Spring MVC conditional GET: validate the representation tag

The downloadable Spring source kit checks this boundary with the named JUnit test and its pinned dependencies.

The tag belongs to the representation

The fixture serves a fixed receipt version with ETag receipt-7-v3. A matching conditional GET returns 304 and an empty body. A stale tag returns 200 and the current body. A missing receipt remains 404; a cache tag cannot turn an absent resource into a valid one.

The code uses Cache-Control: no-cache so a cache may store the representation but must revalidate it before reuse. In a real service, compute a tag from the correct version of the selected representation, including any user-specific visibility boundary. A fixed literal is only a teaching fixture.

Bandwidth is not authorization

An ETag is a cache validator. It is not proof that the caller may see a receipt. A reverse proxy or shared cache must respect authentication and cache-control rules. An If-Match write precondition is a separate mutation contract, not implemented by this GET test.

Checked source

@GetMapping("/contract/receipts/{id}")
ResponseEntity<String> read(@PathVariable int id) {
    if (id != 7) return ResponseEntity.notFound().build();
    return ResponseEntity.ok()
        .cacheControl(CacheControl.noCache())
        .eTag("receipt-7-v3")
        .body("receipt:7:reviewed");
}

Verification boundary

ConditionalReceiptHttpTest.returnsA304OnlyForTheMatchingRepresentationTag runs in the Spring source kit. This excerpt omits imports and surrounding test setup; the downloadable kit contains the complete source.

Costs and boundaries

MockMvc checks the response status, tag and body for current, matching and stale requests. This fixed in-process controller does not measure upstream computation, proxy caching, deployment behavior or private-data isolation.

Common Mistakes

• Do not treat ETag possession as permission to read a resource.
• Do not reuse one tag across different visible representations.
• Do not claim a shallow ETag filter avoids generating the response body.

Read next

Spring MVC request validation: reject invalid commands before mutation, Spring API pagination: bounded requests and immutable snapshots, Spring Security filter chain: authentication, CSRF and request order.

Continue with range and graph boundaries

Continue with Spring If-Match writes: reject a stale receipt revision.