Twelve Best Practices For Spring XML Configurations

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

• The weblog post this article is based upon
• The Spring framework website for further information about Spring XML configurations
• The XML specification for IDREF constraints

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

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