The expression validator is a Struts non-field validator which can be used to validate the relations among fields (or any variables in the value stack) using OGNL expression, rather than validating a single field. For example, checking if an integer field is greater than another, or checking if a date field must be after another. This validator can be used in either of the following forms:

• • XML: using type=”expression” attribute in the <validator> element.
  • Annotation: using @ExpressionValidator annotation type to annotate action method.

1. Struts Expression Validator XML

Because this is a non-field validator so it can be only used with the plain-validator syntax as follows:

<validator type="expression">
	<param name="expression">OGNL expression</param>
	<message>validation error message</message>
</validator> 

Parameters:

Parameter name	Description
expression	The OGNL expression to be evaluated (to a boolean value).

Struts Expression Validator XML Examples:

• • Suppose that we want to check if two integer fields, number1and number2 to ensure that the number1is always less than the number2:
    

    <validator type="expression">
    	<param name="expression">number1 lt number2</param>
    	<message>The number #1 must be less than the number #2</message>
    </validator>

     

  • In the following example, the expression validator ensures that the end date is always after the start date:
    

    <validator type="expression">
    	<param name="expression">startDate lt endDate</param>
    	<message>The start date must be before the end date</message>
    </validator>

     

Some OGNL comparison operators we can use are:

• • lt: less than
  • lte: less than or equal
  • gt: greater than
  • gte: greater than or equal
  • eq: equal
  • neq: not equal
  • etc

 

2. Struts @ExpressionValidator Annotation

Usage: Put the @ExpressionValidatorannotation before the action method in the following form:

@ExpressionValidator (param1 = "param 1 value", param2 = "param 2 value", ...)

Parameters:

Parameter name	Required	Default value	Description
message	Yes		Validation error message.
key	No		i18n key for validation error message.
messageParams	No		Additional parameters to customize the message.
shortCircuit	No	false	Whether this validator is short circuit.
expression	Yes		The OGNL expression to be evaluated (to a boolean value).

Struts @ExpressionValidator Annotation Examples:

• • Annotating the action execute()method:
    

    @ExpressionValidator(
    	expression = "number1 lt number2",
    	message = "The number #1 must be less than the number #2"
    )
    public String execute() {
    	return SUCCESS;
    }

     

  • Specifying i18n key for the message:
    

    @ExpressionValidator(
    	expression = "number1 lt number2",
    	message = "the default message if the i18n key not found",
    	key = "form.validation.numbers"
    )
    public String execute() {
    	return SUCCESS;
    }

     

Related Struts Form Validation Tutorials:

• Struts Form Handling Tutorial
• Struts Form Validation Tutorial
• Struts Date Range Field Validator Example
• Struts Integer Range Field Validator Example
• Struts String Length Field Validator Example
• Struts Required Field Validator Example
• Struts Field Expression Validator Example
• Struts URL Validator Example

 

Other Struts Tutorials:

• Introduction to Struts 2 framework
• Struts beginner tutorial (Eclipse + Tomcat + XML)
• Struts Beginner Tutorial with Annotations
• Struts beginner tutorial with Convention Plugin (zero-configuration)
• How to handle exceptions in Struts
• Send e-mail with attachments in Struts
• Struts File Upload Tutorial
• Struts - Spring - Hibernate Integration Tutorial