Configure embedded Undertow server in Spring Boot Configure embedded Undertow server in Spring Boot

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In this quick tutorial, we’ll configure embedded Undertow server by replacing it with default Tomcat server in Spring Boot web application.

Add Undertow Dependency

We need to do two things here:-

  1. Exclude default dependency spring-boot-starter-tomcat added in spring-boot-start-web
  2. Add spring-boot-starter-undertow dependency.

dependencies {
    implementation('org.springframework.boot:spring-boot-starter-web') {
        exclude group: 'org.springframework.boot', module:'spring-boot-starter-tomcat'
    implementation 'org.springframework.boot:spring-boot-starter-undertow'

That’s it. You have replaced tomcat with Undertow server.

Application Startup Logs

When you start spring boot application, You will in the logs that Undertow is serving your web application now:-

INFO c.e.demo.SpringBootDemoApplication       :  Starting SpringBootDemoApplication using Java 11.0.10 on Ashishs-MBP with PID 5166 (/Users/ashl/IdeaProjects/springboot-examples/springboot-config/build/classes/java/main started by ashl in /Users/ashl/IdeaProjects/springboot-examples/springboot-config)
DEBUG c.e.demo.SpringBootDemoApplication      :  Running with Spring Boot v2.5.0, Spring v5.3.7
INFO c.e.demo.SpringBootDemoApplication       :  No active profile set, falling back to default profiles: default
WARN io.undertow.websockets.jsr               :  UT026010: Buffer pool was not set on WebSocketDeploymentInfo, the default pool will be used
INFO io.undertow.servlet                      :  Initializing Spring embedded WebApplicationContext
INFO w.s.c.ServletWebServerApplicationContext :  Root WebApplicationContext: initialization completed in 753 ms
INFO io.undertow                              :  starting server: Undertow - 2.2.7.Final
INFO org.xnio                                 :  XNIO version 3.8.0.Final
INFO org.xnio.nio                             :  XNIO NIO Implementation Version 3.8.0.Final
INFO org.jboss.threads                        :  JBoss Threads version 3.1.0.Final
INFO o.s.b.w.e.undertow.UndertowWebServer     :  Undertow started on port(s) 8080 (http)
INFO c.e.demo.SpringBootDemoApplication       :  Started SpringBootDemoApplication in 1.858 seconds (JVM running for 2.21)

Add Spring Boot Undertow Configuration

Spring boot also provides Undertow specific configuration which you can configure from application.yml or file.

    allow-encoded-slash: false # Whether the server should decode percent encoded slash characters. Enabling encoded slashes can have security implications due to different servers interpreting the slash differently. Only enable this if you have a legacy application that requires it.
    always-set-keep-alive: true # Whether the 'Connection: keep-alive' header should be added to all responses, even if not required by the HTTP specification.
    buffer-size: # Size of each buffer. The default is derived from the maximum amount of memory that is available to the JVM.
    decode-url: true # Whether the URL should be decoded. When disabled, percent-encoded characters in the URL will be left as-is.
    direct-buffers: # Whether to allocate buffers outside the Java heap. The default is derived from the maximum amount of memory that is available to the JVM.
    eager-filter-init: true # Whether servlet filters should be initialized on startup.
    max-cookies: 200 # Maximum number of cookies that are allowed. This limit exists to prevent hash collision based DOS attacks.
    max-headers: 12 # Maximum number of headers that are allowed. This limit exists to prevent hash collision based DOS attacks.
    max-parameters: # Maximum number of query or path parameters that are allowed. This limit exists to prevent hash collision based DOS attacks.
    max-http-post-size: -1B # Maximum size of the HTTP post content. When the value is -1, the default, the size is unlimited.
    no-request-timeout: # Amount of time a connection can sit idle without processing a request, before it is closed by the server.
    preserve-path-on-forward: false # Whether to preserve the path of a request when it is forwarded.
    url-charset: UTF-8 # Charset used to decode URLs
      enabled: false # Whether to enable the access log
      dir: # Undertow access log directory
      pattern: common # Format pattern for access logs
      rotate: true # Whether to enable access log rotation
      prefix: access_log. # Log file name prefix.
      suffix: log # Log file name suffix
      io: # Number of I/O threads to create for the worker. The default is derived from the number of available processors.
      worker: # Number of worker threads. The default is 8 times the number of I/O threads.   

You can refer to the Spring Boot official documentation for full list of configuration.