Hibernate validator – conditionally apply validator

Hibernate Validator can be used at any layers. However automatic validation will not happen in your case. So you have to trigger the validation programmatically.

Each field constraint should be handled by a distinct validator annotation, or in other words it's not suggested practice to have one field's validation annotation checking against other fields; cross-field validation should be done at the class level. Additionally, the JSR-303 Section 2.2 preferred way to express multiple validations of the same type is via a list of annotations. This allows the error message to be specified per match.

For example, validating a common form:

@FieldMatch.List({
        @FieldMatch(first = "password", second = "confirmPassword", message = "The password fields must match"),
        @FieldMatch(first = "email", second = "confirmEmail", message = "The email fields must match")
})
public class UserRegistrationForm  {
    @NotNull
    @Size(min=8, max=25)
    private String password;

    @NotNull
    @Size(min=8, max=25)
    private String confirmPassword;

    @NotNull
    @Email
    private String email;

    @NotNull
    @Email
    private String confirmEmail;
}

The Annotation:

package constraints;

import constraints.impl.FieldMatchValidator;

import javax.validation.Constraint;
import javax.validation.Payload;
import java.lang.annotation.Documented;
import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
import static java.lang.annotation.ElementType.TYPE;
import java.lang.annotation.Retention;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import java.lang.annotation.Target;

/**
 * Validation annotation to validate that 2 fields have the same value.
 * An array of fields and their matching confirmation fields can be supplied.
 *
 * Example, compare 1 pair of fields:
 * @FieldMatch(first = "password", second = "confirmPassword", message = "The password fields must match")
 * 
 * Example, compare more than 1 pair of fields:
 * @FieldMatch.List({
 *   @FieldMatch(first = "password", second = "confirmPassword", message = "The password fields must match"),
 *   @FieldMatch(first = "email", second = "confirmEmail", message = "The email fields must match")})
 */
@Target({TYPE, ANNOTATION_TYPE})
@Retention(RUNTIME)
@Constraint(validatedBy = FieldMatchValidator.class)
@Documented
public @interface FieldMatch
{
    String message() default "{constraints.fieldmatch}";

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

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

    /**
     * @return The first field
     */
    String first();

    /**
     * @return The second field
     */
    String second();

    /**
     * Defines several <code>@FieldMatch</code> annotations on the same element
     *
     * @see FieldMatch
     */
    @Target({TYPE, ANNOTATION_TYPE})
    @Retention(RUNTIME)
    @Documented
            @interface List
    {
        FieldMatch[] value();
    }
}

The Validator:

package constraints.impl;

import constraints.FieldMatch;
import org.apache.commons.beanutils.BeanUtils;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class FieldMatchValidator implements ConstraintValidator<FieldMatch, Object>
{
    private String firstFieldName;
    private String secondFieldName;

    @Override
    public void initialize(final FieldMatch constraintAnnotation)
    {
        firstFieldName = constraintAnnotation.first();
        secondFieldName = constraintAnnotation.second();
    }

    @Override
    public boolean isValid(final Object value, final ConstraintValidatorContext context)
    {
        try
        {
            final Object firstObj = BeanUtils.getProperty(value, firstFieldName);
            final Object secondObj = BeanUtils.getProperty(value, secondFieldName);

            return firstObj == null && secondObj == null || firstObj != null && firstObj.equals(secondObj);
        }
        catch (final Exception ignore)
        {
            // ignore
        }
        return true;
    }
}