What's the point of Spring MVC's DelegatingFilterProxy?

Question

There's some kind of magic here, but at the end, everything is a deterministic program.

The DelegatingFilterProxy is a Filter as it was explained above, whose goal is "delegating to a Spring-managed bean that implements the Filter interface", that is, it finds a bean ("target bean" or "delegate") in your Spring application context and invokes it. How is it possible? Because this bean implements javax.servlet.Filter, its doFilter method is called.

Which bean is called? the DelegatingFilterProxy "Supports a "targetBeanName" [...], specifying the name of the target bean in the Spring application context."

As you saw in your web.xml that the bean's name is "springSecurityFilterChain".

So, in the context of a web application, a Filter instantiates a bean called "springSecurityFilterChain" in your application context and then delegate to it via the doFilter() method.

Remember, your application context is defined with ALL THE APPLICATION-CONTEXT (XML) files. For instance: applicationContext.xml AND applicationContext-security.xml.

So try to find a bean called "springSecurityFilterChain" in the latter...

...and probably you can't (for instance if you followed a tutorial or if you configured the security using Roo)

Here is the magic: there's a new element for configuring the security, something like

<http auto-config="true" use-expressions="true"> 

as it is allowed by http://www.springframework.org/schema/security/spring-security-3.0.xsd, will do the trick.

When Spring loads the application context using XML files, if it finds a element, it will try to set up the HTTP security, that is, a filter stack and protected URLs and to register the FilterChainProxy named "springSecurityFilterChain".

Alternatively, you can define the bean in the classic way, that is:

<beans:bean id="springSecurityFilterChain" class="org.springframework.security.web.FilterChainProxy">

But it's less recommended, since you need to do a lot of configuration (all the filters that you're going to use. And there are more than a dozen of them)

Do you know what a Servlet Filter is and how it works? It's a very useful piece of the Servlet Spec, allowing us to apply AOP-like concepts to servicing of HTTP requests. Many frameworks use Filter implementations for various things, and it's not uncommon to find custom implementations of them because they're very simple to write and useful. In a Spring app, most of the stuff that your app can do is in your Spring beans. A Filter instance, though, is controlled by the Servlet container. The container instantiates, initializes, and destroys it. The Servlet Spec doesn't require any kind of Spring integration, though, so you're left with a really useful concept (Filters) with no convenient way of tying it to your Spring app and the beans that do the work.

Enter the DelegatingFilterProxy. You write a Filter implementation and make it a Spring bean, but instead of adding your own Filter class to the web.xml, you use the DelegatingFilterProxy, and give it the bean name of your filter in the Spring context. (If you don't explicitly provide a name, it uses the "filter-name".) Then at runtime, the DelegatingFilterProxy handles the complexity of finding the real implementation - the one you wrote and configured in Spring - and routing requests to it. So at runtime, it's as if you had listed your filter in the web.xml, but you get the benefit of being able to wire it like any other Spring bean.

If you take that filter mapping out of your web.xml, everything will continue working, but none of your URLs will be secured. (That's assuming the name "springSecurityFilterChain" accurately describes what it does.) That's because this mapping is filtering every incoming request and handing it off to a security filter that's defined in your Spring context.

What are Servlet Filters?

Servlet filters are, in general, a Java WebApp concept. You can have servlet filters in any webapp, whether or not you use Spring framework in your application.

These filters can intercept requests before they reach the target servlet. You can implement common functionality, like authorization, in servlet filters. Once implemented, you can configure the filter in your web.xml to be applied to a specific servlet, specific request url patterns or all url patterns.

Where servlet filters are used?

Modern web-apps can have dozens of such filters. Things like authorization, caching, ORM session management, and dependency injection are often implemented with the aid of servlet filter. All of these filters need to be registered in web.xml.

Instantiating Servlet Filters - without Spring Framework

Your servlet container creates instances of Filters declared in web.xml and calls them at appropriate times (i.e., when servicing servlet requests). Now if you are like most of the Dependency Injection (DI) fans, you would likely say that creation of instances is what my DI framework (Spring) does better. Can't I get my servlet filters created with Spring so they are amenable to all DI goodness?

DelegatingFilterProxy, so that Spring creates your filter instances

This is where DelegatingFilterProxy steps in. DelegatingFilterProxy is an impelmentation of the javax.servlet.Filter interface provided by Spring Framework. Once you configure DelegatingFilterProxy in web.xml, you can declare the actual beans that do the filtering in your spring configuration. This way, Spring creates the instances of beans that do the actual filtering, and you can use DI to configure these beans.

Note that you need only a single DelegatingFilterProxy declaration in web.xml but you can have several filtering beans chained together in your application context.

The thing is, servlet filters are managed by the servlet container, and not by spring. And you may need to inject some spring components into your filters.

So, if you need something like:

public class FooFilter {

    @Inject
    private FooService service;

    public void doFilter(....) { .. }

}

then you need the delegating filter proxy.

You are right about 'glue' stuff. As written in JavaDocs of FilterChainProxy:

The FilterChainProxy is linked into the servlet container filter chain by adding a standard Spring DelegatingFilterProxy declaration in the application web.xml file.

Please see FIlterChainProxy section of blog Behind the Spring Security Namespace for an excellent explanation.