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Twelve Best Practices For Spring XML Configurations

by Jason Zhicheng Li
01/25/2006

Spring is a powerful Java application framework, used in a wide range of Java applications. It provides enterprise services to Plain Old Java Objects (POJOs). Spring uses dependency injection to achieve simplification and increase testability. Spring beans, dependencies, and the services needed by beans are specified in configuration files, which are typically in an XML format. The XML configuration files, however, are verbose and unwieldy. They can become hard to read and manage when you are working on a large project where many Spring beans are defined.

In this article, I will show you 12 best practices for Spring XML configurations. Some of them are more necessary practices than best practices. Note that other factors, such as domain model design, can impact the XML configuration, but this article focuses on the XML configuration's readability and manageability.

1. Avoid using autowiring

Spring can autowire dependencies through introspection of the bean classes so that you do not have to explicitly specify the bean properties or constructor arguments. Bean properties can be autowired either by property names or matching types. Constructor arguments can be autowired by matching types. You can even specify the autodetect autowiring mode, which lets Spring choose an appropriate mechanism. As an example, consider the following:

    <bean id="orderService"
        class="com.lizjason.spring.OrderService"
        autowire="byName"/>

The property names of the OrderService class are used to match a bean instance in the container. Autowiring can potentially save some typing and reduce clutter. However, you should not use it in real-world projects because it sacrifices the explicitness and maintainability of the configurations. Many tutorials and presentations tout autowiring as a cool feature in Spring without mentioning this implication. In my opinion, like object-pooling in Spring, it is more a marketing feature. It seems like a good idea to make the XML configuration file smaller, but this will actually increase the complexity down the road, especially when you are working on a large project where many beans are defined. Spring allows you mix autowiring and explicit wiring, but the inconsistency will make the XML configurations even more confusing.

2. Use naming conventions

This is the same philosophy as for Java code. Using clear, descriptive, and consistent name conventions across the project is very helpful for developers to understand the XML configurations. For bean ID, for example, you can follow the Java class field name convention. The bean ID for an instance of OrderServiceDAO would be orderServiceDAO.For large projects, you can add the package name as the prefix of the bean ID.

3. Use shortcut forms

The shortcut form is less verbose, since it moves property values and references from child elements into attributes. For example, the following:

    <bean id="orderService"
        class="com.lizjason.spring.OrderService">
        <property name="companyName">
            <value>lizjason</value>
        </property>
        <constructor-arg>
            <ref bean="orderDAO">
        </constructor-arg>
    </bean>

can be rewritten in the shortcut form as:

    <bean id="orderService"
        class="com.lizjason.spring.OrderService">
        <property name="companyName"
            value="lizjason"/>
        <constructor-arg ref="orderDAO"/>
    </bean>

The shortcut form has been available since version 1.2. Note that there is no shortcut form for <ref local="...">.

The shortcut form not only saves you some typing, but also makes the XML configuration files less cluttered. It can noticeably improve readability when many beans are defined in a configuration file.

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4. Prefer type over index for constructor argument matching

Spring allows you to use a zero-based index to solve the ambiguity problem when a constructor has more than one arguments of the same type, or value tags are used. For example, instead of:

    <bean id="billingService"
        class="com.lizjason.spring.BillingService">
        <constructor-arg index="0" value="lizjason"/>
        <constructor-arg index="1" value="100"/>
    </bean>

It is better to use the type attribute like this:

    <bean id="billingService"
        class="com.lizjason.spring.BillingService">
        <constructor-arg type="java.lang.String"
            value="lizjason"/>
        <constructor-arg type="int" value="100"/>
    </bean>

Using index is somewhat less verbose, but it is more error-prone and hard to read compared to using the type attribute. You should only use index when there is an ambiguity problem in the constructor arguments.

5. Reuse bean definitions, if possible

Spring offers an inheritance-like mechanism to reduce the duplication of configuration information and make the XML configuration simpler. A child bean definition can inherit configuration information from its parent bean, which essentially serves as a template for the child beans. This is a must-use feature for large projects. All you need to do is to specify abstract=true for the parent bean, and the parent reference in the child bean. For example:

    <bean id="abstractService" abstract="true"
        class="com.lizjason.spring.AbstractService">
        <property name="companyName"
            value="lizjason"/>
    </bean>

    <bean id="shippingService"
        parent="abstractService"
        class="com.lizjason.spring.ShippingService">
        <property name="shippedBy" value="lizjason"/>
    </bean>

The shippingService bean inherits the value lizjason for the companyName property from the abstractService bean. Note that if you do not specify a class or factory method for a bean definition, the bean is implicitly abstract.

