Embedded Web Servers

Each Spring Boot web application includes an embedded web server. This feature leads to a number of how-to questions, including how to change the embedded server and how to configure the embedded server. This section answers those questions.

Use Another Web Server

Many Spring Boot starters include default embedded containers.

For servlet stack applications, the spring-boot-starter-web includes Tomcat by including spring-boot-starter-tomcat, but you can use spring-boot-starter-jetty or spring-boot-starter-undertow instead.
For reactive stack applications, the spring-boot-starter-webflux includes Reactor Netty by including spring-boot-starter-reactor-netty, but you can use spring-boot-starter-tomcat, spring-boot-starter-jetty, or spring-boot-starter-undertow instead.

When switching to a different HTTP server, you need to swap the default dependencies for those that you need instead. To help with this process, Spring Boot provides a separate starter for each of the supported HTTP servers.

The following Maven example shows how to exclude Tomcat and include Jetty for Spring MVC:

<properties>
	<servlet-api.version>3.1.0</servlet-api.version>
</properties>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
	<exclusions>
		<!-- Exclude the Tomcat dependency -->
		<exclusion>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-tomcat</artifactId>
		</exclusion>
	</exclusions>
</dependency>
<!-- Use Jetty instead -->
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-jetty</artifactId>
</dependency>

Note	The version of the Servlet API has been overridden as, unlike Tomcat 9 and Undertow 2, Jetty 9.4 does not support Servlet 4.0.

If you wish to use Jetty 10, you can do so as shown in the following example:

<properties>
	<jetty.version>10.0.8</jetty.version>
</properties>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
	<exclusions>
		<!-- Exclude the Tomcat dependency -->
		<exclusion>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-tomcat</artifactId>
		</exclusion>
		<!-- Exclude the Jetty-9 specific dependencies -->
		<exclusion>
			<groupId>org.eclipse.jetty.websocket</groupId>
			<artifactId>websocket-server</artifactId>
		</exclusion>
		<exclusion>
			<groupId>org.eclipse.jetty.websocket</groupId>
			<artifactId>javax-websocket-server-impl</artifactId>
		</exclusion>
	</exclusions>
</dependency>
<!-- Use Jetty instead -->
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-jetty</artifactId>
</dependency>

Note that along with excluding the Tomcat starter, a couple of Jetty9-specific dependencies also need to be excluded.

The following Gradle example configures the necessary dependencies and a {gradle-docs}/resolution_rules.html#sec:module_replacement[module replacement] to use Undertow in place of Reactor Netty for Spring WebFlux:

dependencies {
	implementation "org.springframework.boot:spring-boot-starter-undertow"
	implementation "org.springframework.boot:spring-boot-starter-webflux"
	modules {
		module("org.springframework.boot:spring-boot-starter-reactor-netty") {
			replacedBy("org.springframework.boot:spring-boot-starter-undertow", "Use Undertow instead of Reactor Netty")
		}
	}
}

Note	`spring-boot-starter-reactor-netty` is required to use the `WebClient` class, so you may need to keep a dependency on Netty even when you need to include a different HTTP server.

Disabling the Web Server

If your classpath contains the necessary bits to start a web server, Spring Boot will automatically start it. To disable this behavior configure the WebApplicationType in your application.properties, as shown in the following example:

spring:
  main:
    web-application-type: "none"

Change the HTTP Port

In a standalone application, the main HTTP port defaults to 8080 but can be set with configprop:server.port[] (for example, in application.properties or as a System property). Thanks to relaxed binding of Environment values, you can also use configprop:server.port[format=envvar] (for example, as an OS environment variable).

To switch off the HTTP endpoints completely but still create a WebApplicationContext, use server.port=-1 (doing so is sometimes useful for testing).

For more details, see “features.adoc” in the ‘Spring Boot Features’ section, or the {spring-boot-autoconfigure-module-code}/web/ServerProperties.java[ServerProperties] source code.

Use a Random Unassigned HTTP Port

To scan for a free port (using OS natives to prevent clashes) use server.port=0.

Discover the HTTP Port at Runtime

You can access the port the server is running on from log output or from the WebServerApplicationContext through its WebServer. The best way to get that and be sure it has been initialized is to add a @Bean of type ApplicationListener<WebServerInitializedEvent> and pull the container out of the event when it is published.

