When using servlet filters, you obviously need to declare them in your web.xml, or they will be ignored by the servlet container. In Spring Security, the filter classes are also Spring beans defined in the application context and thus able to take advantage of Spring's rich dependency-injection facilities and lifecycle interfaces. Spring's DelegatingFilterProxy provides the link between web.xml and the application context.

When using DelegatingFilterProxy, you will see something like this in the web.xml file:

    
  <filter>
    <filter-name>myFilter</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
  </filter>

  <filter-mapping>
    <filter-name>myFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
      

Notice that the filter is actually a DelegatingFilterProxy, and not the class that will actually implement the logic of the filter. What DelegatingFilterProxy does is delegate the Filter's methods through to a bean which is obtained from the Spring application context. This enables the bean to benefit from the Spring web application context lifecycle support and configuration flexibility. The bean must implement javax.servlet.Filter and it must have the same name as that in the filter-name element. Read the Javadoc for DelegatingFilterProxy for more information

It should now be clear that you can declare each Spring Security filter bean that you require in your application context file and add a corresponding DelegatingFilterProxy entry to web.xml for each filter, making sure that they are ordered correctly. This is a cumbersome approach and clutters up the web.xml file quickly if we have a lot of filters. We would prefer to just add a single entry to web.xml and deal entirely with the application context file for managing our web security beans. This is where Spring Secuiryt's FilterChainProxy comes in. It is wired using a DelegatingFilterProxy, just like in the example above, but with the filter-name set to the bean name “filterChainProxy”. The filter chain is then declared in the application context with the same bean name. Here's an example:

<bean id="filterChainProxy" class="org.springframework.security.web.FilterChainProxy">
  <sec:filter-chain-map path-type="ant">
     <sec:filter-chain pattern="/webServices/**" filters="
           securityContextPersistenceFilterWithASCFalse,
           basicAuthenticationFilter,
           exceptionTranslationFilter,
           filterSecurityInterceptor" />
     <sec:filter-chain pattern="/**" filters="
           securityContextPersistenceFilterWithASCTrue,
           formLoginFilter,
           exceptionTranslationFilter,
           filterSecurityInterceptor" />
  </sec:filter-chain-map>
</bean>
         
    

The namespace element filter-chain-map is used to set up the security filter chain(s) which are required within the application^[4]. It maps a particular URL pattern to a chain of filters built up from the bean names specified in the filters element. Both regular expressions and Ant Paths are supported, and the most specific URIs appear first. At runtime the FilterChainProxy will locate the first URI pattern that matches the current web request and the list of filter beans specified by the filters attribute will be applied to that request. The filters will be invoked in the order they are defined, so you have complete control over the filter chain which is applied to a particular URL.

You may have noticed we have declared two SecurityContextPersistenceFilters in the filter chain (ASC is short for allowSessionCreation, a property of SecurityContextPersistenceFilter). As web services will never present a jsessionid on future requests, creating HttpSessions for such user agents would be wasteful. If you had a high-volume application which required maximum scalability, we recommend you use the approach shown above. For smaller applications, using a single SecurityContextPersistenceFilter (with its default allowSessionCreation as true) would likely be sufficient.

In relation to lifecycle issues, the FilterChainProxy will always delegate init(FilterConfig) and destroy() methods through to the underlaying Filters if such methods are called against FilterChainProxy itself. In this case, FilterChainProxy guarantees to only initialize and destroy each Filter bean once, no matter how many times it is declared in the filter chain(s). You control the overall choice as to whether these methods are called or not via the targetFilterLifecycle initialization parameter of DelegatingFilterProxy. By default this property is false and servlet container lifecycle invocations are not delegated through DelegatingFilterProxy.

When we looked at how to set up web security using namespace configuration, we used a DelegatingFilterProxy with the name “springSecurityFilterChain”. You should now be able to see that this is the name of the FilterChainProxy which is created by the namespace.

The order that filters are defined in the chain is very important. Irrespective of which filters you are actually using, the order should be as follows:

If you're using some other framework that is also filter-based, then you need to make sure that the Spring Security filters come first. This enables the SecurityContextHolder to be populated in time for use by the other filters. Examples are the use of SiteMesh to decorate your web pages or a web framework like Wicket which uses a filter to handle its requests.