[[xml-custom]]
= XML Schema Authoring

[[xsd-custom-introduction]]
Since version 2.0, Spring has featured a mechanism for adding schema-based extensions to the
basic Spring XML format for defining and configuring beans. This section covers
how to write your own custom XML bean definition parsers and
integrate such parsers into the Spring IoC container.

To facilitate authoring configuration files that use a schema-aware XML editor,
Spring's extensible XML configuration mechanism is based on XML Schema. If you are not
familiar with Spring's current XML configuration extensions that come with the standard
Spring distribution, you should first read the previous section on xref:core/appendix/xsd-schemas.adoc[XML Schemas].

To create new XML configuration extensions:

. xref:core/appendix/xml-custom.adoc#core.appendix.xsd-custom-schema[Author] an XML schema to describe your custom element(s).
. xref:core/appendix/xml-custom.adoc#core.appendix.xsd-custom-namespacehandler[Code] a custom `NamespaceHandler` implementation.
. xref:core/appendix/xml-custom.adoc#core.appendix.xsd-custom-parser[Code] one or more `BeanDefinitionParser` implementations
  (this is where the real work is done).
. xref:core/appendix/xml-custom.adoc#core.appendix.xsd-custom-registration[Register] your new artifacts with Spring.

For a unified example, we create an
XML extension (a custom XML element) that lets us configure objects of the type
`SimpleDateFormat` (from the `java.text` package). When we are done,
we will be able to define bean definitions of type `SimpleDateFormat` as follows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<myns:dateformat id="dateFormat"
		pattern="yyyy-MM-dd HH:mm"
		lenient="true"/>
----

(We include much more detailed
examples follow later in this appendix. The intent of this first simple example is to walk you
through the basic steps of making a custom extension.)


[[xsd-custom-schema]]
== Authoring the Schema

Creating an XML configuration extension for use with Spring's IoC container starts with
authoring an XML Schema to describe the extension. For our example, we use the following schema
to configure `SimpleDateFormat` objects:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<!-- myns.xsd (inside package org/springframework/samples/xml) -->

	<?xml version="1.0" encoding="UTF-8"?>
	<xsd:schema xmlns="http://www.mycompany.example/schema/myns"
			xmlns:xsd="http://www.w3.org/2001/XMLSchema"
			xmlns:beans="http://www.springframework.org/schema/beans"
			targetNamespace="http://www.mycompany.example/schema/myns"
			elementFormDefault="qualified"
			attributeFormDefault="unqualified">

		<xsd:import namespace="http://www.springframework.org/schema/beans"/>

		<xsd:element name="dateformat">
			<xsd:complexType>
				<xsd:complexContent>
					<xsd:extension base="beans:identifiedType"> <1>
						<xsd:attribute name="lenient" type="xsd:boolean"/>
						<xsd:attribute name="pattern" type="xsd:string" use="required"/>
					</xsd:extension>
				</xsd:complexContent>
			</xsd:complexType>
		</xsd:element>
	</xsd:schema>
----
<1> The indicated line contains an extension base for all identifiable tags
(meaning they have an `id` attribute that we can use as the bean identifier in the
container). We can use this attribute because we imported the Spring-provided
`beans` namespace.


The preceding schema lets us configure `SimpleDateFormat` objects directly in an
XML application context file by using the `<myns:dateformat/>` element, as the following
example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<myns:dateformat id="dateFormat"
		pattern="yyyy-MM-dd HH:mm"
		lenient="true"/>
----

Note that, after we have created the infrastructure classes, the preceding snippet of XML is
essentially the same as the following XML snippet:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<bean id="dateFormat" class="java.text.SimpleDateFormat">
		<constructor-arg value="yyyy-MM-dd HH:mm"/>
		<property name="lenient" value="true"/>
	</bean>
----

The second of the two preceding snippets
creates a bean in the container (identified by the name `dateFormat` of type
`SimpleDateFormat`) with a couple of properties set.

NOTE: The schema-based approach to creating configuration format allows for tight integration
with an IDE that has a schema-aware XML editor. By using a properly authored schema, you
can use autocompletion to let a user choose between several configuration options
defined in the enumeration.


[[xsd-custom-namespacehandler]]
== Coding a `NamespaceHandler`

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

The `NamespaceHandler` interface features three methods:

* `init()`: Allows for initialization of the `NamespaceHandler` and is 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 itself register bean definitions, return a bean definition, or both.
* `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
  xref:core/beans/factory-scopes.adoc[scopes that Spring supports].
  We start by highlighting a simple example, without using decoration, after which
  we show decoration in a somewhat more advanced example.

