Base Controller interface, representing a component that receives HttpServletRequest and HttpServletResponse instances just like a HttpServlet but is able to participate in an MVC workflow. Controllers are comparable to the notion of a Struts Action.

Any implementation of the Controller interface should be a reusable, thread-safe class, capable of handling multiple HTTP requests throughout the lifecycle of an application. To be able to configure a Controller easily, Controller implementations are encouraged to be (and usually are) JavaBeans.

Workflow

After a DispatcherServlet has received a request and has done its work to resolve locales, themes, and suchlike, it then tries to resolve a Controller, using a HandlerMapping. When a Controller has been found to handle the request, the handleRequest method of the located Controller will be invoked; the located Controller is then responsible for handling the actual request and — if applicable — returning an appropriate ModelAndView. So actually, this method is the main entry point for the DispatcherServlet which delegates requests to controllers.

So basically any direct implementation of the Controller interface just handles HttpServletRequests and should return a ModelAndView, to be further interpreted by the DispatcherServlet. Any additional functionality such as optional validation, form handling, etc. should be obtained through extending AbstractController or one of its subclasses.

Notes on design and testing

The Controller interface is explicitly designed to operate on HttpServletRequest and HttpServletResponse objects, just like an HttpServlet. It does not aim to decouple itself from the Servlet API, in contrast to, for example, WebWork, JSF or Tapestry. Instead, the full power of the Servlet API is available, allowing Controllers to be general-purpose: a Controller is able to not only handle web user interface requests but also to process remoting protocols or to generate reports on demand.

Controllers can easily be tested by passing in mock objects for the HttpServletRequest and HttpServletResponse objects as parameters to the handleRequest method. As a convenience, Spring ships with a set of Servlet API mocks that are suitable for testing any kind of web components, but are particularly suitable for testing Spring web controllers. In contrast to a Struts Action, there is no need to mock the ActionServlet or any other infrastructure; mocking HttpServletRequest and HttpServletResponse is sufficient.

If Controllers need to be aware of specific environment references, they can choose to implement specific awareness interfaces, just like any other bean in a Spring (web) application context can do, for example:

• org.springframework.context.ApplicationContextAware
• org.springframework.context.ResourceLoaderAware
• org.springframework.web.context.ServletContextAware

Such environment references can easily be passed in testing environments, through the corresponding setters defined in the respective awareness interfaces. In general, it is recommended to keep the dependencies as minimal as possible: for example, if all you need is resource loading, implement ResourceLoaderAware only. Alternatively, derive from the WebApplicationObjectSupport base class, which gives you all those references through convenient accessors but requires an ApplicationContext reference on initialization.

Controllers can use the checkNotModified methods on org.springframework.web.context.request.WebRequest for HTTP caching support.