How does Spring Boot embed a web server and how does the embedded Tomcat work?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Add the web starter

   spring-boot-starter-web brings in embedded Tomcat and Spring MVC as transitive dependencies.

2. Step 2

   Boot detects a server factory

   The ServletWebServerApplicationContext finds a TomcatServletWebServerFactory bean during refresh.

3. Step 3

   Create and start the container

   The factory builds a Tomcat instance, binds server.port, and starts its connector and worker thread pool.

4. Step 4

   Register the DispatcherServlet

   Boot wires the DispatcherServlet into the container so incoming requests route to your controllers.

5. Step 5

   Serve requests

   Worker threads accept sockets, parse HTTP, and dispatch each request to the matched handler method.

6. Step 6

   Tune or swap

   Adjust server.* properties, or exclude Tomcat and add Jetty/Undertow to change the container.

Code Example

@SpringBootApplication
public class DemoApplication {
  public static void main(String[] args) {
    // Starts embedded Tomcat in-process
    SpringApplication.run(DemoApplication.class, args);
  }
}

// application.properties
// server.port=8081
// server.tomcat.threads.max=200
// server.tomcat.accept-count=100

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
  <exclusions>
    <exclusion>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
    </exclusion>
  </exclusions>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-undertow</artifactId>
</dependency>

Follow-up Questions

• How do you change the embedded server's port?
• How would you replace Tomcat with Jetty or Undertow?
• What is the DispatcherServlet's role in request handling?
• How does Tomcat's thread pool handle concurrent requests?
• When would you still deploy a traditional WAR instead?