[FIXED] How are hibernate Validators integrated into spring boot starter project

Issue

I often use constraints on fields in spring . The constraint are standard or custom

public  abstract class OrganizationEntity {
@Length(min = 1 , max = 256, message = "Organization name max length is 256 and min length is 1")
@NotNull(message = "Organization name should not be empty")
@Pattern(regexp = "^[a-zA-Z0-9][-a-zA-Z0-9]+[a-zA-Z0-9]$",message = "Organization name can contain Alphanumeric character and Hyphen in the middle")
public abstract String getName();
}

On navigating to Length or Pattern annotation I see the definition of annotation org.hibernate.validator.constraints

    @Documented
@Constraint(
    validatedBy = {}
)
@Target({ElementType.METHOD, ElementType.FIELD, ElementType.ANNOTATION_TYPE, ElementType.CONSTRUCTOR, ElementType.PARAMETER, ElementType.TYPE_USE})
@Retention(RetentionPolicy.RUNTIME)
@Repeatable(List.class)
public @interface Length {
    int min() default 0;

    int max() default Integer.MAX_VALUE;

    String message() default "{org.hibernate.validator.constraints.Length.message}";

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

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

    @Target({ElementType.METHOD, ElementType.FIELD, ElementType.ANNOTATION_TYPE, ElementType.CONSTRUCTOR, ElementType.PARAMETER, ElementType.TYPE_USE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface List {
        Length[] value();
    }
    }

I see validatedBy is left empty which implies there is somebody mechanism/library which has implementation for these checks. Which library provides these implementation? How does spring manage this validation flow.

Solution

Hibernate provides the validation implementation and it is used by Spring.

I may be oversimplifying, but as I understand Spring configures Hibernate's ValidationFactory implementation (ValidationFactoryImpl) with Validator and ExecutableValidator implementation (ValidatorImpl) for Spring's application context usage, exposes it for other Spring components and convert validation results into BindingResult.

At the same time, Hibernate's ValidatorImpl uses validation context, in which ConstraintValidators are registered as a map with different annotations from package javax.validation.constraints as keys, along with Hibernate's own constraints, such as @URL and @ISBN. This map can be found in the ConstraintHelper class.

For example, you can find PatternValidator registered there for validating values annotated with @Pattern.

Then we have the next Hibernate validation flow (again - simplified):

1. ValidatorImpl tries to validate something and creates MetaConstraints (helper class with all information needed for validation);
2. MetaConstraint creates ConstraintTree for multiple constraints check;
3. ConstraintTree calls ConstraintValidatorManager and its getInitializedValidator method, which returns needed ConstraintValidator for a specific annotation from a map, mentioned before.

So the validation framework, used by Spring, already knows which ConstraintValidator instances to use for specific validation annotations.

Answered By - AndrewThomas
Answer Checked By - Willingham (JavaFixing Volunteer)