This example illustrates how you might go about writing the various artifacts required to satisfy a target of the following configuration:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xmlns:foo="http://www.foo.com/schema/component"
      xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.foo.com/schema/component http://www.foo.com/schema/component/component.xsd">

   <foo:component id="bionic-family" name="Bionic-1">
      <foo:component name="Sport-1"/>
      <foo:component name="Rock-1"/>
   </foo:component>

</beans>

The above configuration actually nests custom extensions within each other. The class that is actually configured by the above <foo:component/> element is the Component class (shown directly below). Notice how the Component class does not expose a setter method for the 'components' property; this makes it hard (or rather impossible) to configure a bean definition for the Component class using setter injection.

package com.foo;

import java.util.ArrayList;
import java.util.List;

public class Component {

   private String name;
   private List components = new ArrayList();

   // mmm, there is no setter method for the 'components'
   public void addComponent(Component component) {
      this.components.add(component);
   }

   public List getComponents() {
      return components;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }
}

The typical solution to this issue is to create a custom FactoryBean that exposes a setter property for the 'components' property.

package com.foo;

import org.springframework.beans.factory.FactoryBean;

import java.util.Iterator;
import java.util.List;

public class ComponentFactoryBean implements FactoryBean {

   private Component parent;
   private List children;

   public void setParent(Component parent) {
      this.parent = parent;
   }

   public void setChildren(List children) {
      this.children = children;
   }

   public Object getObject() throws Exception {
      if (this.children != null && this.children.size() > 0) {
         for (Iterator it = children.iterator(); it.hasNext();) {
            Component childComponent = (Component) it.next();
            this.parent.addComponent(childComponent);
         }
      }
      return this.parent;
   }

   public Class getObjectType() {
      return Component.class;
   }

   public boolean isSingleton() {
      return true;
   }
}

This is all very well, and does work nicely, but exposes a lot of Spring plumbing to the end user. What we are going to do is write a custom extension that hides away all of this Spring plumbing. If we stick to the steps described previously, we'll start off by creating the XSD schema to define the structure of our custom tag.

<?xml version="1.0" encoding="UTF-8" standalone="no"?>

<xsd:schema xmlns="http://www.foo.com/schema/component"
         xmlns:xsd="http://www.w3.org/2001/XMLSchema"
         targetNamespace="http://www.foo.com/schema/component"
         elementFormDefault="qualified"
         attributeFormDefault="unqualified">

   <xsd:element name="component">
      <xsd:complexType>
         <xsd:choice minOccurs="0" maxOccurs="unbounded">
            <xsd:element ref="component"/>
         </xsd:choice>
         <xsd:attribute name="id" type="xsd:ID"/>
         <xsd:attribute name="name" use="required" type="xsd:string"/>
      </xsd:complexType>
   </xsd:element>

</xsd:schema>

We'll then create a custom NamespaceHandler.

package com.foo;

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

public class ComponentNamespaceHandler extends NamespaceHandlerSupport {

   public void init() {
      registerBeanDefinitionParser("component", new ComponentBeanDefinitionParser());
   }
}

Next up is the custom BeanDefinitionParser. Remember that what we are creating is a BeanDefinition describing a ComponentFactoryBean.

package com.foo;

import org.springframework.beans.factory.support.AbstractBeanDefinition;
import org.springframework.beans.factory.support.BeanDefinitionBuilder;
import org.springframework.beans.factory.support.ManagedList;
import org.springframework.beans.factory.xml.AbstractBeanDefinitionParser;
import org.springframework.beans.factory.xml.ParserContext;
import org.springframework.util.xml.DomUtils;
import org.w3c.dom.Element;

import java.util.List;

public class ComponentBeanDefinitionParser extends AbstractBeanDefinitionParser {

   protected AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) {
      BeanDefinitionBuilder factory = BeanDefinitionBuilder.rootBeanDefinition(ComponentFactoryBean.class);
      BeanDefinitionBuilder parent = parseComponent(element);
      factory.addPropertyValue("parent", parent.getBeanDefinition());

      List childElements = DomUtils.getChildElementsByTagName(element, "component");
      if (childElements != null && childElements.size() > 0) {
         parseChildComponents(childElements, factory);
      }
      return factory.getBeanDefinition();
   }

   private static BeanDefinitionBuilder parseComponent(Element element) {
      BeanDefinitionBuilder component = BeanDefinitionBuilder.rootBeanDefinition(Component.class);
      component.addPropertyValue("name", element.getAttribute("name"));
      return component;
   }

   private static void parseChildComponents(List childElements, BeanDefinitionBuilder factory) {
      ManagedList children = new ManagedList(childElements.size());
      for (int i = 0; i < childElements.size(); ++i) {
         Element childElement = (Element) childElements.get(i);
         BeanDefinitionBuilder child = parseComponent(childElement);
         children.add(child.getBeanDefinition());
      }
      factory.addPropertyValue("children", children);
   }
}

Lastly, the various artifacts need to be registered with the Spring XML infrastructure.

