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docs: document that SimpleClientHttpRequestFactory is not suitable for WebMVC gateway #4196

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1 change: 1 addition & 0 deletions docs/modules/ROOT/nav.adoc
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* xref:spring-cloud-gateway-server-webmvc.adoc[]
** xref:spring-cloud-gateway-server-webmvc/starter.adoc[]
** xref:spring-cloud-gateway-server-webmvc/http-client.adoc[]
** xref:spring-cloud-gateway-server-webmvc/glossary.adoc[]
** xref:spring-cloud-gateway-server-webmvc/how-it-works.adoc[]
** xref:spring-cloud-gateway-server-webmvc/java-routes-api.adoc[]
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62 changes: 62 additions & 0 deletions docs/modules/ROOT/pages/spring-cloud-gateway-server-webmvc/http-client.adoc
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[[http-client]]
= HTTP Client

Spring Cloud Gateway Server MVC uses Spring Boot's `RestClient` infrastructure to forward requests to backend services.
The underlying `ClientHttpRequestFactory` is resolved from the application context, driven by `spring.http.clients.imperative.factory`.

IMPORTANT: `SimpleClientHttpRequestFactory` (backed by `java.net.HttpURLConnection`) is **not suitable** for the WebMVC gateway.
`HttpURLConnection` throws a `java.io.IOException` (such as `FileNotFoundException` for 404 responses) instead of providing the response body for 4xx/5xx status codes.
The gateway cannot forward error responses correctly in this case, and the client will receive an `HTTP 500` instead of the actual upstream error status.

== Selecting an HTTP Client

Use one of the following supported implementations:

=== JDK HttpClient (recommended, no extra dependency)

@spencergibb spencergibb Jun 9, 2026

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This does not need to be done as it is the default


Set the factory to `jdk` to use Java's built-in `HttpClient` (available since Java 11):

.application.yml
[source,yaml]
----
spring:
http:
clients:
imperative:
factory: jdk
----

=== OkHttp

Add `com.squareup.okhttp3:okhttp` to your classpath. The factory is selected automatically when `okhttp3.OkHttpClient` is on the classpath, or you can set it explicitly:

.application.yml
[source,yaml]
----
spring:
http:
clients:
imperative:
factory: okhttp
----

=== Apache HttpComponents

Add `org.apache.httpcomponents.client5:httpclient5` to your classpath, or set the factory explicitly:

.application.yml
[source,yaml]
----
spring:
http:
clients:
imperative:
factory: apache
----

NOTE: If you have OkHttp or Apache HttpComponents on the classpath, Spring Boot's auto-detection will prefer them over `SimpleClientHttpRequestFactory`, so the IMPORTANT notice above only applies when running without any of those libraries.

== Related Documentation

* xref:spring-cloud-gateway-server-webmvc/tls-and-ssl.adoc[TLS and SSL] – configure SSL bundles for backend calls
* https://docs.spring.io/spring-boot/reference/io/rest-client.html#io.rest-client.restclient[Spring Boot RestClient documentation]