Ease of Development (Eod)

@WebServlet("/foo")
public class MyServlet extends HttpServlet {
    public void doGet(...) {
        ....
    }
}

Pluggability

• Methods to add Servlets and Filters - If a ServletContextListener is registered and wants to add a Servlet or Filter, then at the time of context initialization the event that is fired to the Listener can add Servlets and Filters to the context. The methods are addServlet and addFilter. For more details look for the javadocs when the specification is published.
• Web.xml fragments - Instead of having just one monolithic web.xml that is used by the developer to declare servlets, filters and other configuration for using the framework, the framework can now include a web-fragment.xml with all it's configuration. A web-fragment is almost identical to the web.xml and can contain all the configuration needed by the framework in the META-INF directory of the framework's jar file. The container will use the information to assembe the descriptor for the application and the application needn't have to use any of the boilerplate configuration in their app for the framework. There are rules that are defined int the specification for conflict resolution, overriding and disabling fragment scanning. Along with the annotations which also can be in libraries the feature is very compelling not only for the developers that use frameworks but also for framework authors to be self sufficient in defining the configuration needed.

Asynchronous processing

• This is the biggest change that we have made in the servlet 3.0 specification. In the early draft we had suspend and resume and certain semantics that were defined. However after the early draft the expert group had a lot of discussions on solving various use cases for asynchronous processing and the changes that are made to the specification in this area now also address the various use cases.
• Instead of suspend and resume we now have a startAsync method on request and there are two variations of these. One that takes a request and response in which case the AsyncContext that is created when you call startAsync will be initialized with the request and response passed in. The other is a startAsync() that takes no parameters, in which case the AsyncContext is initialized with the original request and response. Also the Servlets and Filters that support asynchronous processing must be annotated appropriately with asyncSupport attribute set. It is illegal to call startAsync if anywhere in the request processing chain you have have a servlet or a filter that does not support async.
• In addition to the AsyncContext there is also an AsyncListener that can be registered to get notifications on timeout and completion of async processing of a request. The event listener can be used to clean up resources if wrapped request and responses were used to initialize the AsyncContext.