This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Framework 6.2.1!

Filters

See equivalent in the Reactive stack

The spring-web module provides some useful filters:

Form Data
Forwarded Headers
Shallow ETag
CORS
URL Handler

Servlet filters can be configured in the web.xml configuration file or using Servlet annotations. If you are using Spring Boot, you can declare them as beans and configure them as part of your application.

Form Data

Browsers can submit form data only through HTTP GET or HTTP POST but non-browser clients can also use HTTP PUT, PATCH, and DELETE. The Servlet API requires ServletRequest.getParameter*() methods to support form field access only for HTTP POST.

The spring-web module provides FormContentFilter to intercept HTTP PUT, PATCH, and DELETE requests with a content type of application/x-www-form-urlencoded, read the form data from the body of the request, and wrap the ServletRequest to make the form data available through the ServletRequest.getParameter*() family of methods.

Forwarded Headers

See equivalent in the Reactive stack

As a request goes through proxies such as load balancers the host, port, and scheme may change, and that makes it a challenge to create links that point to the correct host, port, and scheme from a client perspective.

RFC 7239 defines the Forwarded HTTP header that proxies can use to provide information about the original request.

Non-standard Headers

There are other non-standard headers, too, including X-Forwarded-Host, X-Forwarded-Port, X-Forwarded-Proto, X-Forwarded-Ssl, and X-Forwarded-Prefix.

X-Forwarded-Host

While not standard, X-Forwarded-Host: <host> is a de-facto standard header that is used to communicate the original host to a downstream server. For example, if a request of example.com/resource is sent to a proxy which forwards the request to localhost:8080/resource, then a header of X-Forwarded-Host: example.com can be sent to inform the server that the original host was example.com.

X-Forwarded-Port

While not standard, X-Forwarded-Port: <port> is a de-facto standard header that is used to communicate the original port to a downstream server. For example, if a request of example.com/resource is sent to a proxy which forwards the request to localhost:8080/resource, then a header of X-Forwarded-Port: 443 can be sent to inform the server that the original port was 443.

X-Forwarded-Proto

While not standard, X-Forwarded-Proto: (https|http) is a de-facto standard header that is used to communicate the original protocol (for example, https / https) to a downstream server. For example, if a request of example.com/resource is sent to a proxy which forwards the request to localhost:8080/resource, then a header of X-Forwarded-Proto: https can be sent to inform the server that the original protocol was https.

X-Forwarded-Ssl

While not standard, X-Forwarded-Ssl: (on|off) is a de-facto standard header that is used to communicate the original protocol (for example, https / https) to a downstream server. For example, if a request of example.com/resource is sent to a proxy which forwards the request to localhost:8080/resource, then a header of X-Forwarded-Ssl: on to inform the server that the original protocol was https.

X-Forwarded-Prefix

While not standard, X-Forwarded-Prefix: <prefix> is a de-facto standard header that is used to communicate the original URL path prefix to a downstream server.

Use of X-Forwarded-Prefix can vary by deployment scenario, and needs to be flexible to allow replacing, removing, or prepending the path prefix of the target server.

Scenario 1: Override path prefix

https://example.com/api/{path} -> http://localhost:8080/app1/{path}

The prefix is the start of the path before the capture group {path}. For the proxy, the prefix is /api while for the server the prefix is /app1. In this case, the proxy can send X-Forwarded-Prefix: /api to have the original prefix /api override the server prefix /app1.

Scenario 2: Remove path prefix

At times, an application may want to have the prefix removed. For example, consider the following proxy to server mapping:

https://app1.example.com/{path} -> http://localhost:8080/app1/{path}
https://app2.example.com/{path} -> http://localhost:8080/app2/{path}

The proxy has no prefix, while applications app1 and app2 have path prefixes /app1 and /app2 respectively. The proxy can send X-Forwarded-Prefix: to have the empty prefix override server prefixes /app1 and /app2.

A common case for this deployment scenario is where licenses are paid per production application server, and it is preferable to deploy multiple applications per server to reduce fees. Another reason is to run more applications on the same server in order to share the resources required by the server to run.

In these scenarios, applications need a non-empty context root because there are multiple applications on the same server. However, this should not be visible in URL paths of the public API where applications may use different subdomains that provides benefits such as:

Added security, for example, same origin policy
Independent scaling of applications (different domain points to different IP address)

Scenario 3: Insert path prefix

In other cases, it may be necessary to prepend a prefix. For example, consider the following proxy to server mapping:

https://example.com/api/app1/{path} -> http://localhost:8080/app1/{path}

In this case, the proxy has a prefix of /api/app1 and the server has a prefix of /app1. The proxy can send X-Forwarded-Prefix: /api/app1 to have the original prefix /api/app1 override the server prefix /app1.