Although you can 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 the following example, we
use the `NamespaceHandlerSupport` class:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package org.springframework.samples.xml;

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

	public class MyNamespaceHandler extends NamespaceHandlerSupport {

		public void init() {
			registerBeanDefinitionParser("dateformat", new SimpleDateFormatBeanDefinitionParser());
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package org.springframework.samples.xml

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

	class MyNamespaceHandler : NamespaceHandlerSupport {

		override fun init() {
			registerBeanDefinitionParser("dateformat", SimpleDateFormatBeanDefinitionParser())
		}
	}
----
======

You may notice that there is not 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 delegates to when it needs to parse an element in its
namespace. This clean separation of concerns lets a `NamespaceHandler` handle the
orchestration of the parsing of all of the custom elements in its namespace while
delegating to `BeanDefinitionParsers` to do the grunt work of the XML parsing. This
means that each `BeanDefinitionParser` contains only the logic for parsing a single
custom element, as we can see in the next step.


[[xsd-custom-parser]]
== Using `BeanDefinitionParser`

A `BeanDefinitionParser` is used if the `NamespaceHandler` encounters an XML
element of the type that has been mapped to the specific bean definition parser
(`dateformat` in this case). In other words, the `BeanDefinitionParser` is
responsible for parsing one distinct top-level XML element defined in the schema. In
the parser, we' have access to the XML element (and thus to its subelements, too) so that
we can parse our custom XML content, as you can see in the following example:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package org.springframework.samples.xml;

	import org.springframework.beans.factory.support.BeanDefinitionBuilder;
	import org.springframework.beans.factory.xml.AbstractSingleBeanDefinitionParser;
	import org.springframework.util.StringUtils;
	import org.w3c.dom.Element;

	import java.text.SimpleDateFormat;

	public class SimpleDateFormatBeanDefinitionParser extends AbstractSingleBeanDefinitionParser { // <1>

		protected Class getBeanClass(Element element) {
			return SimpleDateFormat.class; // <2>
		}

		protected void doParse(Element element, BeanDefinitionBuilder bean) {
			// this will never be null since the schema explicitly requires that a value be supplied
			String pattern = element.getAttribute("pattern");
			bean.addConstructorArgValue(pattern);

			// this however is an optional property
			String lenient = element.getAttribute("lenient");
			if (StringUtils.hasText(lenient)) {
				bean.addPropertyValue("lenient", Boolean.valueOf(lenient));
			}
		}

	}
----
<1> We use the Spring-provided `AbstractSingleBeanDefinitionParser` to handle a lot of
the basic grunt work of creating a single `BeanDefinition`.
<2> We supply the `AbstractSingleBeanDefinitionParser` superclass with the type that our
single `BeanDefinition` represents.

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package org.springframework.samples.xml

	import org.springframework.beans.factory.support.BeanDefinitionBuilder
	import org.springframework.beans.factory.xml.AbstractSingleBeanDefinitionParser
	import org.springframework.util.StringUtils
	import org.w3c.dom.Element

	import java.text.SimpleDateFormat

	class SimpleDateFormatBeanDefinitionParser : AbstractSingleBeanDefinitionParser() { // <1>

		override fun getBeanClass(element: Element): Class<*>? { // <2>
			return SimpleDateFormat::class.java
		}

		override fun doParse(element: Element, bean: BeanDefinitionBuilder) {
			// this will never be null since the schema explicitly requires that a value be supplied
			val pattern = element.getAttribute("pattern")
			bean.addConstructorArgValue(pattern)

			// this however is an optional property
			val lenient = element.getAttribute("lenient")
			if (StringUtils.hasText(lenient)) {
				bean.addPropertyValue("lenient", java.lang.Boolean.valueOf(lenient))
			}
		}
	}
----
<1> We use the Spring-provided `AbstractSingleBeanDefinitionParser` to handle a lot of
the basic grunt work of creating a single `BeanDefinition`.
<2> We supply the `AbstractSingleBeanDefinitionParser` superclass with the type that our
single `BeanDefinition` represents.
======

In this simple case, this is all that we need to do. The creation of our single
`BeanDefinition` is handled by the `AbstractSingleBeanDefinitionParser` superclass, as
is the extraction and setting of the bean definition's unique identifier.


