java 37 lines · 8 steps

Merging default and per-request headers in Spring

A WebClient filter that layers global default headers under optional per-request overrides on every outbound call.

Explained by highlit
Spring Intermediate webclient filters http-headers dependency-injection functional
1@Component
2public class HeaderMergeFilter {
3 
4 private static final String PER_REQUEST_HEADERS = HeaderMergeFilter.class.getName() + ".headers";
5 
6 private final Map<String, String> globalHeaders;
7 
8 public HeaderMergeFilter(HttpHeaderProperties properties) {
9 this.globalHeaders = properties.getDefaults();
10 }
11 
12 public ExchangeFilterFunction asFilter() {
13 return (request, next) -> {
14 ClientRequest merged = ClientRequest.from(request)
15 .headers(headers -> {
16 globalHeaders.forEach(headers::putIfAbsent);
17 HttpHeaders overrides = request.attribute(PER_REQUEST_HEADERS)
18 .map(HttpHeaders.class::cast)
19 .orElseGet(HttpHeaders::new);
20 overrides.forEach((name, values) -> headers.put(name, values));
21 })
22 .build();
23 return next.exchange(merged);
24 };
25 }
26 
27 public static Consumer<ClientRequest.Builder> withHeaders(HttpHeaders headers) {
28 return builder -> builder.attribute(PER_REQUEST_HEADERS, headers);
29 }
30 
31 @Bean
32 public WebClient outboundWebClient(WebClient.Builder builder) {
33 return builder
34 .filter(asFilter())
35 .build();
36 }
37}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Storing per-request data in a request attribute lets you thread configuration through a filter without changing method signatures.
  2. 2putIfAbsent versus put cleanly encodes precedence: defaults fill gaps while overrides win outright.
  3. 3Exposing behavior as an ExchangeFilterFunction keeps header logic in one place and applies it to every WebClient call.

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