[BUG] Spring SDK dynamic headers mutate immutable or template-owned value collections #7771

Current behavior

The Spring contract creates static header values with Collections.singleton, while request/template cloning copies at most the outer header map. RequestHeaderParameterProcessor then appends dynamic values to the existing value collection.

There are two manifestations of the same collection-ownership problem:

1. Normal annotation-created static headers are immutable: combining one with a dynamic parameter for the same header throws before any transport request.
2. If a programmatic/customized template uses mutable value collections, the shallow copy shares those values with the reusable template. Dynamic values accumulate across subsequent invocations.

Locations / reachable flow

• SpringMvcContract.java:123–129 creates singleton static values.
• RequestTemplate.java:81–103 shares/copies only the outer map.
• RequestHeaderParameterProcessor.java:49–77 mutates existing collections.
• CookieValueParameterProcessor.java:54–62 has the same ownership pattern, taking the collection from the template before merging it into request headers.

Reproduction / validation

Define this ordinary Spring SDK method:

@GetMapping(value = "/items", headers = "X-Mode=static")
String call(@RequestHeader("X-Mode") String value);

A probe compiled the current contract, RequestTemplate, response handler and header processor. It parsed the actual interface through SpringMvcContract, then called public ShenyuClientMethodHandler.invoke() with a recording in-process transport:

BUILTIN_CONTRACT_EXCEPTION=UnsupportedOperationException TRANSPORT_CALLS=0

As a separate control, the probe changed only that parsed template's value collection to a mutable [static], then invoked with one and two:

MUTABLE_TEMPLATE_REQUESTS=[[static, one], [static, one, two]]
TEMPLATE_AFTER_CALLS=[static, one, two]

The ordinary built-in case is the first-call exception, not repeated accumulation from a built-in ArrayList. Independent review corrected that initial assumption after inspecting Collections.singleton. The executed reproduction covers the RequestHeader branch; the CookieValue counterpart is source-inspected. No external HTTP service or sensitive header values were involved.

Impact / suggested fix

Combining supported static and dynamic headers fails; mutable reusable templates can also retain prior invocation state. Give each invocation mutable, independently owned value collections before parameter processing, and avoid modifying template-owned collections in the processors. Preserve the intended append/replace contract without duplicating static cookie/header values during merges.

Related existing work

#7711 concerns response status handling, #7712 path replacement syntax, #7464 query encoding and #7465 annotation name aliases. #6254 concerns a different MCP request-template header substitution path. Complete-title/open-PR and live RequestTemplate/header/UnsupportedOperationException searches found no matching collection-ownership report.

Reviewed revision

Apache ShenYu 2.7.2-SNAPSHOT, master@66d7d954ca37d651a5ad916f988a8081157a7ab4. P2 / Medium; high confidence. Unit-test and CI issue categories excluded.