Tests that use @SpringBootTest(webEnvironment=WebEnvironment.RANDOM_PORT) can also inject the actual port into a field by using the @LocalServerPort annotation, as shown in the following example:

link:{docs-java}/howto/webserver/discoverport/MyWebIntegrationTests.java[role=include]

Note

@LocalServerPort is a meta-annotation for @Value("${local.server.port}"). Do not try to inject the port in a regular application. As we just saw, the value is set only after the container has been initialized. Contrary to a test, application code callbacks are processed early (before the value is actually available).

Enable HTTP Response Compression

HTTP response compression is supported by Jetty, Tomcat, Undertow and Netty. It can be enabled in application.properties, as follows:

server:
  compression:
    enabled: true

By default, responses must be at least 2048 bytes in length for compression to be performed. You can configure this behavior by setting the configprop:server.compression.min-response-size[] property.

By default, responses are compressed only if their content type is one of the following:

text/html
text/xml
text/plain
text/css
text/javascript
application/javascript
application/json
application/xml

You can configure this behavior by setting the configprop:server.compression.mime-types[] property.

Configure SSL

SSL can be configured declaratively by setting the various server.ssl.* properties, typically in application.properties or application.yml. The following example shows setting SSL properties in application.properties:

server:
  port: 8443
  ssl:
    key-store: "classpath:keystore.jks"
    key-store-password: "secret"
    key-password: "another-secret"

See {spring-boot-module-code}/web/server/Ssl.java[Ssl] for details of all of the supported properties.

Using configuration such as the preceding example means the application no longer supports a plain HTTP connector at port 8080. Spring Boot does not support the configuration of both an HTTP connector and an HTTPS connector through application.properties. If you want to have both, you need to configure one of them programmatically. We recommend using application.properties to configure HTTPS, as the HTTP connector is the easier of the two to configure programmatically.

Configure HTTP/2

You can enable HTTP/2 support in your Spring Boot application with the configprop:server.http2.enabled[] configuration property. Both h2 (HTTP/2 over TLS) and h2c (HTTP/2 over TCP) are supported. To use h2, SSL must also be enabled. When SSL is not enabled, h2c will be used. The details of the h2 support depend on the chosen web server and the application environment, since that protocol is not supported out-of-the-box by all JDK 8 releases.

HTTP/2 with Tomcat

Spring Boot ships by default with Tomcat 9.0.x which supports h2c out of the box and h2 out of the box when using JDK 9 or later. Alternatively, h2 can be used on JDK 8 if the libtcnative library and its dependencies are installed on the host operating system.

The library directory must be made available, if not already, to the JVM library path. You can do so with a JVM argument such as -Djava.library.path=/usr/local/opt/tomcat-native/lib. More on this in the official Tomcat documentation.

Starting Tomcat 9.0.x on JDK 8 with HTTP/2 and SSL enabled but without that native support logs the following error:

ERROR 8787 --- [           main] o.a.coyote.http11.Http11NioProtocol      : The upgrade handler [org.apache.coyote.http2.Http2Protocol] for [h2] only supports upgrade via ALPN but has been configured for the ["https-jsse-nio-8443"] connector that does not support ALPN.

This error is not fatal, and the application still starts with HTTP/1.1 SSL support.

HTTP/2 with Jetty

For HTTP/2 support, Jetty requires the additional org.eclipse.jetty.http2:http2-server dependency. To use h2c no other dependencies are required. To use h2, you also need to choose one of the following dependencies, depending on your deployment:

org.eclipse.jetty:jetty-alpn-java-server for applications running on JDK9+
org.eclipse.jetty:jetty-alpn-openjdk8-server for applications running on JDK8u252+
org.eclipse.jetty:jetty-alpn-conscrypt-server and the Conscrypt library with no JDK requirement

HTTP/2 with Reactor Netty

The spring-boot-webflux-starter is using by default Reactor Netty as a server. Reactor Netty supports h2c using JDK 8 or later with no additional dependencies. Reactor Netty supports h2 using the JDK support with JDK 9 or later. For JDK 8 environments, or for optimal runtime performance, this server also supports h2 with native libraries. To enable that, your application needs to have an additional dependency.

