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Appendix

XML Schemas

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

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):

<?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>

Using <util:constant/>

Consider the following bean definition:

<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:

<bean id="..." class="...">
	<property name="isolation">
		<util:constant static-field="java.sql.Connection.TRANSACTION_SERIALIZABLE"/>
	</property>
</bean>
Setting a Bean Property or Constructor Argument from a Field Value

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 staticField property:

<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:

<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:

<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 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:

Java
package javax.persistence;

public enum PersistenceContextType {

	TRANSACTION,
	EXTENDED
}
Kotlin
package javax.persistence

enum class PersistenceContextType {

	TRANSACTION,
	EXTENDED
}

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

Java
package example;

public class Client {

	private PersistenceContextType persistenceContextType;

	public void setPersistenceContextType(PersistenceContextType type) {
		this.persistenceContextType = type;
	}
}
Kotlin
package example

class Client {

	lateinit var persistenceContextType: PersistenceContextType
}
<bean class="example.Client">
	<property name="persistenceContextType" value="TRANSACTION"/>
</bean>

Using <util:property-path/>

Consider the following example:

<!-- 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:

<!-- 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.

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:

// 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:

<!-- 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:

<!-- 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:

<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.

Using <util:properties/>

Consider the following example:

<!-- 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 Resource location).

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

<!-- creates a java.util.Properties instance with values loaded from the supplied location -->
<util:properties id="jdbcConfiguration" location="classpath:com/foo/jdbc-production.properties"/>

Using <util:list/>

Consider the following example:

<!-- 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:

<!-- 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:

<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.

Using <util:map/>

Consider the following example:

<!-- 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:

<!-- 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:

<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.

Using <util:set/>

Consider the following example:

<!-- 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:

<!-- 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:

<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.

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 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):

<?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>

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:

<?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>

Using <property-placeholder/>

This element activates the replacement of ${…​} placeholders, which are resolved against a specified properties file (as a Spring resource location). This element is a convenience mechanism that sets up a PropertySourcesPlaceholderConfigurer for you. If you need more control over the specific PropertySourcesPlaceholderConfigurer setup, you can explicitly define it as a bean yourself.

Using <annotation-config/>

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

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

Note
 This element does not activate processing of Spring’s @Transactional annotation; you can use the <tx:annotation-driven/> element for that purpose. Similarly, Spring’s caching annotations need to be explicitly enabled as well.

Using <component-scan/>

This element is detailed in the section on annotation-based container configuration.

Using <load-time-weaver/>

This element is detailed in the section on load-time weaving with AspectJ in the Spring Framework.

Using <spring-configured/>

This element is detailed in the section on using AspectJ to dependency inject domain objects with Spring.

Using <mbean-export/>

This element is detailed in the section on configuring annotation-based MBean export.

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 dependencies and configuration in detail (and, indeed, in that entire 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 Schema Authoring).

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).

<?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 Schema Authoring

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 appendix.adoc.

To create new XML configuration extensions:

  1. Author an XML schema to describe your custom element(s).

  2. Code a custom NamespaceHandler implementation.

  3. Code one or more BeanDefinitionParser implementations (this is where the real work is done).

  4. 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:

<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.)

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:

<!-- 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:

<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:

<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.

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 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:

Java
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
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.

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:

Java
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
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.

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.

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:

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.

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:

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.

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:

<?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.

More Detailed Examples

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

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:

<?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:

Java
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;
	}
}
Kotlin
package com.foo

import java.util.ArrayList

class Component {

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

	// mmm, 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:

Java
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
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 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:

<?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 the process described earlier, we then create a custom NamespaceHandler:

Java
package com.foo;

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

public class ComponentNamespaceHandler extends NamespaceHandlerSupport {

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

Java
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);
	}
}
Kotlin
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:

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

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 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:

<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:

Java
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...
	}
}
Kotlin
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:

<?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:

Java
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
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:

Java
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
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:

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