What is Spring Boot's embedded server and how do you configure it?

# Tomcat (default): no special configuration needed
# application.yml Tomcat tuning:
server:
  port: 8080
  servlet:
    context-path: /api
  tomcat:
    max-threads: 200
    min-spare-threads: 10
    connection-timeout: 20s
    keep-alive-timeout: 60s

# Switch from Tomcat to Jetty:
<!-- pom.xml -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webmvc</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>

# SSL / HTTPS configuration:
server:
  port: 8443
  ssl:
    enabled: true
    key-store: classpath:keystore.p12
    key-store-password: ${SSL_KEYSTORE_PASSWORD}
    key-store-type: PKCS12
    key-alias: my-app

# Virtual threads with Tomcat (Java 21+):
spring:
  threads:
    virtual:
      enabled: true
# Tomcat automatically uses virtual thread executor