[[xsd-custom-registration]]
== Registering the Handler and the Schema

The coding is finished. All that remains to be done is to make the Spring XML
parsing infrastructure aware of our custom element. We do so by registering our custom
`namespaceHandler` and custom XSD file in two special-purpose properties files. These
properties files are both placed in a `META-INF` directory in your application and
can, for example, be distributed alongside your binary classes in a JAR file. The Spring
XML parsing infrastructure automatically picks up your new extension by consuming
these special properties files, the formats of which are detailed in the next two sections.

[[xsd-custom-registration-spring-handlers]]
=== Writing `META-INF/spring.handlers`

The properties file called `spring.handlers` contains a mapping of XML Schema URIs to
namespace handler classes. For our example, we need to write the following:

[literal,subs="verbatim,quotes"]
----
http\://www.mycompany.example/schema/myns=org.springframework.samples.xml.MyNamespaceHandler
----

(The `:` character is a valid delimiter in the Java properties format, so
`:` character in the URI needs to be escaped with a backslash.)

The first part (the key) of the key-value pair is the URI associated with your custom
namespace extension and needs to exactly match exactly the value of the `targetNamespace`
attribute, as specified in your custom XSD schema.

[[xsd-custom-registration-spring-schemas]]
=== Writing 'META-INF/spring.schemas'

The properties file called `spring.schemas` contains a mapping of XML Schema locations
(referred to, along with the schema declaration, in XML files that use the schema as part
of the `xsi:schemaLocation` attribute) to classpath resources. This file is needed
to prevent Spring from absolutely having to use a default `EntityResolver` that requires
Internet access to retrieve the schema file. If you specify the mapping in this
properties file, Spring searches for the schema (in this case,
`myns.xsd` in the `org.springframework.samples.xml` package) on the classpath.
The following snippet shows the line we need to add for our custom schema:

[literal,subs="verbatim,quotes"]
----
http\://www.mycompany.example/schema/myns/myns.xsd=org/springframework/samples/xml/myns.xsd
----

(Remember that the `:` character must be escaped.)

You are encouraged to deploy your XSD file (or files) right alongside
the `NamespaceHandler` and `BeanDefinitionParser` classes on the classpath.


[[xsd-custom-using]]
== Using a Custom Extension in Your Spring XML Configuration

Using a custom extension that you yourself have implemented is no different from using
one of the "`custom`" extensions that Spring provides. The following
example uses the custom `<dateformat/>` element developed in the previous steps
in a Spring XML configuration file:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?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:myns="http://www.mycompany.example/schema/myns"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.mycompany.example/schema/myns http://www.mycompany.com/schema/myns/myns.xsd">

		<!-- as a top-level bean -->
		<myns:dateformat id="defaultDateFormat" pattern="yyyy-MM-dd HH:mm" lenient="true"/> <1>

		<bean id="jobDetailTemplate" abstract="true">
			<property name="dateFormat">
				<!-- as an inner bean -->
				<myns:dateformat pattern="HH:mm MM-dd-yyyy"/>
			</property>
		</bean>

	</beans>
----
<1> Our custom bean.


[[xsd-custom-meat]]
== More Detailed Examples

This section presents some more detailed examples of custom XML extensions.

[[xsd-custom-custom-nested]]
=== Nesting Custom Elements within Custom Elements

The example presented in this section shows how you to write the various artifacts required
to satisfy a target of the following configuration:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?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.example/schema/component"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.foo.example/schema/component http://www.foo.example/schema/component/component.xsd">

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

	</beans>
----

The preceding configuration nests custom extensions within each other. The class
that is actually configured by the `<foo:component/>` element is the `Component`
class (shown in the next example). 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 by using setter injection.
The following listing shows the `Component` class:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo;

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

	public class Component {

		private String name;
		private List<Component> components = new ArrayList<Component> ();

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

		public List<Component> getComponents() {
			return components;
		}

		public String getName() {
			return name;
		}

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

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo

	import java.util.ArrayList

	class Component {

		var name: String? = null
		private val components = ArrayList<Component>()

		// there is no setter method for the 'components'
		fun addComponent(component: Component) {
			this.components.add(component)
		}

		fun getComponents(): List<Component> {
			return components
		}
	}
----
======

The typical solution to this issue is to create a custom `FactoryBean` that exposes a
setter property for the `components` property. The following listing shows such a custom
`FactoryBean`:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo;

	import org.springframework.beans.factory.FactoryBean;

	import java.util.List;

	public class ComponentFactoryBean implements FactoryBean<Component> {

		private Component parent;
		private List<Component> children;

