How to launch jquery modal popup in Struts2 tiles?

I'm developing a shopping cart application. In my application has customer registration form. I'm using Struts2 tiles. When the client clicks Register button, my application pops up a jquery model window. In my NewUser.java action class overridden the validate() method of ActionSupport class. If the email field is empty, How do I popup the register form with field error.

My action class:

@Override
public void validate() {
    if (customer.getEmail().equals("")) {
        addFieldError("email", "First Name is Required.");
    }
}

---

My popup window:

<div id="register" class="modal hide fade" tabindex="-1">
    <form action="CustomerAdd">
      <label class="control-label" for="email">Email address</label>
      <div class="controls">
        <s:textfield name="customer.email" cssClass="input-xlarge"/>
      </div>  
    </form>
</div>
<a href="#register" class="btn btn-small" data-toggle="modal">
    Register now &nbsp; <i class="icon-chevron-right"></i>
</a> 

---

Struts2 action:

 <action name="CustomerAdd" class="com.shopping.op.customer.CustomerAdd">
    <result name="success" type="tiles">success_customer</result>            
    <result name="failed" type="tiles">failed_customer</result>
    <result name="not_available" type="tiles">user_already_added</result>
    <result name="input" type="tiles">validate_customer</result>
 </action>

---

tiles definition:

<definition name="validate_customer" extends="baselayout">        
    <put-attribute name="opr" value="/customer/customer.jsp"/>      
    <put-attribute name="body" value="/login.jsp"/><!--This page has the Register button-->      
</definition> 

---

Thanks in Advance!

In your popup window you need to add actionerror tag to display the errors. Like this,

<form action="CustomerAdd">
  <label class="control-label" for="email">Email address</label>
  <div class="controls">
    <s:textfield name="customer.email" cssClass="input-xlarge"/>
    <s:actionerror/>
  </div>  
</form>

I hope if you properly handled validate method you may not able submit until email is entered. What you missed is display the error in popup window.