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Apache Struts 2 Documentation
Validating Input
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In the Coding Actions lesson, we validated the username and password input with a few lines of Java code. Of course, in a larger application, over time, even these few lines of code can become a maintenance burden.

Happily, the framework provides a validation framework that can validate input "behind the scenes".

## The Code

Validation can be described through an XML document, or using annotations. The XML document is named after the Action being validated with a "-validation" suffix. Since we would like to validate the Logon Action class, our document is named Logon-validation.xml.

Logon-validation.xml

    <!DOCTYPE validators PUBLIC
    "-//OpenSymphony Group//XWork Validator 1.0.2//EN"
    "http://www.opensymphony.com/xwork/xwork-validator-1.0.2.dtd">

    <validators>
        <field name="username">
            <field-validator type="requiredstring">
                <message>Username is required</message>
            </field-validator>
        </field>
        <field name="password">
            <field-validator type="requiredstring">
                <message>Password is required</message>
            </field-validator>
        </field>
    </validators>

Create a file named `Logon-validation.xml` next to the `Logon.java` class, and paste in the code.

The first time a page displays, we wouldn't want the validation to fire. We should have a chance to enter the input before being told it's incorrect. One way to bypass validation is to refer to a special "input" method provided by the base ActionSupport class. To do that, we need to edit the Welcome page and the Logon mapping.

To indicate the changes, we'll use to indicate a line we are removing, and to indicated a line we are adding.

#### Welcome.jsp

`<li><a href="<s:url action="Logon"/>">Sign On</a></li>`
`<li><a href="<s:url action="Logon_input"/>">Sign On</a></li>`

#### struts.xml

`<action name="Logon" class="tutorial.Logon">`
`<action name="Logon_*" method="{1}" class="tutorial.Logon">`

## How the Code Works

To open the Logon form, the Welcome page refers to `Logon_input`.

- The framework matches this reference with the `Logon_*` action mapping.
- The "method={1}" attribute is replaced with "method=input".
- The framework invokes the `input` method on the `Logon` Action class.
- Since "input" is on a special list of methods that bypass validation, the validation framework is not invoked.
- The default `input` method returns the result code "input".
- The framework renders "Logon.jsp" as the response, without any validation messages.

To submit the Logon form, the Login pages refers to `Logon`.

- The framework checks for a validation for the target Action class, `Logon`.
- Finding a `Logon-validation.xml` file, the framework creates a validation object for the class, based on the XML document.
  - Essentially, the validation is a set of Validator objects.
- The Validators are applied to the incoming properties.
- If a Validator fails, its message is added to an internal queue.
- When all the Validators have fired, if the framework sees that errors have been posted, it seeks the "input" result, without invoking the Action class.
- Otherwise, the default Action method fires. Since the input has already been validated, the "success" result code is returned.

## What to Remember

The framework provides a validation framework. A set of validators can be associated with an input field. If validation fails, the framework can return to the input page and display the error messages. To bypass validation, a special "input" Action method can be invoked, instead of the default "execute" method.

For more, see Validation in the Core Developers Guide.

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