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

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

		public Component getObject() throws Exception {
			if (this.children != null && this.children.size() > 0) {
				for (Component child : children) {
					this.parent.addComponent(child);
				}
			}
			return this.parent;
		}

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

		public boolean isSingleton() {
			return true;
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo

	import org.springframework.beans.factory.FactoryBean
	import org.springframework.stereotype.Component

	class ComponentFactoryBean : FactoryBean<Component> {

		private var parent: Component? = null
		private var children: List<Component>? = null

		fun setParent(parent: Component) {
			this.parent = parent
		}

		fun setChildren(children: List<Component>) {
			this.children = children
		}

		override fun getObject(): Component? {
			if (this.children != null && this.children!!.isNotEmpty()) {
				for (child in children!!) {
					this.parent!!.addComponent(child)
				}
			}
			return this.parent
		}

		override fun getObjectType(): Class<Component>? {
			return Component::class.java
		}

		override fun isSingleton(): Boolean {
			return true
		}
	}
----
======

This works nicely, but it 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 xref:core/appendix/xml-custom.adoc#core.appendix.xsd-custom-introduction[the steps described previously], we start off
by creating the XSD schema to define the structure of our custom tag, as the following
listing shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8" standalone="no"?>

	<xsd:schema xmlns="http://www.foo.example/schema/component"
			xmlns:xsd="http://www.w3.org/2001/XMLSchema"
			targetNamespace="http://www.foo.example/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>
----

Again following xref:core/appendix/xml-custom.adoc#core.appendix.xsd-custom-introduction[the process described earlier],
we then create a custom `NamespaceHandler`:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo;

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

	public class ComponentNamespaceHandler extends NamespaceHandlerSupport {

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

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo

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

	class ComponentNamespaceHandler : NamespaceHandlerSupport() {

		override fun init() {
			registerBeanDefinitionParser("component", ComponentBeanDefinitionParser())
		}
	}
----
======

Next up is the custom `BeanDefinitionParser`. Remember that we are creating
a `BeanDefinition` that describes a `ComponentFactoryBean`. The following
listing shows our custom `BeanDefinitionParser` implementation:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo;

	import org.springframework.beans.factory.config.BeanDefinition;
	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) {
			return parseComponentElement(element);
		}

		private static AbstractBeanDefinition parseComponentElement(Element element) {
			BeanDefinitionBuilder factory = BeanDefinitionBuilder.rootBeanDefinition(ComponentFactoryBean.class);
			factory.addPropertyValue("parent", parseComponent(element));

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

			return factory.getBeanDefinition();
		}

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

		private static void parseChildComponents(List<Element> childElements, BeanDefinitionBuilder factory) {
			ManagedList<BeanDefinition> children = new ManagedList<>(childElements.size());
			for (Element element : childElements) {
				children.add(parseComponentElement(element));
			}
			factory.addPropertyValue("children", children);
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo

	import org.springframework.beans.factory.config.BeanDefinition
	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

	class ComponentBeanDefinitionParser : AbstractBeanDefinitionParser() {

		override fun parseInternal(element: Element, parserContext: ParserContext): AbstractBeanDefinition? {
			return parseComponentElement(element)
		}

		private fun parseComponentElement(element: Element): AbstractBeanDefinition {
			val factory = BeanDefinitionBuilder.rootBeanDefinition(ComponentFactoryBean::class.java)
			factory.addPropertyValue("parent", parseComponent(element))

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

			return factory.getBeanDefinition()
		}

		private fun parseComponent(element: Element): BeanDefinition {
			val component = BeanDefinitionBuilder.rootBeanDefinition(Component::class.java)
			component.addPropertyValue("name", element.getAttribute("name"))
			return component.beanDefinition
		}

		private fun parseChildComponents(childElements: List<Element>, factory: BeanDefinitionBuilder) {
			val children = ManagedList<BeanDefinition>(childElements.size)
			for (element in childElements) {
				children.add(parseComponentElement(element))
			}
			factory.addPropertyValue("children", children)
		}
	}
----
======

Finally, the various artifacts need to be registered with the Spring XML infrastructure,
by modifying the `META-INF/spring.handlers` and `META-INF/spring.schemas` files, as follows:

[literal,subs="verbatim,quotes"]
----
# in 'META-INF/spring.handlers'
http\://www.foo.example/schema/component=com.foo.ComponentNamespaceHandler
----

[literal,subs="verbatim,quotes"]
----
# in 'META-INF/spring.schemas'
http\://www.foo.example/schema/component/component.xsd=com/foo/component.xsd
----

[[xsd-custom-custom-just-attributes]]
=== Custom Attributes on "`Normal`" Elements

Writing your own custom parser and the associated artifacts is not hard. However,
it is sometimes not the right thing to do. Consider a scenario where you need to
add metadata to already existing bean definitions. In this case, you certainly
do not want to have to write your own entire custom extension. Rather, you merely
want to add an additional attribute to the existing bean definition element.

