:doc-root: https://docs.spring.io
:api-spring-framework: {doc-root}/spring-framework/docs/{spring-version}/javadoc-api/org/springframework

= Appendix




[[xsd-schemas]]
== XML Schemas

This part of the appendix lists XML schemas related to the core container.



[[xsd-schemas-util]]
=== The `util` Schema

As the name implies, the `util` tags deal with common, utility configuration
issues, such as configuring collections, referencing constants, and so forth.
To use the tags in the `util` schema, you need to have the following preamble at the top
of your Spring XML configuration file (the text in the snippet references the
correct schema so that the tags in the `util` namespace are available to you):

[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:util="http://www.springframework.org/schema/util"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/util https://www.springframework.org/schema/util/spring-util.xsd">

			<!-- bean definitions here -->

	</beans>
----


[[xsd-schemas-util-constant]]
==== Using `<util:constant/>`

Consider the following bean definition:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="..." class="...">
		<property name="isolation">
			<bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
					class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean" />
		</property>
	</bean>
----

The preceding configuration uses a Spring `FactoryBean` implementation (the
`FieldRetrievingFactoryBean`) to set the value of the `isolation` property on a bean
to the value of the `java.sql.Connection.TRANSACTION_SERIALIZABLE` constant. This is
all well and good, but it is verbose and (unnecessarily) exposes Spring's internal
plumbing to the end user.

The following XML Schema-based version is more concise, clearly expresses the
developer's intent ("`inject this constant value`"), and it reads better:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="..." class="...">
		<property name="isolation">
			<util:constant static-field="java.sql.Connection.TRANSACTION_SERIALIZABLE"/>
		</property>
	</bean>
----

[[xsd-schemas-util-frfb]]
===== Setting a Bean Property or Constructor Argument from a Field Value

{api-spring-framework}/beans/factory/config/FieldRetrievingFactoryBean.html[`FieldRetrievingFactoryBean`]
is a `FactoryBean` that retrieves a `static` or non-static field value. It is typically
used for retrieving `public` `static` `final` constants, which may then be used to set a
property value or constructor argument for another bean.

The following example shows how a `static` field is exposed, by using the
{api-spring-framework}/beans/factory/config/FieldRetrievingFactoryBean.html#setStaticField(java.lang.String)[`staticField`]
property:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="myField"
			class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean">
		<property name="staticField" value="java.sql.Connection.TRANSACTION_SERIALIZABLE"/>
	</bean>
----

There is also a convenience usage form where the `static` field is specified as the bean
name, as the following example shows:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
			class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"/>
----

This does mean that there is no longer any choice in what the bean `id` is (so any other
bean that refers to it also has to use this longer name), but this form is very
concise to define and very convenient to use as an inner bean since the `id` does not have
to be specified for the bean reference, as the following example shows:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="..." class="...">
		<property name="isolation">
			<bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
					class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean" />
		</property>
	</bean>
----

You can also access a non-static (instance) field of another bean, as
described in the API documentation for the
{api-spring-framework}/beans/factory/config/FieldRetrievingFactoryBean.html[`FieldRetrievingFactoryBean`]
class.

Injecting enumeration values into beans as either property or constructor arguments is
easy to do in Spring. You do not actually have to do anything or know anything about
the Spring internals (or even about classes such as the `FieldRetrievingFactoryBean`).
The following example enumeration shows how easy injecting an enum value is:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package javax.persistence;

	public enum PersistenceContextType {

		TRANSACTION,
		EXTENDED
	}
----

Now consider the following setter of type `PersistenceContextType` and the corresponding bean definition:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package example;

	public class Client {

		private PersistenceContextType persistenceContextType;

		public void setPersistenceContextType(PersistenceContextType type) {
			this.persistenceContextType = type;
		}
	}
----

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean class="example.Client">
		<property name="persistenceContextType" value="TRANSACTION"/>
	</bean>
----


[[xsd-schemas-util-property-path]]
==== Using `<util:property-path/>`

Consider the following example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- target bean to be referenced by name -->
	<bean id="testBean" class="org.springframework.beans.TestBean" scope="prototype">
		<property name="age" value="10"/>
		<property name="spouse">
			<bean class="org.springframework.beans.TestBean">
				<property name="age" value="11"/>
			</bean>
		</property>
	</bean>

	<!-- results in 10, which is the value of property 'age' of bean 'testBean' -->
	<bean id="testBean.age" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
----

The preceding configuration uses a Spring `FactoryBean` implementation (the
`PropertyPathFactoryBean`) to create a bean (of type `int`) called `testBean.age` that
has a value equal to the `age` property of the `testBean` bean.