6. Prefer assembling bean definitions through ApplicationContext over imports

Like imports in Ant scripts, Spring import elements are useful for assembling modularized bean definitions. For example:

    <beans>
        <import resource="billingServices.xml"/>
        <import resource="shippingServices.xml"/>
        <bean id="orderService"
            class="com.lizjason.spring.OrderService"/>
    <beans>

However, instead of pre-assembling them in the XML configurations using imports, it is more flexible to configure them through the ApplicationContext. Using ApplicationContext also makes the XML configurations easy to manage. You can pass an array of bean definitions to the ApplicationContext constructor as follows:

    String[] serviceResources =
        {"orderServices.xml",
        "billingServices.xml",
        "shippingServices.xml"};
    ApplicationContext orderServiceContext = new
        ClassPathXmlApplicationContext(serviceResources);

7. Use ids as bean identifiers

You can specify either an id or name as the bean identifier. Using ids will not increase readability, but it can leverage the XML parser to validate the bean references. If ids cannot be used due to XML IDREF constraints, you can use names as the bean identifiers. The issue with XML IDREF constraints is that the id must begin with a letter (or one of a few punctuation characters defined in the XML specification) followed by letters, digits, hyphens, underscores, colons, or full stops. In reality, it is very rare to run into the XML IDREF constraint problem.

8. Use dependency-check at the development phase

You can set the dependency-check attribute on a bean definition to a value other than the default none, such as simple, objects, or all, so that the container can do the dependency validation for you. It is useful when all of the properties (or certain categories of properties) of a bean must be set explicitly, or via autowiring.

    <bean id="orderService"
        class="com.lizjason.spring.OrderService"
        dependency-check="objects">
        <property name="companyName"
            value="lizjason"/>
        <constructor-arg ref="orderDAO"/>
    </bean>

In this example, the container will ensure that properties that are not primitives or collections are set for the orderService bean. It is possible to enable the default dependency check for all of the beans, but this feature is rarely used because there can be beans with properties that don't need to be set.

9. Add a header comment to each configuration file

It is preferred to use descriptive ids and names instead of inline comments in the XML configuration files. In addition, it is helpful to add a configuration file header, which summarizes the beans defined in the file. Alternatively, you can add descriptions to the description element. For example:

    <beans>
        <description>
            This file defines billing service
            related beans and it depends on
            baseServices.xml,which provides
            service bean templates...
        </description>
        ...
    </beans>

One advantage of using the description element is that it is easy to for tools to pick up the description from this element.

10. Communicate with team members for changes

When you are refactoring Java source code, you need to make sure to update the configuration files accordingly and notify team members. The XML configurations are still code, and they are critical parts of the application, but they are hard to read and maintain. Most of the time, you need to read both the XML configurations and Java source code to figure out what is going on.

11. Prefer setter injection over constructor injection

Spring provides three types of dependency injection: constructor injection, setter injection, and method injection. Typically we only use the first two types.

    <bean id="orderService"
        class="com.lizjason.spring.OrderService">
        <constructor-arg ref="orderDAO"/>
    </bean>

    <bean id="billingService"
        class="com.lizjason.spring.BillingService">
        <property name="billingDAO"
            ref="billingDAO">
    </bean>

In this example, the orderService bean uses constructor injection, while the BillingService bean uses setter injection. Constructor injection can ensure that a bean cannot be constructed in an invalid state, but setter injection is more flexible and manageable, especially when the class has multiple properties and some of them are optional.

12. Do not abuse dependency injection

As the last point, Spring ApplicationContext can create Java objects for you, but not all Java objects should be created through dependency injection. As an example, domain objects should not be created through ApplicationContext. Spring is an excellent framework, but, as far as the readability and manageability are concerned, the XML-based configuration can become an issue when many beans are defined. Overuse of dependency injection will make the XML configuration more complicated and bloated. Remember, with powerful IDEs, such as Eclipse and IntelliJ, Java code is much easier to read, maintain, and manage than XML files!

Conclusion

XML is the prevailing format for Spring configurations. XML-based configuration can become verbose and unwieldy when many beans are defined. Spring provides a rich set of configuration options. Appropriately using some of the options can make the XML configurations less cluttered, but other options, like autowiring, may reduce readability and maintainability. Following good practices discussed in the article may help you to create clean and readable XML configuration files!

Resources

Jason Zhicheng Li is a senior software engineer with Object Computing, Inc. in St. Louis, MO.


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