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Nested Handler Mappings in Spring

From: "andrew cooke" <andrew@...>

Date: Tue, 14 Mar 2006 19:20:02 -0400 (CLT)

The MVC support in Spring includes the idea of handler mappings - an
incoming HTTP request is passed to an (ordered) set of handlers, and each
in turn attempts to provide a controller.  This is nice, because
controllers are typically associated with URLs, so you can put the URL to
controller login in the handler (note to self - maybe I can do more of
that).

I have generic handlers that do things like pull parameters from the end
of URLs (a simple kind of URL rewriting), for example.  Others that make
decisions based on the state of the database (for URLs associated with
objects that have several significantly different states -
design/preparation and use, for example).

However, there are times when you really want to chain Handlers.  This
isn't possible with standard Spring because (1) the thing returned by a
handler should be a controller, not another handler and (2) every bean
that is an instance of the handler interface is pulled from the context
and called directly - you don't specify the handler list explicitly.

The second point means that there's a risk that your nested handlers are
called directly.

However, one useful wrinkle - handlers return Object.  So there's nothing
in the method signature to stop one handler returning another.

So, what to do?  I tried writing a different interface, but that didn't
work so well - sometimes it's not clear whether a handler should be a top
level handler or a nested level handler (if it's generic, it may be both).
 And a lot of functionality was provided by Spring superclasses, so
starting a new hierarchy lost much of that.

I couldn't find a perfect solution, but in the end I've gone with the
following, which works pretty well:

- an IterationHandler that drives the iterative process.  This is a top
level handler has a subhandler which it calls and, if that returns a
handler itself, it calls that handler in turn.

- the IterationHandler also propogates interceptors along the chain.  This
saves you from having to define the same bunch of interceptors for all
nested handlers - you just put them in the top IteratorHandler.

- a NestableHandler abstract class extends AbstractHandlerMapping and
checks the Session attributes to see if IterationHandler has set a flag. 
This indicates whether or not the subclass is being called at the top
level, and is compared against a configuration parameter.  If a handler is
intended only for nested use then it returns null when called directly -
this is the normal way for handlers to "pass".

- the NestedHandler implements AbstractHandlerMapping's getHandlerInternal
interface as final, passing checked calls to a second, abstract method. 
In this way any subclass automatically (and unavoidably) is checked.

So it works pretty much as required - the only nuisance is that extra
calls may be made from the top level to handlers that will not return
controllers.  I think this can be avoided by making the bean anonymous
(nesting its definition in the IterationHandler bean within Spring). 
However, that is not always desirable.

Andrew

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