Now consider the following example, which adds a `<util:property-path/>` element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- target bean to be referenced by name -->
	<bean id="testBean" class="org.springframework.beans.TestBean" scope="prototype">
		<property name="age" value="10"/>
		<property name="spouse">
			<bean class="org.springframework.beans.TestBean">
				<property name="age" value="11"/>
			</bean>
		</property>
	</bean>

	<!-- results in 10, which is the value of property 'age' of bean 'testBean' -->
	<util:property-path id="name" path="testBean.age"/>
----

The value of the `path` attribute of the `<property-path/>` element follows the form of
`beanName.beanProperty`. In this case, it picks up the `age` property of the bean named
`testBean`. The value of that `age` property is `10`.

[[xsd-schemas-util-property-path-dependency]]
===== Using `<util:property-path/>` to Set a Bean Property or Constructor Argument

`PropertyPathFactoryBean` is a `FactoryBean` that evaluates a property path on a given
target object. The target object can be specified directly or by a bean name. You can then use this
value in another bean definition as a property value or constructor
argument.

The following example shows a path being used against another bean, by name:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	// target bean to be referenced by name
	<bean id="person" class="org.springframework.beans.TestBean" scope="prototype">
		<property name="age" value="10"/>
		<property name="spouse">
			<bean class="org.springframework.beans.TestBean">
				<property name="age" value="11"/>
			</bean>
		</property>
	</bean>

	// results in 11, which is the value of property 'spouse.age' of bean 'person'
	<bean id="theAge"
			class="org.springframework.beans.factory.config.PropertyPathFactoryBean">
		<property name="targetBeanName" value="person"/>
		<property name="propertyPath" value="spouse.age"/>
	</bean>
----

In the following example, a path is evaluated against an inner bean:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- results in 12, which is the value of property 'age' of the inner bean -->
	<bean id="theAge"
			class="org.springframework.beans.factory.config.PropertyPathFactoryBean">
		<property name="targetObject">
			<bean class="org.springframework.beans.TestBean">
				<property name="age" value="12"/>
			</bean>
		</property>
		<property name="propertyPath" value="age"/>
	</bean>
----

There is also a shortcut form, where the bean name is the property path.
The following example shows the shortcut form:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- results in 10, which is the value of property 'age' of bean 'person' -->
	<bean id="person.age"
			class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
----

This form does mean that there is no choice in the name of the bean. Any reference to it
also has to use the same `id`, which is the path. If used as an inner
bean, there is no need to refer to it at all, as the following example shows:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="..." class="...">
		<property name="age">
			<bean id="person.age"
					class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
		</property>
	</bean>
----

You can specifically set the result type in the actual definition. This is not necessary
for most use cases, but it can sometimes be useful. See the javadoc for more info on
this feature.


[[xsd-schemas-util-properties]]
==== Using `<util:properties/>`

Consider the following example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.Properties instance with values loaded from the supplied location -->
	<bean id="jdbcConfiguration" class="org.springframework.beans.factory.config.PropertiesFactoryBean">
		<property name="location" value="classpath:com/foo/jdbc-production.properties"/>
	</bean>
----

The preceding configuration uses a Spring `FactoryBean` implementation (the
`PropertiesFactoryBean`) to instantiate a `java.util.Properties` instance with values
loaded from the supplied <<core.adoc#resources, `Resource`>> location).