# in 'META-INF/spring.handlers'
http\://www.foo.com/schema/component=com.foo.ComponentNamespaceHandler

# in 'META-INF/spring.schemas'
http\://www.foo.com/schema/component/component.xsd=com/foo/component.xsd

Writing your own custom parser and the associated artifacts isn't hard, but sometimes it is not the right thing to do. Consider the scenario where you need to add metadata to already existing bean definitions. In this case you certainly don't want to have to go off and write your own entire custom extension; rather you just want to add an additional attribute to the existing bean definition element.

By way of another example, let's say that the service class that you are defining a bean definition for a service object that will (unknown to it) be accessing a clustered JCache, and you want to ensure that the named JCache instance is eagerly started within the surrounding cluster:

<bean id="checkingAccountService" class="com.foo.DefaultCheckingAccountService"
      jcache:cache-name="checking.account">
   <!-- other dependencies here... -->
</bean>

What we are going to do here is create another BeanDefinition when the 'jcache:cache-name' attribute is parsed; this BeanDefinition will then initialize the named JCache for us. We will also modify the existing BeanDefinition for the 'checkingAccountService' so that it will have a dependency on this new JCache-initializing BeanDefinition.

package com.foo;

public class JCacheInitializer {

   private String name;

   public JCacheInitializer(String name) {
      this.name = name;
   }

   public void initialize() {
      // lots of JCache API calls to initialize the named cache...
   }
}

Now onto the custom extension. Firstly, the authoring of the XSD schema describing the custom attribute (quite easy in this case).

<?xml version="1.0" encoding="UTF-8" standalone="no"?>

<xsd:schema xmlns="http://www.foo.com/schema/jcache"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            targetNamespace="http://www.foo.com/schema/jcache"
            elementFormDefault="qualified">

   <xsd:attribute name="cache-name" type="xsd:string"/>

</xsd:schema>

Next, the associated NamespaceHandler.

package com.foo;

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

public class JCacheNamespaceHandler extends NamespaceHandlerSupport {

   public void init() {
      super.registerBeanDefinitionDecoratorForAttribute("cache-name",
            new JCacheInitializingBeanDefinitionDecorator());
   }
}

Next, the parser. Note that in this case, because we are going to be parsing an XML attribute, we write a BeanDefinitionDecorator rather than a BeanDefinitionParser.

package com.foo;

import org.springframework.beans.factory.config.BeanDefinitionHolder;
import org.springframework.beans.factory.support.AbstractBeanDefinition;
import org.springframework.beans.factory.support.BeanDefinitionBuilder;
import org.springframework.beans.factory.xml.BeanDefinitionDecorator;
import org.springframework.beans.factory.xml.ParserContext;
import org.w3c.dom.Attr;
import org.w3c.dom.Node;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class JCacheInitializingBeanDefinitionDecorator implements BeanDefinitionDecorator {
   
   private static final String[] EMPTY_STRING_ARRAY = new String[0];

   public BeanDefinitionHolder decorate(
         Node source, BeanDefinitionHolder holder, ParserContext ctx) {
      String initializerBeanName = registerJCacheInitializer(source, ctx);
      createDependencyOnJCacheInitializer(holder, initializerBeanName);
      return holder;
   }

   private void createDependencyOnJCacheInitializer(BeanDefinitionHolder holder, String initializerBeanName) {
      AbstractBeanDefinition definition = ((AbstractBeanDefinition) holder.getBeanDefinition());
      String[] dependsOn = definition.getDependsOn();
      if (dependsOn == null) {
         dependsOn = new String[]{initializerBeanName};
      } else {
         List dependencies = new ArrayList(Arrays.asList(dependsOn));
         dependencies.add(initializerBeanName);
         dependsOn = (String[]) dependencies.toArray(EMPTY_STRING_ARRAY);
      }
      definition.setDependsOn(dependsOn);
   }

   private String registerJCacheInitializer(Node source, ParserContext ctx) {
      String cacheName = ((Attr) source).getValue();
      String beanName = cacheName + "-initializer";
      if (!ctx.getRegistry().containsBeanDefinition(beanName)) {
         BeanDefinitionBuilder initializer = BeanDefinitionBuilder.rootBeanDefinition(JCacheInitializer.class);
         initializer.addConstructorArg(cacheName);
         ctx.getRegistry().registerBeanDefinition(beanName, initializer.getBeanDefinition());
      }
      return beanName;
   }
}

Lastly, the various artifacts need to be registered with the Spring XML infrastructure.

# in 'META-INF/spring.handlers'
http\://www.foo.com/schema/jcache=com.foo.JCacheNamespaceHandler

# in 'META-INF/spring.schemas'
http\://www.foo.com/schema/jcache/jcache.xsd=com/foo/jcache.xsd