[Springframework-developer] integrating custom XML spring dialects into the standard Spring XML config file

[<jas...@ma...>]

On the ActiveMQ project we use Spring as our configuration mechanism to 
configure message brokers, their wire formats, transport connectors, 
persistence configurations etc. It works a treat. What we did to make 
things a little easier for end users (who faced with reams of beans 
with generic properties, can find configuration hard) - we made a 
custom Spring XML language, with our own schema, which extends the 
normal Spring XML language but constrains things a little - making it 
easier for folks to see how to configure things and providing better 
IDE-editor smart completion & better validation etc.

e.g. here's the DTD...

http://activemq.org/dtd/activemq.dtd

(as you can see its based on a cut and paste of the Spring DTD to 
extend it; as spring's based on DTDs)

e.g.

<!DOCTYPE beans PUBLIC  "-//ACTIVEMQ//DTD//EN" 
"http://activemq.org/dtd/activemq.dtd">
<beans>
   <broker>
     <connector>
       <tcpServerTransport uri="tcp://localhost:61616" backlog="1000" 
useAsyncSend="true" maxOutstandingMessages="50"/>
     </connector>

     <persistence>
       <!-- you can point this to a different datasource -->
       <jdbcPersistence dataSourceRef="derby-ds"/>

     </persistence>
   </broker>
<beans>


So first step; it'd be good if Spring used an XML Schema so we can just 
extend it :)


Now all we do is provide our own BeanFactory implementation which is 
like a regular XmlBeanFactory but we transform the XML from our 
extended syntax into normal Spring, then let Spring do all the hard 
work.


However here's a bigger thought. What if various different component 
authors wrote their own extended Spring XML languages ('XML marcros' if 
you like) for different subsystems. XML is all about being human/tool 
editable and new custom languages make it easier to edit & add better 
constraints. So the idea of new sub-languages is a good one. (e.g. 
Geronimo's custom XML could be based on Spring too; I'm sure other 
libraries/frameworks could add their own custom XML language - e.g. 
Drools or Hibernate specific stuff or webflow stuff, AOP stuff etc).

What would really rock is if we could have a generic mechanism for 
registering XML languages at different namespaces - then freely mixing 
and matching these different XML configuration languages inside *any* 
standard Spring XML config file using the normal Spring bean factory 
implementations.

e.g. imagine the <broker> element above being the nice, brief way to 
configure an ActiveMQ broker above. Sure we could do it more verbosely 
using <bean> and <property> tags; but the ActiveMQ language is simpler 
& easier to type/validate. But it'd be nice to mix and match this in 
any XML config in any spring use case.

e.g.

<beans>
    <!--- lets use ActiveMQ's custom XML along with some made up Drools 
XML too -->
   <language namespace="http://activemq.org/schema/3.0" 
class="org.activemq.spring.XmlDialectImpl"/>
   <language namespace="http://drools.org/rules" class="org. 
drools.SpringXMLThingy"/>

   <broker xmlns="http://http://activemq.org/schema/3.0" 
xmlns:drools="http://drools.org/rules">
     <connector>
	<!-- lets add a drools rule base -->
	<drools:rulebase>
	    <drools:rule>
		<drools:condition>x > 10</drools:condition>
		<drools:parameter name="foo">
		   <!-- back to regular Spring again here -->
		    <ref>cheese</ref>
		</drools:paramater>
	    </drools:rule>
	</drools:rulebase>
     </connector>
</broker>

<!-- regular Spring again -->
<bean id="cheese" class="com.acme.Foo">
</bean>
	

To give a concrete real use case for this; when using Spring to 
configure ActiveMQ's JCA container for Message Driven POJOs (which 
could use its own simplified XML language too), we often wanna deploy 
an embedded ActiveMQ message broker inside the JCA container. Right now 
rather than including a little XML fragment as above; we have to split 
things into separate files.

Thinking generically; it'd be nice for folks to be able to register 
different XML 'short hand' libraries with Spring, using standard spring 
(rather than custom bean factories) to let all this kinda stuff 
integrate together cleanly.

In principle, registering 'dialect handlers' with namespaces in the 
standard Spring XML configuration file reader is fairly trivial. The 
harder part is how to implement the transformation of some arbitrary 
XML element (say drools:rulebase) to regular Spring configuration stuff 
(say <bean class="org.drools.DefaultRuleBaseImpl">...</bean>). Right 
now in ActiveMQ we ended up using XSLT as the W3C DOM API is such a 
PITA to work with and we didn't wanna add a new dependency; but maybe 
we could come up with a pure Spring based 'XML macro' mechanism we 
could all integrate with?

I'm wondering have others thought along these kinda lines - or is it 
just us :)

James
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http://radio.weblogs.com/0112098/