The following example uses a `util:properties` element to make a more concise representation:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.Properties instance with values loaded from the supplied location -->
	<util:properties id="jdbcConfiguration" location="classpath:com/foo/jdbc-production.properties"/>
----


[[xsd-schemas-util-list]]
==== Using `<util:list/>`

Consider the following example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.List instance with values loaded from the supplied 'sourceList' -->
	<bean id="emails" class="org.springframework.beans.factory.config.ListFactoryBean">
		<property name="sourceList">
			<list>
				<value>pechorin@hero.org</value>
				<value>raskolnikov@slums.org</value>
				<value>stavrogin@gov.org</value>
				<value>porfiry@gov.org</value>
			</list>
		</property>
	</bean>
----

The preceding configuration uses a Spring `FactoryBean` implementation (the
`ListFactoryBean`) to create a `java.util.List` instance and initialize it with values taken
from the supplied `sourceList`.

The following example uses a `<util:list/>` element to make a more concise representation:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.List instance with the supplied values -->
	<util:list id="emails">
		<value>pechorin@hero.org</value>
		<value>raskolnikov@slums.org</value>
		<value>stavrogin@gov.org</value>
		<value>porfiry@gov.org</value>
	</util:list>
----

You can also explicitly control the exact type of `List` that is instantiated and
populated by using the `list-class` attribute on the `<util:list/>` element. For
example, if we really need a `java.util.LinkedList` to be instantiated, we could use the
following configuration:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<util:list id="emails" list-class="java.util.LinkedList">
		<value>jackshaftoe@vagabond.org</value>
		<value>eliza@thinkingmanscrumpet.org</value>
		<value>vanhoek@pirate.org</value>
		<value>d'Arcachon@nemesis.org</value>
	</util:list>
----

If no `list-class` attribute is supplied, the container chooses a `List` implementation.


[[xsd-schemas-util-map]]
==== Using `<util:map/>`

Consider the following example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.Map instance with values loaded from the supplied 'sourceMap' -->
	<bean id="emails" class="org.springframework.beans.factory.config.MapFactoryBean">
		<property name="sourceMap">
			<map>
				<entry key="pechorin" value="pechorin@hero.org"/>
				<entry key="raskolnikov" value="raskolnikov@slums.org"/>
				<entry key="stavrogin" value="stavrogin@gov.org"/>
				<entry key="porfiry" value="porfiry@gov.org"/>
			</map>
		</property>
	</bean>
----

The preceding configuration uses a Spring `FactoryBean` implementation (the
`MapFactoryBean`) to create a `java.util.Map` instance initialized with key-value pairs
taken from the supplied `'sourceMap'`.

The following example uses a `<util:map/>` element to make a more concise representation:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.Map instance with the supplied key-value pairs -->
	<util:map id="emails">
		<entry key="pechorin" value="pechorin@hero.org"/>
		<entry key="raskolnikov" value="raskolnikov@slums.org"/>
		<entry key="stavrogin" value="stavrogin@gov.org"/>
		<entry key="porfiry" value="porfiry@gov.org"/>
	</util:map>
----

You can also explicitly control the exact type of `Map` that is instantiated and
populated by using the `'map-class'` attribute on the `<util:map/>` element. For
example, if we really need a `java.util.TreeMap` to be instantiated, we could use the
following configuration:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<util:map id="emails" map-class="java.util.TreeMap">
		<entry key="pechorin" value="pechorin@hero.org"/>
		<entry key="raskolnikov" value="raskolnikov@slums.org"/>
		<entry key="stavrogin" value="stavrogin@gov.org"/>
		<entry key="porfiry" value="porfiry@gov.org"/>
	</util:map>
----

If no `'map-class'` attribute is supplied, the container chooses a `Map` implementation.


[[xsd-schemas-util-set]]
==== Using `<util:set/>`

Consider the following example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.Set instance with values loaded from the supplied 'sourceSet' -->
	<bean id="emails" class="org.springframework.beans.factory.config.SetFactoryBean">
		<property name="sourceSet">
			<set>
				<value>pechorin@hero.org</value>
				<value>raskolnikov@slums.org</value>
				<value>stavrogin@gov.org</value>
				<value>porfiry@gov.org</value>
			</set>
		</property>
	</bean>
----

The preceding configuration uses a Spring `FactoryBean` implementation (the
`SetFactoryBean`) to create a `java.util.Set` instance initialized with values taken
from the supplied `sourceSet`.