Spring Boot manages the version for the io.netty:netty-tcnative-boringssl-static "uber jar", containing native libraries for all platforms. Developers can choose to import only the required dependencies using a classifier (see the Netty official documentation).

HTTP/2 with Undertow

As of Undertow 1.4.0+, both h2 and h2c are supported on JDK 8 without any additional dependencies.

Configure the Web Server

Generally, you should first consider using one of the many available configuration keys and customize your web server by adding new entries in your application.properties (or application.yml, or environment, etc. see “howto.adoc”). The server.* namespace is quite useful here, and it includes namespaces like server.tomcat.*, server.jetty.* and others, for server-specific features. See the list of application-properties.adoc.

The previous sections covered already many common use cases, such as compression, SSL or HTTP/2. However, if a configuration key doesn’t exist for your use case, you should then look at {spring-boot-module-api}/web/server/WebServerFactoryCustomizer.html[WebServerFactoryCustomizer]. You can declare such a component and get access to the server factory relevant to your choice: you should select the variant for the chosen Server (Tomcat, Jetty, Reactor Netty, Undertow) and the chosen web stack (Servlet or Reactive).

The example below is for Tomcat with the spring-boot-starter-web (Servlet stack):

link:{docs-java}/howto/webserver/configure/MyTomcatWebServerCustomizer.java[role=include]

Note	Spring Boot uses that infrastructure internally to auto-configure the server. Auto-configured `WebServerFactoryCustomizer` beans have an order of `0` and will be processed before any user-defined customizers, unless it has an explicit order that states otherwise.

Once you’ve got access to a WebServerFactory using the customizer, you can use it to configure specific parts, like connectors, server resources, or the server itself - all using server-specific APIs.

In addition Spring Boot provides:

Server	Servlet stack	Reactive stack
Tomcat	`TomcatServletWebServerFactory`	`TomcatReactiveWebServerFactory`
Jetty	`JettyServletWebServerFactory`	`JettyReactiveWebServerFactory`
Undertow	`UndertowServletWebServerFactory`	`UndertowReactiveWebServerFactory`
Reactor	N/A	`NettyReactiveWebServerFactory`

As a last resort, you can also declare your own WebServerFactory bean, which will override the one provided by Spring Boot. When you do so, auto-configured customizers are still applied on your custom factory, so use that option carefully.

Add a Servlet, Filter, or Listener to an Application

In a servlet stack application, i.e. with the spring-boot-starter-web, there are two ways to add Servlet, Filter, ServletContextListener, and the other listeners supported by the Servlet API to your application:

howto.adoc
howto.adoc

Add a Servlet, Filter, or Listener by Using a Spring Bean

To add a Servlet, Filter, or Servlet *Listener by using a Spring bean, you must provide a @Bean definition for it. Doing so can be very useful when you want to inject configuration or dependencies. However, you must be very careful that they do not cause eager initialization of too many other beans, because they have to be installed in the container very early in the application lifecycle. (For example, it is not a good idea to have them depend on your DataSource or JPA configuration.) You can work around such restrictions by initializing the beans lazily when first used instead of on initialization.

In the case of Filters and Servlets, you can also add mappings and init parameters by adding a FilterRegistrationBean or a ServletRegistrationBean instead of or in addition to the underlying component.

Note	If no `dispatcherType` is specified on a filter registration, `REQUEST` is used. This aligns with the Servlet Specification’s default dispatcher type.

Like any other Spring bean, you can define the order of Servlet filter beans; please make sure to check the “features.adoc” section.

Disable Registration of a Servlet or Filter

As described earlier, any Servlet or Filter beans are registered with the servlet container automatically. To disable registration of a particular Filter or Servlet bean, create a registration bean for it and mark it as disabled, as shown in the following example:

link:{docs-java}/howto/webserver/addservletfilterlistener/springbean/disable/MyFilterConfiguration.java[role=include]

Add Servlets, Filters, and Listeners by Using Classpath Scanning

@WebServlet, @WebFilter, and @WebListener annotated classes can be automatically registered with an embedded servlet container by annotating a @Configuration class with @ServletComponentScan and specifying the package(s) containing the components that you want to register. By default, @ServletComponentScan scans from the package of the annotated class.