By way of another example, suppose that you define a bean definition for a
service object that (unknown to it) accesses a clustered
{JSR}107[JCache], and you want to ensure that the
named JCache instance is eagerly started within the surrounding cluster.
The following listing shows such a definition:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<bean id="checkingAccountService" class="com.foo.DefaultCheckingAccountService"
			jcache:cache-name="checking.account">
		<!-- other dependencies here... -->
	</bean>
----

We can then create another `BeanDefinition` when the
`'jcache:cache-name'` attribute is parsed. This `BeanDefinition` then initializes
the named JCache for us. We can also modify the existing `BeanDefinition` for the
`'checkingAccountService'` so that it has a dependency on this new
JCache-initializing `BeanDefinition`. The following listing shows our `JCacheInitializer`:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo;

	public class JCacheInitializer {

		private final String name;

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

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

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo

	class JCacheInitializer(private val name: String) {

		fun initialize() {
			// lots of JCache API calls to initialize the named cache...
		}
	}
----
======

Now we can move onto the custom extension. First, we need to author
the XSD schema that describes the custom attribute, as follows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8" standalone="no"?>

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

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

	</xsd:schema>
----

Next, we need to create the associated `NamespaceHandler`, as follows:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo;

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

	public class JCacheNamespaceHandler extends NamespaceHandlerSupport {

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

	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package com.foo

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

	class JCacheNamespaceHandler : NamespaceHandlerSupport() {

		override fun init() {
			super.registerBeanDefinitionDecoratorForAttribute("cache-name",
					JCacheInitializingBeanDefinitionDecorator())
		}

	}
----
======

Next, we need to create the parser. Note that, in this case, because we are going to parse
an XML attribute, we write a `BeanDefinitionDecorator` rather than a `BeanDefinitionParser`.
The following listing shows our `BeanDefinitionDecorator` implementation:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	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;
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	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

	class JCacheInitializingBeanDefinitionDecorator : BeanDefinitionDecorator {

		override fun decorate(source: Node, holder: BeanDefinitionHolder,
							ctx: ParserContext): BeanDefinitionHolder {
			val initializerBeanName = registerJCacheInitializer(source, ctx)
			createDependencyOnJCacheInitializer(holder, initializerBeanName)
			return holder
		}

		private fun createDependencyOnJCacheInitializer(holder: BeanDefinitionHolder,
														initializerBeanName: String) {
			val definition = holder.beanDefinition as AbstractBeanDefinition
			var dependsOn = definition.dependsOn
			dependsOn = if (dependsOn == null) {
				arrayOf(initializerBeanName)
			} else {
				val dependencies = ArrayList(listOf(*dependsOn))
				dependencies.add(initializerBeanName)
				dependencies.toTypedArray()
			}
			definition.setDependsOn(*dependsOn)
		}

		private fun registerJCacheInitializer(source: Node, ctx: ParserContext): String {
			val cacheName = (source as Attr).value
			val beanName = "$cacheName-initializer"
			if (!ctx.registry.containsBeanDefinition(beanName)) {
				val initializer = BeanDefinitionBuilder.rootBeanDefinition(JCacheInitializer::class.java)
				initializer.addConstructorArg(cacheName)
				ctx.registry.registerBeanDefinition(beanName, initializer.getBeanDefinition())
			}
			return beanName
		}
	}
----
======

Finally, we need to register the various artifacts with the Spring XML infrastructure
by modifying the `META-INF/spring.handlers` and `META-INF/spring.schemas` files, as follows:

[literal,subs="verbatim,quotes"]
----
# in 'META-INF/spring.handlers'
http\://www.foo.example/schema/jcache=com.foo.JCacheNamespaceHandler
----

[literal,subs="verbatim,quotes"]
----
# in 'META-INF/spring.schemas'
http\://www.foo.example/schema/jcache/jcache.xsd=com/foo/jcache.xsd
----
