Using Constraint Annotations

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;

@Builder // lombok annotation
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 
}

Validating Beans/Objects

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

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

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

User user = User.builder().working(true).aboutMe("Its all about me!").age(50).build();
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()); 
}

Annotations Work on Elements of a Collections and Optional

List<@NotBlank String> preferences;

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

Annotations Available

• @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 not 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.
• @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.