How to prepopulate a form in Struts

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The simplest way to prepopulate a form is to have an Action whose sole purpose is to populate an ActionForm and forward to the servlet or JSP to render that form back to the client. A separate Action would then be use to process the submitted form fields, by declaring an instance of the same form bean name.

The struts-example example application that is shipped with Struts illustrates this design pattern nicely. Note the following definitions from the struts-config.xml file:

        ...
        <form-beans>
            ...
            <-- Registration form bean -->
            <form-bean name="registrationForm"
                       type="org.apache.struts.webapp.example.RegistrationForm"/>
            ...
        </form-beans>
        ...
        <action-mappings>
            ...
            <-- Edit user registration -->
            <action path="/editRegistration"
                    type="org.apache.struts.webapp.example.EditRegistrationAction"
                    name="registrationForm"
                   scope="request"
                validate="false"/>
            ...
            <-- Save user registration -->
            <action path="/saveRegistration"
                    type="org.apache.struts.webapp.example.SaveRegistrationAction"
                    name="registrationForm"
                   input="registration"
                   scope="request"/>
            ...
        </action-mappings>

Note the following features of this approach:

Both the /editRegistration and /saveRegistration actions use the same form bean.
When the /editRegistration action is entered, Struts will have pre-created an empty form bean instance, and passed it to the execute() method. The setup action is free to preconfigure the values that will be displayed when the form is rendered, simply by setting the corresponding form bean properties.
When the setup action completes configuring the properties of the form bean, it should return an ActionForm that points at the page which will display this form. If you are using the Struts JSP tag library, the action attribute on your <html:form> tag will be set to /saveRegistration in order for the form to be submitted to the processing action.
Note that the setup action (/editRegistration) turns off validation on the form that is being set up. You will normally want to include this attribute in the configuration of your setup actions, because you are not planning to actually process the results -- you simply want to take advantage of the fact that Struts will precreate a form bean instance of the correct class for you.
The processing action (/saveRegistration), on the other hand, leaves out the validate attribute, which defaults to true. This tells Struts to perform the validations associated with this form bean before invoking the processing action at all. If any validation errors have occurred, Struts will forward back to your input page (technically, it forwards back to an ActionForward named "registration" in this case, because the example webapp uses the inputForward attribute in the <controller> element -- see the documentation describing struts-config.xml for more information) instead of calling your processing action.