What is the ResourceResolver / ResourceChain, and what are the built-in resolvers in Spring MVC?

## The core abstraction `ResourceHttpRequestHandler` doesn't just open a file at the requested path — it runs the request path through a **ResourceResolverChain**, an ordered list of `ResourceResolver` beans. Each resolver's `resolveResource(request, requestPath, locations, chain)` either resolves the resource itself, rewrites the path and delegates to `chain.resolveResource(...)`, or returns null. There is a symmetric `resolveUrlPath` direction used to generate the public URL for a given internal resource (used by `ResourceUrlProvider` / `ResourceUrlEncodingFilter`). ## Enabling the chain ```java registry.addResourceHandler("/assets/**") .addResourceLocations("classpath:/static/") .resourceChain(true); // true = cache resolved resources ``` `resourceChain(cacheResources)` sets up the chain; passing `true` adds a `CachingResourceResolver`/`CachingResourceTransformer` backed by a `ConcurrentMapCache` (or a `Cache` you supply). You can also call `.addResolver(...)`/`.addTransformer(...)` to customize. ## Built-in ResourceResolvers - **PathResourceResolver** — the **terminal** resolver. Locates the file under the configured `Resource` locations and enforces that the resolved path stays inside them (security: blocks `../` traversal). Always last in the chain. - **CachingResourceResolver** — wraps lookups in a `Cache` so repeated requests skip disk access and hash computation. Should be first so cache hits short-circuit the whole chain. - **VersionResourceResolver** — handles **content versioning**. Configured with a `VersionStrategy` per path pattern: `addContentVersionStrategy("/**")` (MD5 content hash embedded in the filename, e.g. `app-9f8a2c1e.js`) or `addFixedVersionStrategy("v3", "/**")` (a fixed token, e.g. a build number as a path prefix). It strips the version from the incoming path before delegating, and validates the version matches the actual content — a mismatch yields 404. - **EncodedResourceResolver** — serves precompressed variants (`.gz`, `.br`) when the request's `Accept-Encoding` allows, setting `Content-Encoding` accordingly. - **WebJarsResourceResolver** — resolves version-agnostic WebJars URLs like `/webjars/jquery/jquery.js` to the versioned classpath location (requires `webjars-locator-core`). ## ResourceTransformers (the parallel pipeline) After a resource is resolved, a chain of `ResourceTransformer`s may rewrite its content: - **CssLinkResourceTransformer** — rewrites `url(...)` and `@import` links inside CSS to their versioned equivalents, so fingerprinting cascades through CSS. - **CachingResourceTransformer** — caches transformed output. ## Generating versioned URLs Resolving an incoming versioned request is only half the job — templates must **emit** versioned URLs. `ResourceUrlProvider` (and in views, the `ResourceUrlEncodingFilter` + Thymeleaf/JSP integration) maps a logical path to its current versioned URL by running the chain in the `resolveUrlPath` direction. ## Gotchas & ordering - Order is significant: Caching first (short-circuit hits), then Version/Encoded, then the terminal Path resolver. - `VersionResourceResolver` with `ContentVersionStrategy` computes an MD5 of file content — pair it with long-lived immutable Cache-Control for real benefit. - Fixed vs content strategy: content strategy busts cache automatically per-file on change; fixed strategy busts everything on each build number bump. - The chain is per-registration; different URL patterns can have different chains.