JSR-303 validation is ignored when custom Validator is annotated with @Component

Question

I've noticed that JSR-303 validation is completely ignored in Spring when a custom Validator bean annotated with @Component is declared. Interestingly enough said custom validator doesn't even have to filled in or used by any of the classes. The fact that its component scanned by Spring appears to be enough to make Spring skip JSR-303 validation during object binding altogether. Removing @Component from custom Validator and restarting web application enables JSR-303 validation as expected. Annotating custom validators with @Component has its uses eg to have Spring Autowire dependencies.

Consider simple example below:

/* Simple JSR-303 annotated User object */
public class User {

    @NotNull @Size(min = 2, max = 5)
    protected String username;

    @Size(min = 2, max = 32)
    protected String firstName;

    @Size(min = 2, max = 32)
    protected String lastName;

    @NotNull @Past @DateTimeFormat(pattern="dd/MM/yyyy")
    protected Date dateOfBirth;

    @NotNull @Email
    protected String email;

    protected String phone;

    //getters and setters
}

/* Controller example */
@RestController
public class UserController {

    @PostMapping("/users/register")
    public ResponseEntity postUser(@Valid @RequestBody User user, BindingResult result) {
        if (result.hasErrors()) {
            return new ResponseEntity(result.getAllErrors(), HttpStatus.INTERNAL_SERVER_ERROR);
        }
        return new ResponseEntity(user, HttpStatus.CREATED);
    }

}

/* Custom validator (doesn't even have to be in use) */
@Component //commenting out @Component annotation enables JSR-303 again
public class SomeValidator implements Validator {

    @Override
    public boolean supports(Class<?> clazz) {
        //just an example
        return false;
    }

    @Override
    public void validate(Object target, Errors errors) {
        //empty
    }
}

I was scratching my head over this one and couldn't figure out why my JSR-303 objects weren't validated but I managed to narrow it down and replicate this with a Spring Boot project containing above classes. Why is that? Am I missing something here or is this a Spring bug?

---

Edit

See demo Spring Boot project on GitHub

Answer 1

So there are a number of problems going on in your code. First, your custom validator doesn't support anything, so it's never going to be invoked. You probably realize that, so I'll leave it up to you to fix it.

Your real problem, though, is that by creating a validator bean like that, Spring will not create the defaultValidator bean (which is a LocalValidatorFactoryBean) nor will it create the methodValidationPostProcessor bean (which is needed to validate method parameters). This is a common problem people run into with Spring: once you do something to disturb the auto-configure process, you have to define things manually. The solution is simple: create a config class that defines these beans. Example:

@Configuration
public class ValidationConfig {
    @Bean public LocalValidatorFactoryBean defaultValidator() {
        return new LocalValidatorFactoryBean();
    }

    @Bean public MethodValidationPostProcessor methodValidationPostProcessor() {
        return new MethodValidationPostProcessor();
    }
}