Featured Post

Applying Email Validation to a JavaFX TextField Using Binding

This example uses the same controller as in a previous post but adds a use case to support email validation.  A Commons Validator object is ...

Friday, April 15, 2011

A Class Constructor Argument to a Spring Bean using XML

Most example Spring config files I've seen deal with bean references or simple types.  This XML fragment is used to pass in a Class object to a Spring bean.


In most cases, you can use the <value> element to pass a simple type into a Spring bean. For example


<bean id="validator" class="pkg.ValidatorImpl">
   <constructor-arg index="0">
    <property name="className" value="pkg.DomainClass" />
   </constructor-arg>
</bean>


If the class doesn't support a simple type, like String in the previous example, you'll either need to expand the constructor definition to allow for a String or use a different Spring configuration based on MethodInvokingFactoryBean.

This example wraps up the Class.forName() call into a nested Spring bean.  "staticMethod" is a fully-qualified method name.  "arguments" is an ordered list of arguments for the static method.  The Spring-provided MethodInvokingFactoryBean is used to control the invoking.

<bean id="validator" class="pkg.ValidatorImpl">
   <constructor-arg index="0">
     <bean class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
      <property name="staticMethod">
       <value>java.lang.Class.forName</value>
      </property>
      <property name="arguments">
       <list>
        <value>pkg.DomainClass</value>
       </list>
    </property>
   </bean>
  </constructor-arg>
</bean>


Using simple types in your bean's definition constructor makes configuring the beans easier.  However, if you aren't able to expand the bean's constructors -- say it's from a third party library -- take a factory approach.

3 comments:

  1. Hi, Thank you for the useful information.

    I have one doubt regd. "pkg.DomainClass" - What if the package "pkg" is defined in a different project? Can Spring will be able to locate that "DomainClass" since it is not annotated with @bean?

    Thank you

    ReplyDelete
    Replies
    1. Hi,

      pkg.DomainClass is not a Spring bean class, but a .class file that appears on the classpath. This is a workaround for non-Java Bean classes, in this case java.lang.Class which doesn't have a default constructor.

      Delete