A MethodInterceptor that performs conditional HTTP revalidation using ETag and Last-Modified headers. On the first call, the decoded response is stored alongside its validators. Subsequent calls send If-None-Match / If-Modified-Since headers, and a 304 Not Modified response short-circuits to the cached value.

Marked @Experimental while the underlying feign.interceptor.MethodInterceptor API stabilises.

<dependency>
  <groupId>io.github.openfeign</groupId>
  <artifactId>feign-http-cache</artifactId>
  <version>${feign.version}</version>
</dependency>

HttpCacheStore store = new InMemoryHttpCacheStore();

Api api = Feign.builder()
    .methodInterceptor(new HttpCacheInterceptor(store))
    .target(Api.class, "https://example.com");

Plug in a different store (Caffeine, Redis, etc.) by implementing HttpCacheStore.

• keyFn: derive the cache key from an Invocation (e.g. include a tenant header).
• cacheable: decide which RequestTemplates participate. The default is GET / HEAD only.

HttpCacheInterceptor configured = new HttpCacheInterceptor(store)
    .key(invocation -> invocation.methodMetadata().configKey()
        + "|" + invocation.requestTemplate().url()
        + "|" + invocation.requestTemplate().headers().get("X-Tenant"))
    .cacheable(template -> "GET".equalsIgnoreCase(template.method()));

• 304 detection relies on the configured ErrorDecoder raising a FeignException for non-2xx responses (the default). A custom error decoder that swallows or transforms 304 will break cache hits.
• The store retains the decoded response object. Decoding happens once, on the first 200; on a 304 the cached object is returned as-is. Mutations on the returned object propagate to subsequent callers — return immutable values from your decoder if that matters.
• Cache-Control directives beyond no-store are not interpreted; freshness windows (max-age, etc.) are not enforced. Every cached entry triggers revalidation.