= OAuth 2.0 Resource Server Bearer Tokens

[[webflux-oauth2resourceserver-bearertoken-resolver]]
== Bearer Token Resolution

By default, Resource Server looks for a bearer token in the `Authorization` header.
This, however, can be customized.

For example, you may have a need to read the bearer token from a custom header.
To achieve this, you can wire an instance of `ServerBearerTokenAuthenticationConverter` into the DSL, as you can see in the following example:

.Custom Bearer Token Header
====
.Java
[source,java,role="primary"]
----
ServerBearerTokenAuthenticationConverter converter = new ServerBearerTokenAuthenticationConverter();
converter.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION);
http
    .oauth2ResourceServer(oauth2 -> oauth2
        .bearerTokenConverter(converter)
    );
----

.Kotlin
[source,kotlin,role="secondary"]
----
val converter = ServerBearerTokenAuthenticationConverter()
converter.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION)
return http {
    oauth2ResourceServer {
        bearerTokenConverter = converter
    }
}
----
====

== Bearer Token Propagation

Now that you're in possession of a bearer token, it might be handy to pass that to downstream services.
This is quite simple with `{security-api-url}org/springframework/security/oauth2/server/resource/web/reactive/function/client/ServerBearerExchangeFilterFunction.html[ServerBearerExchangeFilterFunction]`, which you can see in the following example:

====
.Java
[source,java,role="primary"]
----
@Bean
public WebClient rest() {
    return WebClient.builder()
            .filter(new ServerBearerExchangeFilterFunction())
            .build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun rest(): WebClient {
    return WebClient.builder()
            .filter(ServerBearerExchangeFilterFunction())
            .build()
}
----
====

When the above `WebClient` is used to perform requests, Spring Security will look up the current `Authentication` and extract any `{security-api-url}org/springframework/security/oauth2/core/AbstractOAuth2Token.html[AbstractOAuth2Token]` credential.
Then, it will propagate that token in the `Authorization` header.

For example:

====
.Java
[source,java,role="primary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .retrieve()
        .bodyToMono(String.class)
----

.Kotlin
[source,kotlin,role="secondary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .retrieve()
        .bodyToMono<String>()
----
====

Will invoke the `https://other-service.example.com/endpoint`, adding the bearer token `Authorization` header for you.

In places where you need to override this behavior, it's a simple matter of supplying the header yourself, like so:

====
.Java
[source,java,role="primary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .headers(headers -> headers.setBearerAuth(overridingToken))
        .retrieve()
        .bodyToMono(String.class)
----

.Kotlin
[source,kotlin,role="secondary"]
----
rest.get()
        .uri("https://other-service.example.com/endpoint")
        .headers { it.setBearerAuth(overridingToken) }
        .retrieve()
        .bodyToMono<String>()
----
====

In this case, the filter will fall back and simply forward the request onto the rest of the web filter chain.

[NOTE]
Unlike the https://docs.spring.io/spring-security/site/docs/current-SNAPSHOT/api/org/springframework/security/oauth2/client/web/reactive/function/client/ServerOAuth2AuthorizedClientExchangeFilterFunction.html[OAuth 2.0 Client filter function], this filter function makes no attempt to renew the token, should it be expired.
To obtain this level of support, please use the OAuth 2.0 Client filter.