Allow AuthenticationProvider Bean in Java Config

This commit adds support for defaulting java configuration's
authentication by providing an AuthenticationProvider Bean.

Fixes gh-3091

docs/manual/src/docs/asciidoc/index.adoc

@@ -378,6 +378,7 @@ This will give you access to the entire project history (including all releases
 * <<el-access-web-path-variables,Path Variables in Web Security Expressions>>
 * <<test-method-withanonymoususer,@WithAnonymousUser>>
 * <<jc-authentication-userdetailsservice,Simplified UserDetailsService Java Configuration>>
+* <<jc-authentication-authenticationprovider,Simplified AuthenticationProvider Java Configuration>>
 
 === What's new in Spring Security 4.0
 
@@ -916,17 +917,35 @@ cn: admin
 uniqueMember: uid=admin,ou=people,dc=springframework,dc=org
 ----
 
+[[jc-authentication-authenticationprovider]]
+==== AuthenticationProvider
+
+You can define custom authentication by exposing a custom `AuthenticationProvider` as a bean.
+For example, the following will customize authentication assuming that `SpringAuthenticationProvider` implements `AuthenticationProvider`:
+
+NOTE: This is only used if the `AuthenticationManagerBuilder` has not been populated
+
+[source,java]
+----
+@Bean
+public SpringAuthenticationProvider springAuthenticationProvider() {
+	return new SpringAuthenticationProvider();
+}
+----
+
 [[jc-authentication-userdetailsservice]]
 ==== UserDetailsService
 
 You can define custom authentication by exposing a custom `UserDetailsService` as a bean.
 For example, the following will customize authentication assuming that `SpringDataUserDetailsService` implements `UserDetailsService`:
 
+NOTE: This is only used if the `AuthenticationManagerBuilder` has not been populated and no `AuthenticationProviderBean` is defined.
+
 [source,java]
 ----
 @Bean
 public SpringDataUserDetailsService springDataUserDetailsService() {
-		return new SpringDataUserDetailsService();
+	return new SpringDataUserDetailsService();
 }
 ----
 
@@ -937,7 +956,7 @@ For example, if you use bcrypt you can add a bean definition as shown below:
 ----
 @Bean
 public BCryptPasswordEncoder passwordEncoder() {
-		return new BCryptPasswordEncoder();
+	return new BCryptPasswordEncoder();
 }
 ----