B.3. Coding a NamespaceHandler

Spring Framework

B.3. Coding a NamespaceHandler

In addition to the schema, we need a NamespaceHandler that will parse all elements of this specific namespace Spring encounters while parsing configuration files. The NamespaceHandler should in our case take care of the parsing of the myns:dateformat element.

The NamespaceHandler interface is pretty simple in that it features just three methods:

  • init() - allows for initialization of the NamespaceHandler and will be called by Spring before the handler is used

  • BeanDefinition parse(Element, ParserContext) - called when Spring encounters a top-level element (not nested inside a bean definition or a different namespace). This method can register bean definitions itself and/or return a bean definition.

  • BeanDefinitionHolder decorate(Node, BeanDefinitionHolder, ParserContext) - called when Spring encounters an attribute or nested element of a different namespace. The decoration of one or more bean definitions is used for example with the out-of-the-box scopes Spring 2.0 supports. We'll start by highlighting a simple example, without using decoration, after which we will show decoration in a somewhat more advanced example.

Although it is perfectly possible to code your own NamespaceHandler for the entire namespace (and hence provide code that parses each and every element in the namespace), it is often the case that each top-level XML element in a Spring XML configuration file results in a single bean definition (as in our case, where a single <myns:dateformat/> element results in a single SimpleDateFormat bean definition). Spring features a number of convenience classes that support this scenario. In this example, we'll make use the NamespaceHandlerSupport class:

package org.springframework.samples.xml;

import org.springframework.beans.factory.xml.NamespaceHandlerSupport;

public class MyNamespaceHandler extends NamespaceHandlerSupport {
    public void init() {
        registerBeanDefinitionParser("dateformat", new SimpleDateFormatBeanDefinitionParser());        

The observant reader will notice that there isn't actually a whole lot of parsing logic in this class. Indeed... the NamespaceHandlerSupport class has a built in notion of delegation. It supports the registration of any number of BeanDefinitionParser instances, to which it will delegate to when it needs to parse an element in it's namespace. This clean separation of concerns allows a NamespaceHandler to handle the orchestration of the parsing of all of the custom elements in it's namespace, while delegating to BeanDefinitionParsers to do the grunt work of the XML parsing; this means that each BeanDefinitionParser will contain just the logic for parsing a single custom element, as we can see in the next step