Configure Access Logging

Access logs can be configured for Tomcat, Undertow, and Jetty through their respective namespaces.

For instance, the following settings log access on Tomcat with a {tomcat-docs}/config/valve.html#Access_Logging[custom pattern].

server:
  tomcat:
    basedir: "my-tomcat"
    accesslog:
      enabled: true
      pattern: "%t %a %r %s (%D ms)"

Note

The default location for logs is a logs directory relative to the Tomcat base directory. By default, the logs directory is a temporary directory, so you may want to fix Tomcat’s base directory or use an absolute path for the logs. In the preceding example, the logs are available in my-tomcat/logs relative to the working directory of the application.

Access logging for Undertow can be configured in a similar fashion, as shown in the following example:

server:
  undertow:
    accesslog:
      enabled: true
      pattern: "%t %a %r %s (%D ms)"

Logs are stored in a logs directory relative to the working directory of the application. You can customize this location by setting the configprop:server.undertow.accesslog.dir[] property.

Finally, access logging for Jetty can also be configured as follows:

server:
  jetty:
    accesslog:
      enabled: true
      filename: "/var/log/jetty-access.log"

By default, logs are redirected to System.err. For more details, see the Jetty documentation.

Running Behind a Front-end Proxy Server

If your application is running behind a proxy, a load-balancer or in the cloud, the request information (like the host, port, scheme…) might change along the way. Your application may be running on 10.10.10.10:8080, but HTTP clients should only see example.org.

RFC7239 "Forwarded Headers" defines the Forwarded HTTP header; proxies can use this header to provide information about the original request. You can configure your application to read those headers and automatically use that information when creating links and sending them to clients in HTTP 302 responses, JSON documents or HTML pages. There are also non-standard headers, like X-Forwarded-Host, X-Forwarded-Port, X-Forwarded-Proto, X-Forwarded-Ssl, and X-Forwarded-Prefix.

If the proxy adds the commonly used X-Forwarded-For and X-Forwarded-Proto headers, setting server.forward-headers-strategy to NATIVE is enough to support those. With this option, the Web servers themselves natively support this feature; you can check their specific documentation to learn about specific behavior.

If this is not enough, Spring Framework provides a {spring-framework-docs}/web.html#filters-forwarded-headers[ForwardedHeaderFilter]. You can register it as a Servlet Filter in your application by setting server.forward-headers-strategy is set to FRAMEWORK.

Tip	If you are using Tomcat and terminating SSL at the proxy, configprop:server.tomcat.redirect-context-root[] should be set to `false`. This allows the `X-Forwarded-Proto` header to be honored before any redirects are performed.

Note	If your application runs in Cloud Foundry or Heroku, the configprop:server.forward-headers-strategy[] property defaults to `NATIVE`. In all other instances, it defaults to `NONE`.

Customize Tomcat’s Proxy Configuration

If you use Tomcat, you can additionally configure the names of the headers used to carry “forwarded” information, as shown in the following example:

server:
  tomcat:
    remoteip:
      remote-ip-header: "x-your-remote-ip-header"
      protocol-header: "x-your-protocol-header"

Tomcat is also configured with a default regular expression that matches internal proxies that are to be trusted. By default, IP addresses in 10/8, 192.168/16, 169.254/16 and 127/8 are trusted. You can customize the valve’s configuration by adding an entry to application.properties, as shown in the following example:

server:
  tomcat:
    remoteip:
      internal-proxies: "192\\.168\\.\\d{1,3}\\.\\d{1,3}"

Note	You can trust all proxies by setting the `internal-proxies` to empty (but do not do so in production).

You can take complete control of the configuration of Tomcat’s RemoteIpValve by switching the automatic one off (to do so, set server.forward-headers-strategy=NONE) and adding a new valve instance using a WebServerFactoryCustomizer bean.

