How to perform JSF validation in actionListener or action method?

Introduction

You can do it, but JSF ajax/action/listener methods are semantically the wrong place to do validation. You actually don't want to get that far in JSF lifecycle if you've wrong input values in the form. You want the JSF lifecycle to stop after JSF validations phase.

You want to use a JSR303 Bean Validation annotation (@NotNull and friends) and/or constraint validator, or use a JSF Validator (required="true", <f:validateXxx>, etc) for that instead. It will be properly invoked during JSF validations phase. This way, when validation fails, the model values aren't updated and the business action isn't invoked and you stay in the same page/view.

As there isn't a standard Bean Validation annotation or JSF Validator for the purpose of checking if a given input value is unique according the database, you'd need to homegrow a custom validator for that.

I'll for both ways show how to create a custom validator which checks the uniqueness of the username.

Custom JSR303 Bean Validation Annotation

First create a custom @Username constraint annotation:

@Constraint(validatedBy = UsernameValidator.class)
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE})
public @interface Username {
    String message() default "Username already exists";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

With this constraint validator (note: @EJB or @Inject inside a ConstraintValidator works only since CDI 1.1; so if you're still on CDI 1.0 then you'd need to manually grab it from JNDI):

public class UsernameValidator implements ConstraintValidator<Username, String> {

    @EJB
    private UserService service;

    @Override
    public void initialize(Username constraintAnnotation) {
        // If not on CDI 1.1 yet, then you need to manually grab EJB from JNDI here.
    }

    Override
    public boolean isValid(String username, ConstraintValidatorContext context) {
        return !service.exist(username);
    }

}

Finally use it as follows in model:

@Username
private String username;

Custom JSF Validator

An alternative is to use a custom JSF validator. Just implement the JSF Validator interface:

@ManagedBean
@RequestScoped
public class UsernameValidator implements Validator {

    @EJB
    private UserService userService;

    @Override
    public void validate(FacesContext context, UIComponent component, Object submittedAndConvertedValue) throws ValidatorException {
        String username = (String) submittedAndConvertedValue;

        if (username == null || username.isEmpty()) {
            return; // Let required="true" or @NotNull handle it.
        }

        if (userService.exist(username)) {
            throw new ValidatorException(new FacesMessage("Username already in use, choose another"));
        }
    }

}

Finally use it as follows in view:

<h:inputText id="username" ... validator="#{usernameValidator}" />
<h:message for="username" />

Note that you'd normally use a @FacesValidator annotation on the Validator class, but until the upcoming JSF 2.3, it doesn't support @EJB or @Inject. See also How to inject in @FacesValidator with @EJB, @PersistenceContext, @Inject, @Autowired.

Yes you can. You can do validation in action listener method, add faces messages if your custom validation failed, then call FacesContext.validationFailed() just before return.

The only problem with this solution is, it happens after the JSF validation and bean validation. I.e., it is after the validation phase. If you have multiple action listeners, say listener1 and listener2: if your custom validation in listener1 failed, it will continue to execute listener2. But after all, you'll get validationFailed in AJAX response.