The following example uses a `<util:set/>` element to make a more concise representation:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!-- creates a java.util.Set instance with the supplied values -->
	<util:set id="emails">
		<value>pechorin@hero.org</value>
		<value>raskolnikov@slums.org</value>
		<value>stavrogin@gov.org</value>
		<value>porfiry@gov.org</value>
	</util:set>
----

You can also explicitly control the exact type of `Set` that is instantiated and
populated by using the `set-class` attribute on the `<util:set/>` element. For
example, if we really need a `java.util.TreeSet` to be instantiated, we could use the
following configuration:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<util:set id="emails" set-class="java.util.TreeSet">
		<value>pechorin@hero.org</value>
		<value>raskolnikov@slums.org</value>
		<value>stavrogin@gov.org</value>
		<value>porfiry@gov.org</value>
	</util:set>
----

If no `set-class` attribute is supplied, the container chooses a `Set` implementation.



[[xsd-schemas-aop]]
=== The `aop` Schema

The `aop` tags deal with configuring all things AOP in Spring, including Spring's
own proxy-based AOP framework and Spring's integration with the AspectJ AOP framework.
These tags are comprehensively covered in the chapter entitled <<core.adoc#aop,
Aspect Oriented Programming with Spring>>.

In the interest of completeness, to use the tags in the `aop` schema, you need to have
the following preamble at the top of your Spring XML configuration file (the text in the
snippet references the correct schema so that the tags in the `aop` namespace
are available to you):

[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:aop="http://www.springframework.org/schema/aop"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/aop https://www.springframework.org/schema/aop/spring-aop.xsd">

		<!-- bean definitions here -->

	</beans>
----



[[xsd-schemas-context]]
=== The `context` Schema

The `context` tags deal with `ApplicationContext` configuration that relates to plumbing
-- that is, not usually beans that are important to an end-user but rather beans that do
a lot of the "`grunt`" work in Spring, such as `BeanfactoryPostProcessors`. The following
snippet references the correct schema so that the elements in the `context` namespace are
available to you:

[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:context="http://www.springframework.org/schema/context"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">

		<!-- bean definitions here -->

	</beans>
----


[[xsd-schemas-context-pphc]]
==== Using `<property-placeholder/>`

This element activates the replacement of `${...}` placeholders, which are resolved against a
specified properties file (as a <<core.adoc#resources, Spring resource location>>). This element
is a convenience mechanism that sets up a <<core.adoc#beans-factory-placeholderconfigurer,
`PropertySourcesPlaceholderConfigurer`>> for you. If you need more control over the specific
`PropertySourcesPlaceholderConfigurer` setup, you can explicitly define it as a bean yourself.


[[xsd-schemas-context-ac]]
==== Using `<annotation-config/>`

This element activates the Spring infrastructure to detect annotations in bean classes:

* Spring's <<core.adoc#beans-factory-metadata, `@Configuration`>> model
* <<core.adoc#beans-annotation-config, `@Autowired`/`@Inject`>> and `@Value`
* JSR-250's `@Resource`, `@PostConstruct` and `@PreDestroy` (if available)
* JPA's `@PersistenceContext` and `@PersistenceUnit` (if available)
* Spring's <<core.adoc#context-functionality-events-annotation, `@EventListener`>>

Alternatively, you can choose to explicitly activate the individual `BeanPostProcessors`
for those annotations.

NOTE: This element does not activate processing of Spring's
<<data-access.adoc#transaction-declarative-annotations, `@Transactional`>> annotation;
you can use the <<data-access.adoc#tx-decl-explained, `<tx:annotation-driven/>`>>
element for that purpose. Similarly, Spring's
<<integration.adoc#cache-annotations, caching annotations>> need to be explicitly
<<integration.adoc#cache-annotation-enable, enabled>> as well.


