自定义验证的注解:

/**
 * This constraint is to be put on object level for which need to validate on the Product SKU.
 * It calls {@link ValidProductSKUValidator} to perform validation.
 *
 * @author System-In-Montion
 *
 */
@Constraint(validatedBy = {ValidProductSKUValidator.class})
//This constraint annotation can be used only on class.
@Target({ElementType.TYPE})
@Retention(value = RetentionPolicy.RUNTIME)
@Documented
public @interface ValidProductSKU {

/** The message to return when the instance of product SKU fails the validation.*/
  String message() default "{product.validation.SKU}";

Class<?>[] groups() default {};
  Class<? extends Payload>[] payload() default {};
}

自定义验证的类：

/**
 * The product sku validator
 *
 * @author System-In-Motion
 *
 */
@Component
public class ValidProductSKUValidator implements
        ConstraintValidator<ValidProductSKU, Product> {

@Autowired
    private ProductRepository productRepository;
    @Autowired
    private EventRepository<Event> eventRepository;
    /**
     * Nothing to be done for initialization
     */
    @Override
    public void initialize(ValidProductSKU constraintAnnotation) {
        // TODO Auto-generated method stub
    }

/**
     * Perform validation on the {@link productSKU} via the instance for
     * {@link ValidProductSKUValidator}
     */
    @Override
    public boolean isValid(Product product, ConstraintValidatorContext context) {
        if(product.getId()==null){    //validate create
            return productRepository.countBySku(product.getSku()) > 0?false:true;
        }else if(eventRepository.countByProduct(product.getId()) > 0){   //validate delete
            return false;
        }else if(productRepository.countBySkuIsAndIdIsNot(product.getSku(), product.getId()) > 0){  //validate update
            return false;
        }
        return true;
            
    }

}

在product这个entity上添加注解@ValidProductSKU

经过这样的设置后，每次在增加，删除和修改时就能验证是否合法了。

另外：在实现过程中也遇到一个小问题，就是isValid中传递的product值始终为空，原因是在更新的form表单里没有隐藏携带id值，只需添加<input    th:field="*{id}" type="hidden"/>即可。