Java Bean Validation API is a specification which

  • Providing data validation model via built-in and custom defined constraint annotations placed on class, field and method

  • Providing APIs to validate objects, object graphs, parameters and return values of methods and constructors

  • Defined in javax.validation package and available as a part of Java EE since Java 6

Hibernate Validator is an implementation of the API

In this tutorial, you will learn to implement an Hibernate Validator example to validate a data model

What you'll need

  • JDK 8+ or OpenJDK 8+
  • Maven 3+

Project structure

├── src
│   └── main
│       ├── java
│       │   └── com
│       │       └── hellokoding
│       │           └── validation
│       │               ├── Main.java
│       │               ├── Product.java
│       │               ├── ProductCodeExisting.java
│       │               └── ProductCodeExistingValidator.java
│       └── resources
│           └── ValidationMessages.properties
└── pom.xml

Dependencies

Include hibernate-validator into your pom.xml

<dependency>  
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>6.0.16.Final</version>
</dependency>

Unified Expression Language (EL) is also required for evaluating dynamic expressions in constraint violation messages when you run your application in Java SE. In Java EE, it is already provided by container

<dependency>  
    <groupId>org.glassfish</groupId>
    <artifactId>javax.el</artifactId>
    <version>3.0.1-b09</version>
</dependency>

Full content of pom.xml as below

Define data model and validation constraints

Define data model

@NotNull, @Size and @Min are built-in constraints

@ProductCodeExisting is a custom constraint defined as below

{Size.name} and {Min.price} are message templates, theirs value will be replaced at run time by the below ValidationMessages.properties

Define custom constraint

An annotation type is defined using the @interface keyword

All attributes of an annotation type are declared in a method-like manner, the following are required

  • an attribute message that returns the default violation messages

  • an attribute groups to restrict the set of constraints applied during validation

  • an attribute payload used by clients of the Bean Validation API to assign custom payload objects to a constraint

{ProductCodeExisting} is a message template. Its value will be replaced at run time by the below ValidationMessages.properties

Define custom constraint validator

Interpolate / customize the validation messages

Provide the message value when you express the constraint on defining data model, for example the constraint @NotNull(message = "must be not null") of Product.java, or add a file named ValidationMessages.properties to the classpath

Bean Validation API uses the Unified Expression Language (EL) in constraint violation messages. The following objects available in the EL context

  • the attribute values of the constraint mapped to the attribute names, for example {min} and {max} of the message Size.name mapped to @Size constraint annotation in Product.java

  • ${validatedValue} mapped to the current validated value of field, bean or method parameter

  • a bean named formatter exposing the var-arg method format(String format, Object…​ args) which behaves like java.util.Formatter.format(String format, Object…​ args)

Validating the constraints

Use the built-in ValidatorFactory of javax.validation to get a Validator instance

Validator validator = Validation.buildDefaultValidatorFactory().getValidator();

Use validator to validate all constraints on object, for example

Set<ConstraintViolation<Product>> constraintViolations = validator.validate(product);

Type command mvn clean package exec:java at your project root directory to run, expected output

PRICE 0.79 is less than 1  
ID is required  
NAME must be between 1 and 10 character long  
DESCRIPTION must be not null  
CODE "Hello Koding" is not existing

Source code

https://github.com/hellokoding/hellokoding-courses/tree/master/java-examples/java-bean-validation

Java