[[xsd-schemas-context-component-scan]]
==== Using `<component-scan/>`

This element is detailed in the section on <<core.adoc#beans-annotation-config,
annotation-based container configuration>>.


[[xsd-schemas-context-ltw]]
==== Using `<load-time-weaver/>`

This element is detailed in the section on <<core.adoc#aop-aj-ltw,
load-time weaving with AspectJ in the Spring Framework>>.


[[xsd-schemas-context-sc]]
==== Using `<spring-configured/>`

This element is detailed in the section on <<core.adoc#aop-atconfigurable,
using AspectJ to dependency inject domain objects with Spring>>.


[[xsd-schemas-context-mbe]]
==== Using `<mbean-export/>`

This element is detailed in the section on <<integration.adoc#jmx-context-mbeanexport,
configuring annotation-based MBean export>>.



[[xsd-schemas-beans]]
=== The Beans Schema

Last but not least, we have the elements in the `beans` schema. These elements
have been in Spring since the very dawn of the framework. Examples of the various elements
in the `beans` schema are not shown here because they are quite comprehensively covered
in <<core.adoc#beans-factory-properties-detailed, dependencies and configuration in detail>>
(and, indeed, in that entire <<core.adoc#beans, chapter>>).

Note that you can add zero or more key-value pairs to `<bean/>` XML definitions.
What, if anything, is done with this extra metadata is totally up to your own custom
logic (and so is typically only of use if you write your own custom elements as described
in the appendix entitled <<xml-custom>>).

The following example shows the `<meta/>` element in the context of a surrounding `<bean/>`
(note that, without any logic to interpret it, the metadata is effectively useless
as it stands).

[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"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd">

		<bean id="foo" class="x.y.Foo">
			<meta key="cacheName" value="foo"/> <1>
			<property name="name" value="Rick"/>
		</bean>

	</beans>
----
<1> This is the example `meta` element

In the case of the preceding example, you could assume that there is some logic that consumes
the bean definition and sets up some caching infrastructure that uses the supplied metadata.




[[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 appendix entitled <<xsd-config>>.

To create new XML configuration extensions:

. <<xsd-custom-schema, Author>> an XML schema to describe your custom element(s).
. <<xsd-custom-namespacehandler, Code>> a custom `NamespaceHandler` implementation.
. <<xsd-custom-parser, Code>> one or more `BeanDefinitionParser` implementations
  (this is where the real work is done).
. <<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.com/schema/myns"
			xmlns:xsd="http://www.w3.org/2001/XMLSchema"
			xmlns:beans="http://www.springframework.org/schema/beans"
			targetNamespace="http://www.mycompany.com/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-HH-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
  <<core.adoc#beans-factory-scopes, 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:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package org.springframework.samples.xml;

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

	public class MyNamespaceHandler extends NamespaceHandlerSupport {

		public void init() {
			registerBeanDefinitionParser("dateformat", new 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:

[source,java,indent=0]
----
	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.addConstructorArg(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.

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.com/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.com/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.com/schema/myns"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.mycompany.com/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.com/schema/component"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.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="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:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package com.foo;

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

	public class Component {

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

		// mmm, 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;
		}
	}
----

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`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	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;
		}
	}
----

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 <<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.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>
----

Again following <<xsd-custom-introduction, the process described earlier>>,
we then create a custom `NamespaceHandler`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	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 we are creating
a `BeanDefinition` that describes a `ComponentFactoryBean`. The following
listing shows our custom `BeanDefinitionParser` implementation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	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<BeanDefinition>(childElements.size());
			for (Element element : 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.com/schema/component=com.foo.ComponentNamespaceHandler
----

[literal]
[subs="verbatim,quotes"]
----
# in 'META-INF/spring.schemas'
http\://www.foo.com/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
https://jcp.org/en/jsr/detail?id=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`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	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 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.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, we need to create the associated `NamespaceHandler`, as follows:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	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, 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:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	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;
		}
	}
----

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.com/schema/jcache=com.foo.JCacheNamespaceHandler
----

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