[Spring 3] Autowiring a service into a validator

Question

This example is a bit contrived; I've simplified it to remove extraneous details and to focus on the problem I am having. I have a validator that looks like this:

@Component
public class UniqueUsernameValidator implements ConstraintValidator<UniqueUsername, String> {

    @Autowired
    UsernameService usernameService;

    @Override
    public void initialize(UniqueUsername uniqueUsername) {
    }

    @Override
    public boolean isValid(String s, ConstraintValidatorContext constraintValidatorContext) {
        return !usernameService.exists(s);
    }   
}

I call the validator from my controller like this:

@RequestMapping
public void checkUsername(Model model, User user) {
    ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
    Validator validator = factory.getValidator();

    Set<ConstraintViolation<User>> constraintViolations = validator.validateProperty(user, "username");
    model.addAttribute("error", constraintViolations.size() > 0);
}

However, I keep getting an NullPointerException exception. I added a breakpoint in my validator and saw that usernameService was null. Why isn't it getting autowired? Initially I thought it was because I hadn't annotated the validator with @Component, but I still have the same problem even after annotating it. The UsernameService class has already been annotated with @Service and I can verify that its constructor is getting called.

I am new to Spring, so I'm not even sure if it is alright to wire a service into a validator. What am I doing wrong?

Answer 1

Instead of creating a new validator, you have to autowire or inject it to controller. The NPE from the service not being injected as your validator is not being created/managed by spring.

Answer 2

In Spring, you need to obtain ValidatorFactory (or Validator itself) via LocalValidatorFactoryBean instead of Validation.buildDefaultValidatorFactory(), as described in the reference.

@Autowired
Validator validator;

@RequestMapping 
public void checkUsername(Model model, User user) { 
    Set<ConstraintViolation<User>> constraintViolations = validator.validateProperty(user, "username"); 
    model.addAttribute("error", constraintViolations.size() > 0); 
} 

-

<bean id="validator"
    class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>

EDIT: But perhaps the better way to do it is to use Spring MVC's automatic validation with @Valid annotation:

@RequestMapping  
public void checkUsername(Model model, @Valid User user, BindingResult result) {  
    if (result.hasErrors()) {   
        ...
    }
}

This also requires <mvc:annotation-driven/> in the config.