Spring Boot: Map Hibernate Validator ConstraintValidator to `@Constraint` annotation without having both in the same project

README.md

A @Constraint annotation is normally mapped to ConstraintValidator(s) using the @Constraint#validatedBy annotation parameter.

This however has the limitation that the @Constraint annotation has to know about the ConstraintValidator(s) during compilation, which may not always be feasible (for example, if the annotation is used at the DTO layer, but a ConstraintValidator depends on the service layer, and the DTO layer is in a separate JAR file).

Most examples will simply create a new Validator bean, which will override the one created by Spring Boot.

The ValidationConfigurerAdapter abstract class provided can be extended as a @Configuration to have the same behavior as Spring Boot's ValidationAutoConfiguration, but allows the mapping of @Constraint annotations to ConstraintValidators to be defined by overriding the mapConstraints(ConstraintMapping) method. See https://docs.jboss.org/hibernate/validator/6.0/reference/en-US/html_single/#section-programmatic-constraint-definition for instructions on how to configure the mappings programmatically.

pom.xml

<parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>2.2.4.RELEASE</version>
  <relativePath/>
</parent>

<properties>
  <java.version>11</java.version>
</properties>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
    <version>2.2.4.RELEASE</version>
</dependency>

ValidationConfigurerAdapter.java

import javax.validation.Configuration;
import javax.validation.Constraint;
import javax.validation.Validator;
import org.hibernate.validator.HibernateValidatorConfiguration;
import org.hibernate.validator.cfg.ConstraintMapping;
import org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration;
import org.springframework.boot.validation.MessageInterpolatorFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean;

/**
 * Provides a convenient base class for creating a {@link org.springframework.context.annotation.Configuration} to map
 * {@link Constraint} annotations to validators for Hibernate Validator.
 *
 * @see ValidationAutoConfiguration
 **/
public abstract class ValidationConfigurerAdapter {
  protected void mapConstraints(ConstraintMapping constraintMapping);

  @Bean
  public Validator validator() {
    var factoryBean = new LocalValidatorFactoryBean() {
      @Override
      protected void postProcessConfiguration(Configuration<?> configuration) {
        if (configuration instanceof HibernateValidatorConfiguration) {
          var hibernateValidatorConfiguration = (HibernateValidatorConfiguration) configuration;
          var constraintMapping = hibernateValidatorConfiguration.createConstraintMapping();
          mapConstraints(constraintMapping);
          hibernateValidatorConfiguration.addMapping(constraintMapping);
        }
      }
    };
    var interpolatorFactory = new MessageInterpolatorFactory();
    factoryBean.setMessageInterpolator(interpolatorFactory.getObject());
    return factoryBean;
  }
}