Difference between modelAttribute and commandName attributes in form tag in spring?

In Spring 3, I have seen two different attribute in form tag in jsp

<form:form method="post" modelAttribute="login">

in this the attribute modelAttribute is the name of the form object whose properties are used to populate the form. And I used it in posting a form and in controller I have used @ModelAttribute to capture value, calling validator, applying business logic. Everything is fine here. Now

<form:form method="post" commandName="login">

What is expected by this attribute, is it also a form object whose properties we are going to populate?

If you look at the source code of FormTag (4.3.x) which backs your <form> element, you'll notice this

/**
 * Set the name of the form attribute in the model.
 * <p>May be a runtime expression.
 */
public void setModelAttribute(String modelAttribute) {
	this.modelAttribute = modelAttribute;
}

/**
 * Get the name of the form attribute in the model.
 */
protected String getModelAttribute() {
	return this.modelAttribute;
}

/**
 * Set the name of the form attribute in the model.
 * <p>May be a runtime expression.
 * @see #setModelAttribute
 */
public void setCommandName(String commandName) {
	this.modelAttribute = commandName;
}

/**
 * Get the name of the form attribute in the model.
 * @see #getModelAttribute
 */
protected String getCommandName() {
	return this.modelAttribute;
}

They are both referring to the same field, thus having same effect.

But, as the field name indicates, modelAttribute should be preferred, as others have also pointed out.

OLD WAY = commandName

...
<spring:url value="/manage/add.do" var="action" />
	<form:form action="${action}" commandName="employee">
		<div>
			<table>
....

NEW WAY = modelAttribute

..
<spring:url value="/manage/add.do" var="action" />
	<form:form action="${action}" modelAttribute="employee">
		<div>
			<table>
..

I had the same question a while ago, I can't remember the exact differences but from research I ascertained that commandName was the old way of doing it and in new applications you should be using modelAttribute

commandName = name of a variable in the request scope or session scope that contains the information about this form,or this is model for this view. Tt should be a been.

In xml based config, we will use command class to pass an object between controller and views. Now in annotation we are using modelattribute.