@Component
public class CustomAuthenticationProvider
 implements AuthenticationProvider {
 
 @Override
 public Authentication authenticate(Authentication authentication) 
  throws AuthenticationException {
 
  String name = authentication.getName();
  String password = authentication.getCredentials().toString();
   
  if (shouldAuthenticateAgainstThirdPartySystem()) {
 
   // use the credentials
   // and authenticate against the third-party system
   return new UsernamePasswordAuthenticationToken(
    name, password, new ArrayList<>());
  } else {
   return null;
  }
 }
 
 @Override
 public boolean supports(Class<?> authentication) {
  return authentication.equals(
   UsernamePasswordAuthenticationToken.class);
 }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans
 xmlns="http://www.springframework.org/schema/security"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns:beans="http://www.springframework.org/schema/beans"
 xsi:schemaLocation="
 http://www.springframework.org/schema/security 
 http://www.springframework.org/schema/security/spring-security-4.0.xsd
 http://www.springframework.org/schema/beans 
 http://www.springframework.org/schema/beans/spring-beans-4.2.xsd">
 
 <http use-expressions="true">
  <intercept-url pattern="/**" access="isAuthenticated()"/>
  <http-basic/>
 </http>
 
 <authentication-manager>
  <authentication-provider
   ref="customAuthenticationProvider" />
 </authentication-manager>
 
</beans:beans>

@Configuration
@EnableWebSecurity
@ComponentScan("org.baeldung.security")
public class SecurityConfig extends WebSecurityConfigurerAdapter {
 
 @Autowired
 private CustomAuthenticationProvider authProvider;
 
 @Override
 protected void configure(
  AuthenticationManagerBuilder auth) throws Exception {
 
  auth.authenticationProvider(authProvider);
 }
 
 @Override
 protected void configure(HttpSecurity http) throws Exception {
  http.authorizeRequests().anyRequest().authenticated()
   .and()
   .httpBasic();
 }
}

curl --header "Accept:application/json" -i --user user1:user1Pass 
 http://localhost:8080/spring-security-custom/api/foo/1

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Set-Cookie: JSESSIONID=B8F0EFA81B78DE968088EBB9AFD85A60; Path=/spring-security-custom/; HttpOnly
Content-Type: application/json;charset=UTF-8
Transfer-Encoding: chunked
Date: Sun, 02 Jun 2013 17:50:40 GMT