Spring Security provides three classes that together provide an anonymous authentication feature. AnonymousAuthenticationToken is an implementation of Authentication, and stores the GrantedAuthority[]s which apply to the anonymous principal. There is a corresponding AnonymousAuthenticationProvider, which is chained into the ProviderManager so that AnonymousAuthenticationTokens are accepted. Finally, there is an AnonymousProcessingFilter, which is chained after the normal authentication mechanisms and automatically add an AnonymousAuthenticationToken to the SecurityContextHolder if there is no existing Authentication held there. The definition of the filter and authentication provider appears as follows:

<bean id="anonymousProcessingFilter"
    class="org.springframework.security.providers.anonymous.AnonymousProcessingFilter">
  <property name="key" value="foobar"/>
  <property name="userAttribute" value="anonymousUser,ROLE_ANONYMOUS"/>
</bean>

<bean id="anonymousAuthenticationProvider"
    class="org.springframework.security.providers.anonymous.AnonymousAuthenticationProvider">
  <property name="key" value="foobar"/>
</bean>
    

The key is shared between the filter and authentication provider, so that tokens created by the former are accepted by the latter. The userAttribute is expressed in the form of usernameInTheAuthenticationToken,grantedAuthority[,grantedAuthority]. This is the same syntax as used after the equals sign for InMemoryDaoImpl's userMap property.

As explained earlier, the benefit of anonymous authentication is that all URI patterns can have security applied to them. For example:

<bean id="filterInvocationInterceptor"
    class="org.springframework.security.intercept.web.FilterSecurityInterceptor">
  <property name="authenticationManager" ref="authenticationManager"/>
  <property name="accessDecisionManager" ref="httpRequestAccessDecisionManager"/>
  <property name="objectDefinitionSource">
    <security:filter-invocation-definition-source>
      <security:intercept-url pattern='/index.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/hello.htm' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/logoff.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/login.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/**' access='ROLE_USER'/>
    </security:filter-invocation-definition-source>" +
  </property>
</bean>
    

Rounding out the anonymous authentication discussion is the AuthenticationTrustResolver interface, with its corresponding AuthenticationTrustResolverImpl implementation. This interface provides an isAnonymous(Authentication) method, which allows interested classes to take into account this special type of authentication status. The ExceptionTranslationFilter uses this interface in processing AccessDeniedExceptions. If an AccessDeniedException is thrown, and the authentication is of an anonymous type, instead of throwing a 403 (forbidden) response, the filter will instead commence the AuthenticationEntryPoint so the principal can authenticate properly. This is a necessary distinction, otherwise principals would always be deemed "authenticated" and never be given an opportunity to login via form, basic, digest or some other normal authentication mechanism