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Spring Boot: Embedded Server

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Definitions

Embedded Server: Web server (Tomcat, Jetty, Undertow) packaged within Spring Boot application JAR. Eliminates external server deployment; application runs as standalone executable JAR.

Standalone Application: Self-contained executable JAR containing application code, dependencies, and embedded server. Deploys and runs without external application server infrastructure.

Servlet Container: Java component managing servlet lifecycle, HTTP request handling, and session management. Tomcat, Jetty, Undertow are servlet containers.

Server Configuration: Settings controlling embedded server behavior (port, context path, SSL, thread pools, session timeout). Configured via application.properties or application.yml.

Fat JAR: Executable JAR containing application code, all dependencies, and embedded server. Produced by Spring Boot’s Maven/Gradle plugin.

Q&A

Why does Spring Boot include an embedded server?

Embedded servers eliminate complex deployment infrastructure, enabling Spring Boot applications to run with single java -jar command. Simplifies DevOps, enables containerization (Docker), supports microservices architecture where each service is independently deployable and runnable.

How do you change the embedded server port?

Set server.port property in application.properties or application.yml. Default is 8080. Example: server.port=9090. Can also pass as command-line argument: java -jar app.jar --server.port=9090. Zero value (server.port=0) assigns random available port, useful for testing.

Can you switch from Tomcat to Jetty or Undertow?

Yes, exclude spring-boot-starter-tomcat from spring-boot-starter-web and add preferred server starter. For Jetty: add spring-boot-starter-jetty. For Undertow: add spring-boot-starter-undertow. Only one embedded server can be active; inclusion of multiple causes Spring Boot to choose based on classpath priority.

How do you configure SSL/HTTPS for embedded server?

Generate keystore, then configure in application.properties: server.ssl.key-store=classpath:keystore.p12, server.ssl.key-store-password=password, server.ssl.key-store-type=PKCS12. Application runs on HTTPS by default. Useful for testing; production typically uses reverse proxy (Nginx, AWS ELB) for SSL termination.

What are the performance differences between embedded servers?

Tomcat (default) is mature, widely used, good general-purpose performance. Jetty is lightweight, lower memory footprint, good for resource-constrained environments. Undertow (Red Hat) is high-performance, lowest latency, uses non-blocking I/O. Choice depends on performance requirements, team familiarity, and resource constraints.

Can you disable embedded server and use external server?

Yes, set spring.main.web-application-type=none to disable embedded server, or set it to servlet for embedding but reactive for reactive stack. For external deployment, package as WAR instead of JAR, add spring-boot-starter-tomcat with provided scope, and deploy to standalone server.

Code Examples

Example - Configure Embedded Server (application.yml):

server:
  port: 9090
  servlet:
    context-path: /api
  tomcat:
    threads:
      max: 200
      min-spare: 10
    max-connections: 10000

Example - Switch to Jetty (Maven pom.xml):

<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-jetty</artifactId>
</dependency>

Example - Enable SSL (application.properties):

server.ssl.key-store=classpath:keystore.p12
server.ssl.key-store-password=mypassword
server.ssl.key-store-type=PKCS12
server.port=8443

Example - Customize Server Port (Command Line):

java -jar myapp.jar --server.port=8888

Key Points

  • Simplicity: Single JAR simplifies deployment; no external server setup required.
  • Immutability: Fat JAR contains exact dependencies; consistent across environments.
  • Container-Friendly: Embedded server enables easy containerization with Docker.
  • Thread Pool Tuning: Configure tomcat.threads.max based on expected concurrency and server resources.
  • Production Deployment: For high-traffic, use reverse proxy (Nginx) in front of embedded server for SSL, load balancing, caching.

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