## 7.4 Validation and Internationalization

Another important thing to note about errors in Grails is that the messages that the errors display are not hard coded anywhere. The FieldError class in Spring essentially resolves messages from message bundles using Grails' i18n support.

#### Constraints and Message Codes

The codes themselves are dictated by a convention. For example consider the constraints we looked at earlier:

    package com.mycompany.myappclass User {
        ...    static constraints = {
            login(size:5..15, blank:false, unique:true)
            password(size:5..15, blank:false)
            email(email:true, blank:false)
            age(min:18, nullable:false)
        }
    }

If the `blank` constraint was violated Grails will, by convention, look for a message code in the form:

    [Class Name].[Property Name].[Constraint Code]

In the case of the `blank` constraint this would be `user.login.blank` so you would need a message such as the following in your `grails-app/i18n/messages.properties` file:

    user.login.blank=Your login name must be specified!

The class name is looked for both with and without a package, with the packaged version taking precedence. So for example, com.mycompany.myapp.User.login.blank will be used before user.login.blank. This allows for cases where you domain class encounters message code clashes with plugins.

For a reference on what codes are for which constraints refer to the reference guide for each constraint.

#### Displaying Messages

The renderErrors tag will automatically deal with looking up messages for you using the message tag. However, if you need more control of rendering you will need to do this yourself:

    <g:hasErrors bean="${user}">
      <ul>
       <g:eachError var="err" bean="${user}">
           <li><g:message error="${err}" /></li>
       </g:eachError>
      </ul>
    </g:hasErrors>

In this example within the body of the eachError tag we use the message tag in combination with its `error` argument to read the message for the given error.