BearerTokenResolver Docs

Fixes gh-6254

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/access-token.adoc

@@ -33,3 +33,46 @@ SecurityWebFilterChain configure(ServerHttpSecurity http) throws Exception {
 ----
 
 You can now leverage Spring Security's <<webclient>> or <<webflux-roac,@RegisteredOAuth2AuthorizedClient>> support to obtain and use the access token.
+
+== BearerTokenResolver
+
+With interface BearerTokenResolver you can provide a strategy to resolve a bearer token.
+
+The interface provides the next method:
+
+[source,java]
+----
+/**
+* Resolve any <a href="https://tools.ietf.org/html/rfc6750#section-1.2" target="_blank">Bearer Token</a>
+* value from the request.
+*
+* @param request the request
+* @return the Bearer Token value or {@code null} if none found
+* @throws OAuth2AuthenticationException if the found token is invalid
+*/
+String resolve(HttpServletRequest request);
+----
+
+In code base, you can find two implementation of this interface:
+HeaderBearerTokenResolver and DefaultBearerTokenResolver (based on RFC 6750).
+
+Below you can see HeaderBearerTokenResolver, it takes a bearer token from request by header
+which was passed in constructor
+
+[source,java]
+----
+public class HeaderBearerTokenResolver implements BearerTokenResolver {
+
+	private String header;
+
+	public HeaderBearerTokenResolver(String header) {
+		Assert.hasText(header, "header cannot be empty");
+		this.header = header;
+	}
+
+	@Override
+	public String resolve(HttpServletRequest request) {
+		return request.getHeader(this.header);
+	}
+}
+----