Building Scalable Applications with Thymeleaf and Spring Boot/

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Custom Validator

Custom Validator

Learn how to create custom validations that work for the whole object.

We'll cover the following...

We’ve been using the built-in constraints from the javax.validation.constraints package until now.

Creating custom validations

When those are not enough, we can create our own custom validation. Such validation can validate a single field or the whole class. Check out the Custom Validator to check if, say, a String contains an XML blog post as an example of a custom validation on a single field.

We will create a custom validator that validates the whole object. For example, it would be good to validate if there is already a known user with the given email address since the email address of each user should be unique.

Create annotation

We start by creating our own annotation:

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package com.tamingthymeleaf.application.user.web;
import javax.validation.Constraint;
import javax.validation.Payload;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.TYPE) //<.>
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = NotExistingUserValidator.class) //<.>
public @interface NotExistingUser {
    String message() default "{UserAlreadyExisting}";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}
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