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+====== Validator example in primefaces ======
+Here is the example of a validator in primefaces. The example is based in the following use case: suppose we have a box calle "username" that has to be validated to see if the user entered new username exists previously in the database.
+This example is run with primefaces in the presentation layer.
+<code xhtml>
+        <p:inputText id="txtUsername"
+                    value="#{profileForm.username}"
+                    required="true"
+                    requiredMessage="#{msg.get('fieldIsRequired')}"
+                    validator="#{profileForm.isUsernameValid}"/>
+        <p:message for="txtUsername"/>
+</code>
+The ''message'' tag wil hold the messages related with the validation errors. You can also use a <p:growl> tag for this.
+The backing bean has to have a method called ''isUsernameValid'' to properly validate the username. Here is an example:
+<code java>
+import javax.faces.component.UIComponent;
+import javax.faces.context.FacesContext;
+import javax.faces.validator.ValidatorException;
+	/**
+	 * Validates if the new username is available or not
+	 */
+	public void isUsernameValid(FacesContext ctx, UIComponent component, Object value)
+										throws ValidatorException
+	{
+		String newUsername = value.toString();
+		// if the username hasn't changed, is valid
+		if( newUsername.equals( initialUsername ) )
+			return;
+		// if the username has changed, but it is
+		// available in the database, is correct
+		if( !usersController.userExists(newUsername) )
+		{
+			// if it contains an space in the username,
+			// throw exception
+			if( newUsername.contains( " " ) )
+			{
+				fma.addErrorMessage( "usernameIncorrectFormat" );
+				throw new ValidatorException( fma.getLastMessage() );
+			} // username.contains
+		}
+		else
+		{
+			// if exists in the database, throw exception
+			fma.addErrorMessage( "usernameAlreadyExists" );
+			throw new ValidatorException( fma.getLastMessage() );
+		}
+	} // isUsernameValid
+</code>
+The ''fma'' object is a helper object that has a method to create a new faces message out of the resources of the application, instead of using a constant literal which will have problems for translating into various languages.
+~~DISQUS~~