Frequently many handlers are combined together to handle different aspects of
a request. A handler may:
- handle the request and completely generate the response
- partially handle the request, but defer response generation to another
handler.
- select another handler to pass the request to.
- use business logic to decide to do one of the above.
Multiple handlers may be combined with:
HandlerWrapper which will nest one handler inside another. In
this example, the HelloHandler is nested inside a HandlerWrapper that sets
the greeting as a request attribute.
HandlerList which will call a collection of handlers until the
request is marked as handled. In this example, a list is used to combine the
param handler (which only handles the request if there are parameters) and
the wrapper handler. Frequently handler lists are terminated with the
DefaultHandler, which will generate a suitable 404 response if the
request has not been handled.
HandlerCollection which will call each handler regardless if the
request has been handled or not. Typically this is used to always pass a
request to the logging handler.