Jakarta Struts Cookbook

8.1. Reusing Validator Attribute Values

Problem

You want to define, in one place, a common value you can reference wherever needed in a Validator form.

Solution

Define the value as a global or form-set constant in one of your validation documents, as shown in Example 8-3.

Example 8-3. Defining global and form-set validator constants

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE form-validation PUBLIC
 "-//Apache Software Foundation//DTD Commons Validator Rules 
 Configuration 1.1//EN"
 "http://jakarta.apache.org/commons/dtds/validator_1_1_3.dtd">
<form-validation>
    <global>
        <constant>
            <constant-name>globalVarName</constant-name>
            <constant-value>globalVarValue</constant-value>
        </constant>
    </global>
    <formset>
        <constant>
            <constant-name>formsetVarName</constant-name>
            <constant-value>formsetVarValue</constant-value>
        </constant>       
        <form name="MyForm">
            <field property="myfield"
                    depends="someRule,anotherRule">
                <var>
                    <var-name>someRule</var-name>
                    <var-value>${globalVarName}</var-value>
                </var>
                <var>
                    <var-name>anotherRule</var-name>
                    <var-value>${formsetVarName}</var-value>
                </var>
            </field>
        </form>
    <formset>
<form-validation>

Discussion

Good developers understand the Don't Repeat Yourself (DRY) principle of software engineering. But if they fail to follow this principle, they could end up all wet. The Validator embraces this guideline, making it an excellent choice for input validation. If you have an attribute value shared by multiple field elements, you can define the value as a named constant. ...