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YAVI (Yet Another ValIdation)

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YAVI (pronounced jɑ-vάɪ) is a lambda based type safe validation for Java.


YAVI sounds as same as a Japanese slang "YABAI (ヤバイ)" that means awesome or awful depending on the context (like "Crazy"). If you use YAVI, you will surely understand that it means the former.

The concepts are

  • No reflection!
  • No (runtime) annotation!
  • Not only Java Beans!
  • Zero dependency!

If you are not a fan of Bean Validation, YAVI will be an awesome alternative.

YAVI has the following features:

  • Type-safe constraints, unsupported constraints cannot be applied to the wrong type
  • Fluent and intuitive API
  • Constraints on any object. Java Beans, Records, Protocol Buffers, Immutables and anything else.
  • Lots of powerful built-in constraints
  • Easy custom constraints
  • Validation for groups, conditional validation
  • Validation for arguments before creating an object
  • Support for API and combination of validation results and validators that incorporate the concept of functional programming

See the reference documentation for details.


Getting Started

This content is derived from

Welcome to YAVI.

The following paragraphs will guide you through the initial steps required to integrate YAVI into your application.


Project set up

In order to use YAVI within a Maven project, simply add the following dependency to your pom.xml:


This tutorial uses JUnit 5 and AssertJ. Add the following dependencies as needed:


Applying constraints

Let’s dive into an example to see how to apply constraints:

Create src/main/java/com/example/ and write the following code.

package com.example;

import am.ik.yavi.builder.ValidatorBuilder;
import am.ik.yavi.core.Validator;

public class Car {
    private final String manufacturer;

    private final String licensePlate;

    private final int seatCount;

    public static final Validator<Car> validator = ValidatorBuilder.<Car>of()
            .constraint(Car::getManufacturer, "manufacturer", c -> c.notNull())
            .constraint(Car::getLicensePlate, "licensePlate", c -> c.notNull().greaterThanOrEqual(2).lessThanOrEqual(14))
            .constraint(Car::getSeatCount, "seatCount", c -> c.greaterThanOrEqual(2))

    public Car(String manufacturer, String licencePlate, int seatCount) {
        this.manufacturer = manufacturer;
        this.licensePlate = licencePlate;
        this.seatCount = seatCount;

    public String getManufacturer() {
        return manufacturer;

    public String getLicensePlate() {
        return licensePlate;

    public int getSeatCount() {
        return seatCount;

The ValidatorBuilder.constraint is used to declare the constraints which should be applied to the return values of getter for the Car instance:

  • manufacturer must never be null
  • licensePlate must never be null and must be between 2 and 14 characters long
  • seatCount must be at least 2

You can find the complete source code on GitHub.

Validating constraints

To perform a validation of these constraints, you use a Validator instance. To demonstrate this, let’s have a look at a simple unit test:

Create src/test/java/com/example/ and write the following code.

package com.example;

import am.ik.yavi.core.ConstraintViolations;
import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.assertThat;

class CarTest {

    void manufacturerIsNull() {
        final Car car = new Car(null, "DD-AB-123", 4);
        final ConstraintViolations violations = Car.validator.validate(car);

        assertThat(violations.get(0).message()).isEqualTo("\"manufacturer\" must not be null");

    void licensePlateTooShort() {
        final Car car = new Car("Morris", "D", 4);
        final ConstraintViolations violations = Car.validator.validate(car);

        assertThat(violations.get(0).message()).isEqualTo("The size of \"licensePlate\" must be greater than or equal to 2. The given size is 1");

    void seatCountTooLow() {
        final Car car = new Car("Morris", "DD-AB-123", 1);
        final ConstraintViolations violations = Car.validator.validate(car);

        assertThat(violations.get(0).message()).isEqualTo("\"seatCount\" must be greater than or equal to 2");

    void carIsValid() {
        final Car car = new Car("Morris", "DD-AB-123", 2);
        final ConstraintViolations violations = Car.validator.validate(car);


Validator instances are thread-safe and may be reused multiple times.

The validate() method returns a ConstraintViolations instance, which you can iterate in order to see which validation errors occurred. The first three test methods show some expected constraint violations:

  • The notNull() constraint on manufacturer is violated in manufacturerIsNull()
  • The greaterThanOrEqual(int) constraint on licensePlate is violated in licensePlateTooShort()
  • The greaterThanOrEqual(int) constraint on seatCount is violated in seatCountTooLow()

If the object validates successfully, validate() returns an empty ConstraintViolations as you can see in carIsValid(). You can also check if the validation was successful with the ConstraintViolations.isValid method.

Where to go next?

That concludes the 5 minutes tour through the world of YAVI. If you want a more complete introduction, it is recommended to read "Using YAVI" in the reference document.


  • Java 8+


Licensed under the Apache License, Version 2.0.