$form_state->cleanValues();
$this->entity = $this->buildEntity($form, $form_state);

use SymfonyComponentValidatorConstraints as Assert;
use SymfonyComponentValidatorContextExecutionContextInterface;
use SymfonyComponentValidatorMappingClassMetadata;
use SymfonyComponentValidatorValidation;

/**
 * Validates my config entity.
 *
 * @return SymfonyComponentValidatorConstraintViolationList
 *   List of violations. Empty if valid.
 */
public function validate() {
  $validator = Validation::createValidator();
    ->addMethodMapping('loadValidatorMetadata')
    ->getValidator();
  return $validator->validate($this);
}

/**
 * Defines validation rules for this config entity.
 */
public static function loadValidatorMetadata(ClassMetadata $metadata) {
  // Same validator rules.
  // Add your own as required.
  // Note this is static, so you don't have $this in scope.
  $metadata->addPropertyConstraint('someProperty', new AssertNotBlank(['message' => 'Some property cannot be blank']));
  // But if you add a validation callback, you can operate in the context
  // of an instance.
  $metadata->addConstraint(new AssertCallback(['methods' => ['someValidationCallback']]));
}

/**
 * Validation callback for some field.
 */
public function someValidationCallback(ExecutionContextInterface $context) {
  if ($this->something === 'something it should not be') {
      $context->buildViolation("Can't let you do that Dave.")
      ->atPath('something')
      ->addViolation();
    }
}