How does the Spring framework process XML namespace based syntax for configuring spring components? - spring

Question

While reading about spring-rabbit, I stumble upon the following syntax of configuring spring components:
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:rabbit="http://www.springframework.org/schema/rabbit"
xsi:schemaLocation="http://www.springframework.org/schema/rabbit
http://www.springframework.org/schema/rabbit/spring-rabbit.xsd
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<rabbit:connection-factory id="connectionFactory"/>
<rabbit:template id="amqpTemplate" connection-factory="connectionFactory"/>
<rabbit:admin connection-factory="connectionFactory"/>
<rabbit:queue name="myqueue"/>
</beans>
I am only aware of bean way of configuring components, where you declare a bean with id and class and set appropriate properties. I am not clear about the above way, by making use of XML namespace, of configuring components and how it is internally handled by Spring.
Thanks.

Answer 1

The use of namespaces is really nothing more than shortcut configurations. Behind the scenes, Spring is doing all of your bean creating like you mentioned you are accustomed to doing yourself. As for HOW, well, there are a few components. First you need a registered NamespaceHandler. Going with your spring-rabbit example, you'll find in the spring-rabbit jar a file at a META-INF/spring/spring.handlers with something like this:
http\://www.springframework.org/schema/rabbit=org.springframework.amqp.rabbit.config.RabbitNamespaceHandler
This is registering RabbitNamespaceHandler for the rabbit namespace configurations. Alongside that file you will see one called spring.schemas, and its purpose is to register the schemas that constrain the rabbit namespace.
The next step is to have one or more BeanDefinitionParser instances that do the "magic" of turning your shortcut configurations into full-blown bean creation definitions, so to speak. In this rabbit example, you will find ConnectionFactoryParser which is responsible for parsing the <rabbit:connection-factory /> element. It simply looks at all of the attributes, sub-elements, etc. that your declare in xml and creates the necessary beans for you.
You can follow this formula to create your own namespace configuration as well.

Answer 2

Spring's XML namespace support provides a way to register several components at once, in a more succinct and meta-data driven (or indeed domain-specific) approach. Anything you can do with a namespace handler, could also be done with (probably quite a lot more) bean definitions.
Creating a namespace handler is a fairly simple process - and Spring, by its extensible nature easily allows you to create your own, if you wish.
Here's a some detailed documentation on how to do that:
http://docs.spring.io/spring/docs/3.0.0.M3/reference/html/apbs03.html
http://cscarioni.blogspot.com/2012/04/creating-custom-spring-3-xml-namespace.html