Understanding The Struts Validator Framework

To validate the form data,Struts Framework provides the functionality.It can be use to validate the data on client side as well as on the server side. Struts Framework can be used to validate the form data on the client browser and emits java scripts.By sub classing your From Bean with the DynaValidatorForm class,server side validation of the form can be accomplished .

David Winterfeldt developed the Validator framework as third-party add-on to the Struts.Now the Validator framework can be used with or without the Struts and it is a part of Jakarta Commons project .The Validator framework can be used without doing any extra settings and comes integrated with Struts Framework.

Using Validator Framework

To pickup validation rules to be applied to an form,validator uses the XML file. In XML validation requirements are applied to the form and aslo can be defined.In validator framework,we can plug in our own custom validations into Validator.

The Validator Framework uses two XML configuration files called validator-rules.xml and validation.xml. Standard validation routines are defined by the validator-rules.xml and used in validation.xml. To define form specific validations,validation.xml is used.To define the validations applied to a form bean,the validation.xml

Structure of validator-rule.xml

The validation-rules.xmldeclares and assigns the logical names to the validation routines and is provided with Validator Framework. For each validation routine,it also contains client-side javascript code . To perform specific validations,the validation routines are the java methods plugged into the system .

Following table contains the details of the elements in this file:

Element	Attributes and Description
form-validation	This is the root node. It contains nested elements for all of the other configuration settings.
global	The validator details specified within this, are global and are accessed by all forms.
validator	The validator element defines what validators objects can be used with the fields referenced by the formset elements. The attributes are: name: Contains a logical name for the validation routine classname: Name of the Form Bean class that extends the subclass of ActionForm class method: Name of the method of the Form Bean class methodParams: parameters passed to the method msg:Validator uses Struts' Resource Bundle mechanism for externalizing error messages. Instead of having hard-coded error messages in the framework, Validator allows you to specify a key to a message in the ApplicationResources.properties file that should be returned if a validation fails. Each validation routine in the validator-rules.xml file specifies an error message key as value for this attribute. depends: If validation is required, the value here is specified as 'required' for this attribute. jsFunctionName: Name of the javascript function is specified here.
javascript	Contains the code of the javascript function used for client-side validation. Starting in Struts 1.2.0 the default javascript definitions have been consolidated to commons-validator. The default can be overridden by supplying a <javascript> element with a CDATA section, just as in struts 1.1.

The Validator plug-in which is also known as validator-rules.xml is supplied with the predefined set of commonly used validation rules such as Required, Minimum Length, Maximum length, Date Validation, Email Address validation and more. If required,this basic set of rules can also be extended with the custom validators .

Structure of validation.xml

This validation.xml configuration file define which validation routines that is used to validate Form Beans and also you can define validation logic for any number of Form Beans in this configuration file . Inside that definition, you specify validations you want to apply to the Form Bean's fields and the definition in this file use the logical names of Form Beans from the struts-config.xml file along with the logical names of validation routines from the validator-rules.xml file to tie the two together.

The following table shows the elements of the validation.xml file

Element	Attributes and Description
form-validation	This is the root node. It contains nested elements for all of the other configuration settings
global	The constant details are specified in <constant> element within this element.
constant	Constant properties are specified within this element for pattern matching.
constant-name	Name of the constant property is specified here
constant-value	Value of the constant property is specified here.
formset	This element contains multiple <form> elements
form	This element contains the form details. The attributes are: name: Contains the form name. Validator uses this logical name to map the validations to a Form Bean defined in the struts-config.xml file
field	This element is inside the form element, and it defines the validations to apply to specified Form Bean fields. The attributes are: property: Contains the name of a field in the specified Form Bean depends: Specifies the logical names of validation routines from the validator-rules.xml file that should be applied to the field.
arg	A key for the error message to be thrown incase the validation fails, is specified here
var	Contains the variable names and their values as nested elements within this element.
var-name	The name of the criteria against which a field is validated is specified here as a variable
var-value	The value of the field is specified here

Example of form in the validation.xml file is as follows:

<form name="logonForm">
<field property="username"
depends="required">
<arg key="logonForm.username"/>
</field>
<field property="password"
depends="required,mask">
<arg key="logonForm.password"/>
<var>
<var-name>mask</var-name>
<var-value>^[0-9a-zA-Z]*$</var-value>
</var>
</field>
</form>

To allow front-end validation based on xml in validation.xml,the <html:javascript> tag is used.In the validation.xml file,for example the code: <html:javascript formName="logonForm" dynamicJavascript="true" staticJavascript="true" /> generate the client side java script for the form "logonForm" . The <html:javascript> generates the client site validation script when added in jsp file.

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