WebClient ExchangeFilterFunction unable to add header java.lang.UnsupportedOperationException ReadOnlyHttpHeaders - spring-boot

Question

I am trying to add a simple ExchangeFilterFunction to a WebClient request. However, I am seeing the following exception:
java.lang.UnsupportedOperationException
at org.springframework.http.ReadOnlyHttpHeaders.add(ReadOnlyHttpHeaders.java:84)
at com.ecs.springframework.reactive.web.filter.AddHeaderExchangeFilter.filter(AddHeaderExchangeFilter.java:24)
at org.springframework.web.reactive.function.client.ExchangeFilterFunction.lambda$apply$2(ExchangeFilterFunction.java:66)
at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.exchange(DefaultWebClient.java:319)
at com.ecs.springframework.reactive.web.filter.AddHeaderExchangeFilterTest.whenExchangeFilterFunctionInjectedIntoWebClient_thenWebClientShouldPropagate(AddHeaderExchangeFilterTest.java:57)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:532)
at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:115)
at
ExchangeFilterFunction
public class AddHeaderExchangeFilter implements ExchangeFilterFunction {
#Override
public Mono<ClientResponse> filter(ClientRequest request, ExchangeFunction next) {
ClientRequest newRequest = ClientRequest.create(request.method(), request.url()).build();
newRequest.headers().add("EFF-TEST-HEADER", "EFF-TEST-VALUE");
return next.exchange(newRequest);
}
}
WebClient webTestClient = WebClient.builder().filter(new AddHeaderExchangeFilter()).build();
webTestClient.get().uri("http://httpbin.org/get").exchange();
I am relatively new to WebFlux so any pointers would be appreciated.

Answer 1

You need to add the headers when you creating newRequest:
public class AddHeaderExchangeFilter implements ExchangeFilterFunction {
#Override
public Mono<ClientResponse> filter(ClientRequest request, ExchangeFunction next) {
ClientRequest newRequest = ClientRequest.from(request)
.header("EFF-TEST-HEADER", "EFF-TEST-VALUE")
.build();
return next.exchange(newRequest);
}
}
The reason for your current behavior is that ClientRequest.create initiating new DefaultClientRequestBuilder.
You can see in the Spring's source code that once you build() your request the builder building the request with read only headers, and therefore you can't add the headers after you built the ClientRequest.