring MVC to handle the real business logic.

How Servlet work on request:

• Servlet container creates and manage threads, and the thread execute the single servlet instance.

  • There is ONE DispatcherServlet object
  • There are MANY threads
  • Each thread calls the same servlet instance

  // so same single servlet instances handles all concurrent requests using multiple threads

• So controller must be stateless, since multiple threads are accessing instance var concurrently

• If the shared state is mutable, will cause race condition

Request 1 → Thread-1 → DispatcherServlet instance
Request 2 → Thread-2 → DispatcherServlet instance
Request 3 → Thread-3 → DispatcherServlet instance

• Each request runs on a separate thread (one request one thread)

  • Thread will wait during:

    • DB call
    • External API call
    • File I/O

    // traditional Servlet model, I/O operations are blocking instead of wating

  • Many slow requests (long wait):

    • Thread pool can be exhausted
    • Server stops accepting new requests

  • Example:

    1. If thread pool size = 200
    2. There is 200 slow request (like DB calls) same time
    3. So 200 threads are currently all occupied
    4. The 201st request will wait in request queue
    5. If queue is full, the request will be rejected
    6. Timeout / degraded performance
• Hence, must avoid mutable shared state

Servlet run inside Servlet Container

• One Tomcat instance creates one servlet instance.
• Tomcat’s thread pool uses multiple threads to execute that single servlet instance concurrently for different requests.

Servlet Container (Tomcat)

This is the real HTTP engine: