Understanding the concept of data binding in a Spring-MVC app

Question

I'm trying to understand the concept of data binding in Spring-MVC with Velocity (I'm learning this framework and porting an app to this platform).

I'm used to getting form variables using request.getParameter("username") , in the Spring world it seems that I can perform validation and such against " form objects " eg a datamodel style object that represent all the fields of a form.

The concept of a validator makes sense, but marshaling the data from a query string to these objects is fuzzy for me still. This is the concept of "Data Binding" correct?

If I'm correct to this point a few specific questions:

• When a "binding" is made between a form variable (say "username" for example) and the the field of an object (say org.abMyNewUserFormObj.username ) is that "binding" a permanent definition such that all subsequent http posts of that form cause the username form variable to be assigned to org.abMyNewUserFormObj.username ?
• How in the world do I accomplish the above binding definition? (if what I've said up to now is correct I feel like Costello in 'Who's on First', I don't even know what I just said!), I just need a conceptual picture.

Thanks for setting straight a brain gone astray.

Answer 1

There is no magic in data binding.

Actually, Spring simply populate properties of @ModelAttribute object with the values of request parameters with the corresponding names (in the simpliest case request parameter have the same name as a property, but nested properties are also supported).

So, if you have

<input type = "text" name = "firstName" />

and

public class Person {
    private String firstName;
    ... getters, setters ...
}

you get a value from the form field.

Spring also provides convenient method for creating HTML forms. So, instead of creating form fields manually, you can write in JSP:

<form:form modelAttribute = "person" ...>
    <form:input path = "firstName" />
</form:form>

or in Velocity (note that in this case <form> is created manually and property path is prefixed with the model attribute name):

<form ...>
    #springFormInput("person.firstName" "")
</form>

Fields of the forms generated this way will be prepopulated with the values of the corresponding properties of the model attribute (that's why model attribute name is needed).