I have a Spring based webapp. I am using several repository classes with annotations @Repository, @Transactional in my Controllers. That part works fine.

I created a Custom Constraint Validator which has to access the repository. Now I am not able to understand why the Repository is null. I tried annotating the validator with @Component annotation. My base package containing all these classes is in the part of the xml. So what else should I do to ensure the repository dependency injection works. This is how my Constraint validator looks like.

public class DonorTypeExistsConstraintValidator implements
    ConstraintValidator<DonorTypeExists, DonorType> {

  @Autowired
  private DonorTypeRepository donorTypeRepository;

  @Override
  public void initialize(DonorTypeExists constraint) {

  }

  public boolean isValid(DonorType target, ConstraintValidatorContext context) {

   System.out.println("donorType: " + target);
   if (target == null)
     return true;

   try {
    if (donorTypeRepository.isDonorTypeValid(target.getDonorType()))
     return true;
   } catch (Exception e) {
    e.printStackTrace();
   }
   return false;
  }

  public void setDonorRepository(DonorTypeRepository donorTypeRepository) {
    this.donorTypeRepository = donorTypeRepository;
  }
}

Update

Package scan configuration:

<context:annotation-config />

<!-- Configures the @Controller programming model -->
<mvc:annotation-driven />

<context:component-scan base-package="controller" />
<context:component-scan base-package="repository" />
<context:component-scan base-package="model" />
<context:component-scan base-package="viewmodel" />

This class is in the package model.donortype.
The repository is in the package repository.

Update

Something more puzzling, removing all preinsert listeners (in this case only BeanValidationEventListener) fixes the autowiring issue. This has complicated the matter even more.

I am not even inserting the BeanValidationEventListener as described in the answer below. This is infact contrary to the suggestion here:
JSR-303 dependency injection and Hibernate

  @Override
  public void integrate(Configuration configuration, SessionFactoryImplementor implementor,
                        SessionFactoryServiceRegistry registry) {
      logger.info("Registering event listeners");
      System.out.println("here");
      // you can add duplication strategory for duplicate registrations

      final EventListenerRegistry eventRegistry = registry.getService(EventListenerRegistry.class);
      // prepend to register before or append to register after
      // this example will register a persist event listener

      eventRegistry.prependListeners(EventType.PERSIST, EntitySaveListener.class);
      eventRegistry.appendListeners(EventType.MERGE, EntitySaveListener.class);
      Iterator<PreInsertEventListener> iter = eventRegistry.getEventListenerGroup(EventType.PRE_INSERT).listeners().iterator();
      while (iter.hasNext()) {
        PreInsertEventListener el = iter.next();
        System.out.println(el.getClass());
        BeanValidationEventListener listener = (BeanValidationEventListener) el;
        System.out.println(listener);
        iter.remove();
      }
  }

Update

I am using JPA. I have a backing form for the JPA entities. In the controller I use InitBinder to instantiate a custom validator binder.setValidator(new EntityBackingFormValidator(binder.getValidator()));.
Here is the code for this:
https://github.com/C4G/V2V/blob/87dc266043f6d623c101d947a88fa0b0ad536355/src/controller/CollectedSampleController.java

This custom validator invokes the validation on the default validator and leaves scope to do some custom validations.
https://github.com/C4G/V2V/blob/87dc266043f6d623c101d947a88fa0b0ad536355/src/model/collectedsample/CollectedSampleBackingFormValidator.java

I am using annotations within the entity to create constraints. Apart from the default constraints, I need to define constraints that check whether a entity at the other side of a many-to-one mapping exists or not.
Here is the custom constraint validator.
https://github.com/C4G/V2V/blob/87dc266043f6d623c101d947a88fa0b0ad536355/src/model/donor/DonorExistsConstraintValidator.java

Now the autowired repository in this custom constraint validator is null. I am trying to customize code in my CustomIntegrator which is present here
https://github.com/C4G/V2V/blob/87dc266043f6d623c101d947a88fa0b0ad536355/src/interceptor/CustomIntegrator.java

and the xml configuration file present here
https://github.com/C4G/V2V/blob/87dc266043f6d623c101d947a88fa0b0ad536355/war/WEB-INF/v2v-servlet.xml

Hopefully the actual code will help you to answer my question.
Thanks!