Bean Validation is a Java specification that basically helps you to protect your data. Through its API, you can validate fields and parameters, express constraints using annotations, and extend your customs' validation rules.

It can be used both with Java SE and Java EE.

In this recipe, you will have a glimpse of Bean Validation 2.0. It doesn't matter whether you are new to it or already using version 1.1; this content will help you get familiar with some of its new features.

First, you need to add the right Bean Validation dependency to your project, as follows:

<dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.hamcrest</groupId>
            <artifactId>hamcrest-core</artifactId>
            <version>1.3</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>javax</groupId>
            <artifactId>javaee-api</artifactId>
            <version>8.0</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.hibernate.validator</groupId>
            <artifactId>hibernate-validator</artifactId>
            <version>6.0.8.Final</version>
        </dependency>
        <dependency>
            <groupId>org.glassfish</groupId>
            <artifactId>javax.el</artifactId>
            <version>3.0.1-b10</version>
        </dependency>
</dependencies>

1. First, we need to create an object with some fields to be validated:

public class User {

    @NotBlank
    private String name;
    
    @Email
    private String email;
    
    @NotEmpty
    private List<@PositiveOrZero Integer> profileId;
    
    public User(String name, String email, List<Integer> profileId) {
        this.name = name;
        this.email = email;
        this.profileId = profileId;
    }
}

2. Then we create a test class to validate those constraints:

public class UserTest {
 
    private static Validator validator;
 
    @BeforeClass
    public static void setUpClass() {
        validator = Validation.buildDefaultValidatorFactory()
        .getValidator();
    }
 
    @Test
    public void validUser() {
        User user = new User(
            "elder", 
            "[email protected]", 
            asList(1,2));
 
            Set<ConstraintViolation<User>> cv = validator
            .validate(user);
            assertTrue(cv.isEmpty());
    }
 
    @Test
    public void invalidName() {
        User user = new User(
            "", 
            "[email protected]", 
            asList(1,2));
 
            Set<ConstraintViolation<User>> cv = validator
            .validate(user);
            assertEquals(1, cv.size());
    }

    @Test
    public void invalidEmail() {
        User user = new User(
        "elder", 
        "elder-eldermoraes_com", 
        asList(1,2));
 
        Set<ConstraintViolation<User>> cv = validator
        .validate(user);
        assertEquals(1, cv.size());
    } 
 
    @Test
    public void invalidId() {
        User user = new User(
            "elder", 
            "[email protected]", 
            asList(-1,-2,1,2));
 
            Set<ConstraintViolation<User>> cv = validator
            .validate(user);
            assertEquals(2, cv.size());
    } 
}

Our User class uses three of the new constraints introduced by Bean Validation 2.0:

• @NotBlank: Assures that the value is not null, empty, or an empty string (it trims the value before evaluation, to make sure there aren't spaces).
• @Email: Allows only a valid email format. Forget those crazy JavaScript functions!
• @NotEmpty: Ensures that a list has at least one item.
• @PositiveOrZero: Guarantees that a number is equal or greater than zero.

Then we create a test class (using JUnit) to test our validations. It first instantiates Validator:

@BeforeClass
public static void setUpClass() {
    validator = Validation.buildDefaultValidatorFactory().getValidator();
}

Validator is an API that validates beans according to the constraints defined for them.

Our first test method tests a valid user, which is a User object that has:

• Name not empty
• Valid email
• profileId list only with integers greater than zero:

User user = new User(
   "elder", 
   "[email protected]", 
   asList(1,2));

And finally, the validation:

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

The validate() method from Validator returns a set of constraint violations found, if any, or an empty set if there are no violations at all.

So, for a valid user it should return an empty set:

assertTrue(cv.isEmpty());

And the other methods work with variations around this model:

• invalidName(): Uses an empty name
• invalidEmail(): Uses a malformed email
• invalidId(): Adds some negative numbers to the list

Note that the invalidId() method adds two negative numbers to the list:

asList(-1,-2,1,2));

So, we expect two constraint violations:

assertEquals(2, cv.size());

In other words, Validator checks not only the constraints violated, but how many times they are violated.