HTTP Caching

HTTP 缓存可以显著提高 Web 应用程序的性能。HTTP 缓存围绕`Cache-Control`响应头和后续条件请求头旋转，例如`Last-Modified`和`ETag`。Cache-Control`指示私有（例如浏览器）和公共（例如代理）缓存如何缓存和重复使用响应。`ETag`头用于发出条件请求，如果内容没有更改，则可能导致 304(NOT_MODIFIED，没有正文)。`ETag 可以看作是 Last-Modified 头的更高级的继承者。

HTTP caching can significantly improve the performance of a web application. HTTP caching revolves around the Cache-Control response header and subsequent conditional request headers, such as Last-Modified and ETag. Cache-Control advises private (for example, browser) and public (for example, proxy) caches 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.

本节介绍 Spring WebFlux 中可用的与 HTTP 缓存相关的选项。

This section describes the HTTP caching related options available in Spring WebFlux.

CacheControl

CacheControl 提供对与 Cache-Control 头部相关的设置的支持，并且被当作参数接受在多个位置：

CacheControl provides support for configuring settings related to the Cache-Control header and is accepted as an argument in a number of places:

尽管https://datatracker.ietf.org/doc/html/rfc7234#section-5.2.2[RFC 7234]描述了`Cache-Control`响应标头的所有可能的指令，但`CacheControl`类型采用了面向用例的方法，重点在于常见的场景，如下图所示：

While 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, as the following example shows:

  • Java

  • Kotlin

// 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();
// 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()

Controllers

控制器可以为 HTTP 缓存添加显式支持。我们建议这样做，因为在与条件请求头比较资源的`lastModified`或`ETag`值之前需要计算该值。控制器可以向`ResponseEntity`添加`ETag`和`Cache-Control`设置，如下面的示例所示：

Controllers can add explicit support for HTTP caching. We recommend 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 and Cache-Control settings to a ResponseEntity, as the following example shows:

  • Java

  • Kotlin

@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);
}
@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)
}

前面的示例以空正文发送 304 (NOT_MODIFIED) 响应，如果与条件请求头的比较表明内容没有更改。否则，`ETag`和`Cache-Control`将会被添加到响应。

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

也可以在控制器中对条件请求标头进行检查，如下例所示：

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

Java
@RequestMapping
public String myHandleMethod(ServerWebExchange exchange, Model model) {

	long eTag = ... (1)

	if (exchange.checkNotModified(eTag)) {
		return null; (2)
	}

	model.addAttribute(...); (3)
	return "myViewName";
}
1 Application-specific calculation.
2 Response has been set to 304 (NOT_MODIFIED). No further processing.
3 Continue with request processing.
Kotlin
@RequestMapping
fun myHandleMethod(exchange: ServerWebExchange, model: Model): String? {

	val eTag: Long = ... (1)

	if (exchange.checkNotModified(eTag)) {
		return null(2)
	}

	model.addAttribute(...) (3)
	return "myViewName"
}
4 Application-specific calculation.
5 Response has been set to 304 (NOT_MODIFIED). No further processing.
6 Continue with request processing.

有针对`eTag`值、lastModified`值或两者检查条件请求的三个变体。对于条件`GET`和`HEAD`请求，你可以将响应设置为 304 (NOT_MODIFIED)。对于条件`POST、PUT`和`DELETE，你可以将响应设置为 412 (PRECONDITION_FAILED) 以防止并发修改。

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.

Static Resources

您应该使用 Cache-Control 和条件响应标头提供静态资源以获得最佳性能。请参见有关配置 Static Resources 的部分。

You should serve static resources with a Cache-Control and conditional response headers for optimal performance. See the section on configuring Static Resources.