Generic Top

I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:

>> CHECK OUT THE COURSE

1. Overview

In this quick tutorial, we cover the basics of validating a Java bean with the standard framework — JSR 380, also known as Bean Validation 2.0.

Validating user input is a super common requirement in most applications. And the Java Bean Validation framework has become the de facto standard for handling this kind of logic.

Further reading:

Validation in Spring Boot

Learn how to validate domain objects in Spring Boot using Hibernate Validator, the reference implementation of the Bean Validation framework.

Method Constraints with Bean Validation 2.0

An introduction to method constraints using Bean Validation 2.0.

2. JSR 380

JSR 380 is a specification of the Java API for bean validation, part of Jakarta EE and JavaSE. This ensures that the properties of a bean meet specific criteria, using annotations such as @NotNull, @Min, and @Max.

This version requires Java 8 or higher, and takes advantage of new features added in Java 8, such as type annotations and support for new types like Optional and LocalDate.

For full information on the specifications, go ahead and read through the JSR 380.

3. Dependencies

We're going to use a Maven example to show the required dependencies. But of course, these jars can be added in various ways.

3.1. Validation API

Per the JSR 380 specification, the validation-api dependency contains the standard validation APIs:

<dependency>
    <groupId>javax.validation</groupId>
    <artifactId>validation-api</artifactId>
    <version>2.0.1.Final</version>
</dependency>

3.2. Validation API Reference Implementation

Hibernate Validator is the reference implementation of the validation API.

To use it, we need to add the following dependency:

<dependency>
    <groupId>org.hibernate.validator</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>6.0.13.Final</version>
</dependency>

A quick note: hibernate-validator is entirely separate from the persistence aspects of Hibernate. So, by adding it as a dependency, we're not adding these persistence aspects into the project.

3.3. Expression Language Dependencies

JSR 380 supports variable interpolation, allowing expressions inside the violation messages.

To parse these expressions, we'll add the javax.el dependency from GlassFish, that contains  an implementation of the Expression Language specification:

<dependency>
    <groupId>org.glassfish</groupId>
    <artifactId>javax.el</artifactId>
    <version>3.0.0</version>
</dependency>

4. Using Validation Annotations

Here, we'll take a User bean and work on adding some simple validation to it:

import javax.validation.constraints.AssertTrue;
import javax.validation.constraints.Max;
import javax.validation.constraints.Min;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
import javax.validation.constraints.Email;

public class User {

    @NotNull(message = "Name cannot be null")
    private String name;

    @AssertTrue
    private boolean working;

    @Size(min = 10, max = 200, message 
      = "About Me must be between 10 and 200 characters")
    private String aboutMe;

    @Min(value = 18, message = "Age should not be less than 18")
    @Max(value = 150, message = "Age should not be greater than 150")
    private int age;

    @Email(message = "Email should be valid")
    private String email;

    // standard setters and getters 
}

All of the annotations used in the example are standard JSR annotations:

  • @NotNull validates that the annotated property value is not null.
  • @AssertTrue validates that the annotated property value is true.
  • @Size validates that the annotated property value has a size between the attributes min and max; can be applied to String, Collection, Map, and array properties.
  • @Min validates that the annotated property has a value no smaller than the value attribute.
  • @Max validates that the annotated property has a value no larger than the value attribute.
  • @Email validates that the annotated property is a valid email address.

Some annotations accept additional attributes, but the message attribute is common to all of them. This is the message that will usually be rendered when the value of the respective property fails validation.

And some additional annotations that can be found in the JSR:

  • @NotEmpty validates that the property is not null or empty; can be applied to String, Collection, Map or Array values.
  • @NotBlank can be applied only to text values and validates that the property is not null or whitespace.
  • @Positive and @PositiveOrZero apply to numeric values and validate that they are strictly positive, or positive including 0.
  • @Negative and @NegativeOrZero apply to numeric values and validate that they are strictly negative, or negative including 0.
  • @Past and @PastOrPresent validate that a date value is in the past or the past including the present; can be applied to date types including those added in Java 8.
  • @Future and @FutureOrPresent validate that a date value is in the future, or in the future including the present.

The validation annotations can also be applied to elements of a collection:

List<@NotBlank String> preferences;

In this case, any value added to the preferences list will be validated.

Also, the specification supports the new Optional type in Java 8:

private LocalDate dateOfBirth;

public Optional<@Past LocalDate> getDateOfBirth() {
    return Optional.of(dateOfBirth);
}

Here, the validation framework will automatically unwrap the LocalDate value and validate it.

5. Programmatic Validation

Some frameworks — such as Spring — have simple ways to trigger the validation process by just using annotations. This is mainly so that we don't have to interact with the programmatic validation API.

Now let's go the manual route and set things up programmatically:

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

To validate a bean, we first need a Validator object, which is built using a ValidatorFactory.

5.1. Defining the Bean

We're now going to set up this invalid user — with a null name value:

User user = new User();
user.setWorking(true);
user.setAboutMe("Its all about me!");
user.setAge(50);

5.2. Validate the Bean

Now that we have a Validator, we can validate our bean by passing it to the validate method.

Any violations of the constraints defined in the User object will be returned as a Set:

Set<ConstraintViolation<User>> violations = validator.validate(user);

By iterating over the violations, we can get all the violation messages using the getMessage method:

for (ConstraintViolation<User> violation : violations) {
    log.error(violation.getMessage()); 
}

In our example (ifNameIsNull_nameValidationFails), the set would contain a single ConstraintViolation with the message “Name cannot be null”.

6. Conclusion

This article focused on a simple pass through the standard Java Validation API. We showed the basics of bean validation using javax.validation annotations and APIs.

As usual, an implementation of the concepts in this article and all code snippets can be found over on GitHub.

Generic bottom

I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:

>> CHECK OUT THE COURSE
Comments are closed on this article!