ForwardedHeaderFilter

ForwardedHeaderFilter is a Servlet filter that modifies the request in order to a) change the host, port, and scheme based on Forwarded headers, and b) to remove those headers to eliminate further impact. The filter relies on wrapping the request, and therefore it must be ordered ahead of other filters, such as RequestContextFilter, that should work with the modified and not the original request.

Security Considerations

There are security considerations for forwarded headers since an application cannot know if the headers were added by a proxy, as intended, or by a malicious client. This is why a proxy at the boundary of trust should be configured to remove untrusted Forwarded headers that come from the outside. You can also configure the ForwardedHeaderFilter with removeOnly=true, in which case it removes but does not use the headers.

Dispatcher Types

In order to support asynchronous requests and error dispatches this filter should be mapped with DispatcherType.ASYNC and also DispatcherType.ERROR. If using Spring Framework’s AbstractAnnotationConfigDispatcherServletInitializer (see Servlet Config) all filters are automatically registered for all dispatch types. However if registering the filter via web.xml or in Spring Boot via a FilterRegistrationBean be sure to include DispatcherType.ASYNC and DispatcherType.ERROR in addition to DispatcherType.REQUEST.

Shallow ETag

The ShallowEtagHeaderFilter filter creates a “shallow” ETag by caching the content written to the response and computing an MD5 hash from it. The next time a client sends, it does the same, but it also compares the computed value against the If-None-Match request header and, if the two are equal, returns a 304 (NOT_MODIFIED).

This strategy saves network bandwidth but not CPU, as the full response must be computed for each request. State-changing HTTP methods and other HTTP conditional request headers such as If-Match and If-Unmodified-Since are outside the scope of this filter. Other strategies at the controller level can avoid the computation and have a broader support for HTTP conditional requests. See HTTP Caching.

This filter has a writeWeakETag parameter that configures the filter to write weak ETags similar to the following: W/"02a2d595e6ed9a0b24f027f2b63b134d6" (as defined in RFC 7232 Section 2.3).

In order to support asynchronous requests this filter must be mapped with DispatcherType.ASYNC so that the filter can delay and successfully generate an ETag to the end of the last async dispatch. If using Spring Framework’s AbstractAnnotationConfigDispatcherServletInitializer (see Servlet Config) all filters are automatically registered for all dispatch types. However if registering the filter via web.xml or in Spring Boot via a FilterRegistrationBean be sure to include DispatcherType.ASYNC.

CORS

See equivalent in the Reactive stack

Spring MVC provides fine-grained support for CORS configuration through annotations on controllers. However, when used with Spring Security, we advise relying on the built-in CorsFilter that must be ordered ahead of Spring Security’s chain of filters.

See the sections on CORS and the CORS Filter for more details.

URL Handler

See equivalent in the Reactive stack

In previous Spring Framework versions, Spring MVC could be configured to ignore trailing slashes in URL paths when mapping incoming requests on controller methods. This could be done by enabling the setUseTrailingSlashMatch option on the PathMatchConfigurer. This means that sending a "GET /home/" request would be handled by a controller method annotated with @GetMapping("/home").

This option has been retired, but applications are still expected to handle such requests in a safe way. The UrlHandlerFilter Servlet filter has been designed for this purpose. It can be configured to:

respond with an HTTP redirect status when receiving URLs with trailing slashes, sending browsers to the non-trailing slash URL variant.
wrap the request to act as if the request was sent without a trailing slash and continue the processing of the request.

Here is how you can instantiate and configure a UrlHandlerFilter for a blog application:

Java
Kotlin

UrlHandlerFilter urlHandlerFilter = UrlHandlerFilter
		// will HTTP 308 redirect "/blog/my-blog-post/" -> "/blog/my-blog-post"
		.trailingSlashHandler("/blog/**").redirect(HttpStatus.PERMANENT_REDIRECT)
		// will wrap the request to "/admin/user/account/" and make it as "/admin/user/account"
		.trailingSlashHandler("/admin/**").wrapRequest()
		.build();

val urlHandlerFilter = UrlHandlerFilter
		// will HTTP 308 redirect "/blog/my-blog-post/" -> "/blog/my-blog-post"
		.trailingSlashHandler("/blog/**").redirect(HttpStatus.PERMANENT_REDIRECT)
		// will wrap the request to "/admin/user/account/" and make it as "/admin/user/account"
		.trailingSlashHandler("/admin/**").wrapRequest()
		.build()