[[mvc-caching]]
= HTTP Caching

[.small]#xref:web/webflux/caching.adoc[See equivalent in the Reactive stack]#

HTTP caching can significantly improve the performance of a web application. HTTP caching
revolves around the `Cache-Control` response header and, subsequently, conditional request
headers (such as `Last-Modified` and `ETag`). `Cache-Control` advises private (for example, browser)
and public (for example, proxy) caches on how to cache and re-use responses. An `ETag` header is used
to make a conditional request that may result in a 304 (NOT_MODIFIED) without a body,
if the content has not changed. `ETag` can be seen as a more sophisticated successor to
the `Last-Modified` header.

This section describes the HTTP caching-related options that are available in Spring Web MVC.



[[mvc-caching-cachecontrol]]
== `CacheControl`
[.small]#xref:web/webflux/caching.adoc#webflux-caching-cachecontrol[See equivalent in the Reactive stack]#

{api-spring-framework}/http/CacheControl.html[`CacheControl`] provides support for
configuring settings related to the `Cache-Control` header and is accepted as an argument
in a number of places:

* {api-spring-framework}/web/servlet/mvc/WebContentInterceptor.html[`WebContentInterceptor`]
* {api-spring-framework}/web/servlet/support/WebContentGenerator.html[`WebContentGenerator`]
* xref:web/webmvc/mvc-caching.adoc#mvc-caching-etag-lastmodified[Controllers]
* xref:web/webmvc/mvc-caching.adoc#mvc-caching-static-resources[Static Resources]

While https://tools.ietf.org/html/rfc7234#section-5.2.2[RFC 7234] describes all possible
directives for the `Cache-Control` response header, the `CacheControl` type takes a
use case-oriented approach that focuses on the common scenarios:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	CacheControl ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS);

	// Prevent caching - "Cache-Control: no-store"
	CacheControl ccNoStore = CacheControl.noStore();

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	CacheControl ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic();
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	val ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS)

	// Prevent caching - "Cache-Control: no-store"
	val ccNoStore = CacheControl.noStore()

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	val ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic()
----
======

`WebContentGenerator` also accepts a simpler `cachePeriod` property (defined in seconds) that
works as follows:

* A `-1` value does not generate a `Cache-Control` response header.
* A `0` value prevents caching by using the `'Cache-Control: no-store'` directive.
* An `n > 0` value caches the given response for `n` seconds by using the
`'Cache-Control: max-age=n'` directive.



[[mvc-caching-etag-lastmodified]]
== Controllers
[.small]#xref:web/webflux/caching.adoc#webflux-caching-etag-lastmodified[See equivalent in the Reactive stack]#

Controllers can add explicit support for HTTP caching. We recommended doing so, since the
`lastModified` or `ETag` value for a resource needs to be calculated before it can be compared
against conditional request headers. A controller can add an `ETag` header and `Cache-Control`
settings to a `ResponseEntity`, as the following example shows:

--
[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@GetMapping("/book/{id}")
	public ResponseEntity<Book> showBook(@PathVariable Long id) {

		Book book = findBook(id);
		String version = book.getVersion();

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book);
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	@GetMapping("/book/{id}")
	fun showBook(@PathVariable id: Long): ResponseEntity<Book> {

		val book = findBook(id);
		val version = book.getVersion()

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book)
	}
----
======
--

The preceding example sends a 304 (NOT_MODIFIED) response with an empty body if the comparison
to the conditional request headers indicates that the content has not changed. Otherwise, the
`ETag` and `Cache-Control` headers are added to the response.

You can also make the check against conditional request headers in the controller,
as the following example shows:

--
[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@RequestMapping
	public String myHandleMethod(WebRequest request, Model model) {

		long eTag = ... // <1>

		if (request.checkNotModified(eTag)) {
			return null; // <2>
		}

		model.addAttribute(...); // <3>
		return "myViewName";
	}
----
<1> Application-specific calculation.
<2> The response has been set to 304 (NOT_MODIFIED) -- no further processing.
<3> Continue with the request processing.

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	@RequestMapping
	fun myHandleMethod(request: WebRequest, model: Model): String? {

		val eTag: Long = ... // <1>

		if (request.checkNotModified(eTag)) {
			return null // <2>
		}

		model[...] = ... // <3>
		return "myViewName"
	}
----
<1> Application-specific calculation.
<2> The response has been set to 304 (NOT_MODIFIED) -- no further processing.
<3> Continue with the request processing.
======
--


There are three variants for checking conditional requests against `eTag` values, `lastModified`
values, or both. For conditional `GET` and `HEAD` requests, you can set the response to
304 (NOT_MODIFIED). For conditional `POST`, `PUT`, and `DELETE`, you can instead set the response
to 412 (PRECONDITION_FAILED), to prevent concurrent modification.



[[mvc-caching-static-resources]]
== Static Resources
[.small]#xref:web/webflux/caching.adoc#webflux-caching-static-resources[See equivalent in the Reactive stack]#

You should serve static resources with a `Cache-Control` and conditional response headers
for optimal performance. See the section on configuring xref:web/webmvc/mvc-config/static-resources.adoc[Static Resources].



[[mvc-httpcaching-shallowetag]]
== `ETag` Filter

You can use the `ShallowEtagHeaderFilter` to add "`shallow`" `eTag` values that are computed from the
response content and, thus, save bandwidth but not CPU time. See xref:web/webmvc/filters.adoc#filters-shallow-etag[Shallow ETag].