Enable Multiple Connectors with Tomcat

You can add an org.apache.catalina.connector.Connector to the TomcatServletWebServerFactory, which can allow multiple connectors, including HTTP and HTTPS connectors, as shown in the following example:

link:{docs-java}/howto/webserver/enablemultipleconnectorsintomcat/MyTomcatConfiguration.java[role=include]

Use Tomcat’s LegacyCookieProcessor

By default, the embedded Tomcat used by Spring Boot does not support "Version 0" of the Cookie format, so you may see the following error:

java.lang.IllegalArgumentException: An invalid character [32] was present in the Cookie value

If at all possible, you should consider updating your code to only store values compliant with later Cookie specifications. If, however, you cannot change the way that cookies are written, you can instead configure Tomcat to use a LegacyCookieProcessor. To switch to the LegacyCookieProcessor, use an WebServerFactoryCustomizer bean that adds a TomcatContextCustomizer, as shown in the following example:

link:{docs-java}/howto/webserver/usetomcatlegacycookieprocessor/MyLegacyCookieProcessorConfiguration.java[role=include]

Enable Tomcat’s MBean Registry

Embedded Tomcat’s MBean registry is disabled by default. This minimizes Tomcat’s memory footprint. If you want to use Tomcat’s MBeans, for example so that they can be used to expose metrics via Micrometer, you must use the configprop:server.tomcat.mbeanregistry.enabled[] property to do so, as shown in the following example:

server:
  tomcat:
    mbeanregistry:
      enabled: true

Enable Multiple Listeners with Undertow

Add an UndertowBuilderCustomizer to the UndertowServletWebServerFactory and add a listener to the Builder, as shown in the following example:

link:{docs-java}/howto/webserver/enablemultiplelistenersinundertow/MyUndertowConfiguration.java[role=include]

Create WebSocket Endpoints Using @ServerEndpoint

If you want to use @ServerEndpoint in a Spring Boot application that used an embedded container, you must declare a single ServerEndpointExporter @Bean, as shown in the following example:

link:{docs-java}/howto/webserver/createwebsocketendpointsusingserverendpoint/MyWebSocketConfiguration.java[role=include]

The bean shown in the preceding example registers any @ServerEndpoint annotated beans with the underlying WebSocket container. When deployed to a standalone servlet container, this role is performed by a servlet container initializer, and the ServerEndpointExporter bean is not required.

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webserver.adoc

webserver.adoc

Embedded Web Servers

Use Another Web Server

Disabling the Web Server

Change the HTTP Port

Use a Random Unassigned HTTP Port

Discover the HTTP Port at Runtime

Enable HTTP Response Compression

Configure SSL

Configure HTTP/2

HTTP/2 with Tomcat

HTTP/2 with Jetty

HTTP/2 with Reactor Netty

HTTP/2 with Undertow

Configure the Web Server

Add a Servlet, Filter, or Listener to an Application

Add a Servlet, Filter, or Listener by Using a Spring Bean

Disable Registration of a Servlet or Filter

Add Servlets, Filters, and Listeners by Using Classpath Scanning

Configure Access Logging

Running Behind a Front-end Proxy Server

Customize Tomcat’s Proxy Configuration

Enable Multiple Connectors with Tomcat

Use Tomcat’s LegacyCookieProcessor

Enable Tomcat’s MBean Registry

Enable Multiple Listeners with Undertow

Create WebSocket Endpoints Using @ServerEndpoint

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webserver.adoc

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webserver.adoc

File metadata and controls

Embedded Web Servers

Use Another Web Server

Disabling the Web Server

Change the HTTP Port

Use a Random Unassigned HTTP Port

Discover the HTTP Port at Runtime

Enable HTTP Response Compression

Configure SSL

Configure HTTP/2

HTTP/2 with Tomcat

HTTP/2 with Jetty

HTTP/2 with Reactor Netty

HTTP/2 with Undertow

Configure the Web Server

Add a Servlet, Filter, or Listener to an Application

Add a Servlet, Filter, or Listener by Using a Spring Bean

Disable Registration of a Servlet or Filter

Add Servlets, Filters, and Listeners by Using Classpath Scanning

Configure Access Logging

Running Behind a Front-end Proxy Server

Customize Tomcat’s Proxy Configuration

Enable Multiple Connectors with Tomcat

Use Tomcat’s LegacyCookieProcessor

Enable Tomcat’s MBean Registry

Enable Multiple Listeners with Undertow

Create WebSocket Endpoints Using @ServerEndpoint