Programming Jakarta Struts, 2nd Edition [Electronic resources] نسخه متنی

7.4 Performing Presentation Validation

This chapter has touched on performing your
application's input
validation
in the validate() method of the
ActionForm. You can create whatever presentation
validation rules you need in this method. For example, the
LoginForm from Example 7-2
validated that the email and password fields were entered and were
not empty strings. Although this is a trivial example, you can
validate anything you like. A common validation rule is to ensure
that a string value that should be a number is in fact a string
representation of a valid number. The validate()
routine for this rule might look like the one in Example 7-5.

Example 7-5. Performing a number validation rule

public ActionErrors validate(ActionMapping mapping, HttpServletRequest request) {
ActionErrors errors = new ActionErrors( );
String orderQtyStr = getQuantity( );
if( orderQtyStr == null || orderQtyStr.length( ) < 1 ){
errors.add( ActionMessages.GLOBAL_MESSAGE,
new ActionMessage( "order.quantity.required" ));
}
// Validate that the qty entered was in fact a number
try{
// Integer.parse was not used because it's not really I18N-safe
java.text.Format format = java.text.NumberFormat.getNumberInstance( );
Number orderQty = (Number)format.parseObject( orderQtyStr );
}catch( Exception ex ){
// The quantity entered by the user was not a valid qty
errors.add( ActionMessages.GLOBAL_MESSAGE,
new ActionMessage( "order.quantity.invalid" ));
}
return errors;
}

As you can imagine, web applications often need to check for required
values or validate that data entered fits a certain format or is of a
certain type. Because all data that is retrieved from a request is of
type String, you must ensure that the data is not
going to corrupt your business components.

Although you can perform the validation programmatically, the Struts
framework provides an alternative that can be external to the
ActionForm and the validate()
method. In fact, most of the standard validation rules already are
defined, so you don't have to write any code for
them. All you need to do is to declaratively configure the rules that
you need in an extra XML file. The Struts Validator will be covered
in Chapter 11.