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I am trying to understand the purpose of Java's ConstraintValidator interface.

It's an interface, however how does it make coding more quick or more efficient? Trying to understand benefits of using it with our team.

From Baeldung's Spring MVC Custom Validation:

The validation class implements the ConstraintValidator interface, and must also implement the isValid method; it's in this method that we defined our validation rules.

Naturally, we're going with a simple validation rule here in order to show how the validator works.

ConstraintValidator defines the logic to validate a given constraint for a given object. Implementations must comply with the following restrictions:

Code Example:

public class ContactNumberValidator implements 
  ConstraintValidator<ContactNumberConstraint, String> {

    @Override
    public void initialize(ContactNumberConstraint contactNumber) {
    }

    @Override
    public boolean isValid(String contactField,
      ConstraintValidatorContext cxt) {
        return contactField != null && contactField.matches("[0-9]+")
          && (contactField.length() > 8) && (contactField.length() < 14);
    }

}
hc_dev
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mattsmith5
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    You create a `ConstraintValidator` implementation as part of a custom constraint annotation. – Slaw Jun 26 '22 at 18:59
  • Are you asking the more fundamental question as to [why Java has interfaces](https://stackoverflow.com/q/3528420/12567365) in the first place? (If so, I would say its not primarily to "make coding more quick or more efficient"). – andrewJames Jun 26 '22 at 19:09
  • Minor point: If your platform supports it, you may want to use the newer [Jakarta API](https://jakarta.ee/specifications/webprofile/9/apidocs/jakarta/validation/constraintvalidator) instead of the older [javax API](https://docs.oracle.com/javaee/7/api/javax/validation/ConstraintValidator.html). See [Maven](https://mvnrepository.com/artifact/jakarta.validation/jakarta.validation-api) - this is used by the latest versions of [Hibernate Validator](https://mvnrepository.com/artifact/org.hibernate.validator/hibernate-validator/7.0.4.Final). – andrewJames Jun 26 '22 at 20:33

1 Answers1

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The purpose is to define custom validation-logic for a custom annotation.

Purpose of ConstraintValidator

In the given example the ConstraintValidator implementation can be used as annotation on your property (assuming ContactNumberConstraint is a public @interface, defined as annotation for fields):

@ContactNumberConstraint
String contactField;

As such shorthand it combines several validations like otherwise have to be listed separately:

@NotNull  // contactField != null
@Pattern(regexp="[0-9]+" )  // contactField.matches("[0-9]+")
@Length(min=9, max=13)  // (contactField.length() > 8) && (contactField.length() < 14)
String contactField;

Here the implemented ConstraintValidator is used to validate the property, for example if used as parameter (in a REST-controller, or any other validated method).

How does it make coding more quick or more efficient?

With this pair of annotation and validator you can simply declare a rather complex validation at any field or class by just annotating the field - by adding one line. Spring would care about initiating the validation. It executes the logic defined in validator and handles errors.

This predefined validation component as pair of annotation-interface and validator can be reused easily at many places (reduce code duplication), it can be composed and allows giving complex validations a name (using an expressive annotation name). Through its declarative way (annotation) it is loosely coupled.

More on benefits of Java Bean Validation (JSR 303, 380)

See also

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