[Springframework-developer] Ordering HandlerMappings and other beans

[<jue...@we...>]

Hi Rod, everyone,

I've just developed a custom HandlerMapping strategy for one of our =
applications and faced the issue that I wanted to retrieve beans of a =
certain class from the ApplicationContext in a specifyable order. =
Looking for a solution, I've recognized that exactly the same issue =
occurs with HandlerMappings themselves too. Currently, ControllerServlet =
solves this via sorting by bean name ("a.urlMap", "b.urlMap"), but =
frankly, I've never particularly liked that solution.

So I've applied a new approach to both HandlerMapping order and the =
problem in the custom application. Details of the HandlerMapping =
solution follow:

- I've introduced a Ordered interface to com.interface21.core, featuring =
one single method "int getOrder()", and an associated OrderComparator. =
The getOrder() method simply returns an int value, higher value meaning =
greater in terms of sorting resp. lower priority. This is somewhat =
analogous to Servlet load-on-startup values. OrderComparator sorts by =
order value, treating non-Ordered objects as greatest resp. lowest =
priority.

- ControllerServlet now sorts HandlerMappings (and HandlerAdapters, for =
consistency's sake) by sorting them with OrderComparator, rather than by =
bean name. Non-Ordered instances are thus applied in an arbitrary order, =
but instances can choose to implement Ordered to be able to specify a =
higher priority. Of course, the obvious way is adding an "order" bean =
property with getter and setter, to allow specifying the order value in =
the ApplicationContext configuration.

- There's AbstractUrlHandlerMapping now, an abstract base class for =
url-based HandlerMapping implementations. In addition to refactored =
things like unified handler initialization code (propagating =
ApplicationContext and LocaleResolver) for both =
BeanNameUrlHandlerMapping and UrlHandlerMapping, it offers a =
property-based Ordered implementation.

An example snippet from an applicationContext.xml file:

	<bean name=3D"myUrlMap" =
class=3D"com.interface21.web.servlet.handler.UrlHandlerMapping">
		<property name=3D"order">0</property>
		<property name=3D"mappings">
			/example.do=3DexampleController
		</property>
	</bean>

	<bean name=3D"yourUrlMap" =
class=3D"com.interface21.web.servlet.handler.UrlHandlerMapping">
		<property name=3D"order">1</property>
		<property name=3D"mappings">
			/example.do=3D/exampleFormController
			/exampleForm.do=3DexampleFormController
		</property>
	</bean>

According to the order values, myUrlMap gets applied first, thus =
"/example.do" will lead to exampleController's execution. If one sets =
myUrlMap's order value to "2" or omits its order line, yourUrlMap will =
get applied first, thus "/example.do" will lead to =
exampleFormController's execution.

I consider this more elegant and more flexible than sorting by bean =
name. After all, servlets don't get started in the order of their name =
for a reason... ;-) Fortunately, an application developer doesn't have =
to care about the Ordered interface if he doesn't want to: A single =
HandlerMapping definition doesn't need to specify an order value, and a =
HandlerMapping implementation doesn't need to implement Ordered.

If noone objects, I will check in the changes promptly, as they go hand =
in hand with other things like the UrlHandlerMapping refactoring and =
some ControllerServlet polishing.

Juergen


DI J=FCrgen H=F6ller
Senior System Architect
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