Validation

public interface Validator {
    <T> Set<ConstraintViolation<T>> validate(T object, Class<?>... groups);
    <T> Set<ConstraintViolation<T>> validateProperty(T object, String propertyName, Class<?>... groups);
    <T> Set<ConstraintViolation<T>> validateValue(Class<T> beanType, String propertyName, Object value, Class<?>... groups);
    BeanDescriptor getConstraintsForClass(Class<?> clazz);
    <T> T unwrap(Class<T> type);
    ExecutableValidator forExecutables();
}

public interface ValidatorFactory extends AutoCloseable {
    Validator getValidator();
    ValidatorContext usingContext();
    MessageInterpolator getMessageInterpolator();
    TraversableResolver getTraversableResolver();
    ConstraintValidatorFactory getConstraintValidatorFactory();
    ParameterNameProvider getParameterNameProvider();
    ClockProvider getClockProvider();
    <T> T unwrap(Class<T> type);
    void close();
}

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = {})
public @interface NotNull {
    String message() default "{javax.validation.constraints.NotNull.message}";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = {})
public @interface NotEmpty {
    String message() default "{javax.validation.constraints.NotEmpty.message}";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = {})
public @interface Size {
    String message() default "{javax.validation.constraints.Size.message}";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
    int min() default 0;
    int max() default Integer.MAX_VALUE;
}

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = {})
public @interface Email {
    String message() default "{javax.validation.constraints.Email.message}";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
    String regexp() default ".*";
    Pattern.Flag[] flags() default {};
}

public class User {
    @NotNull
    @Size(min = 2, max = 50)
    private String name;
    
    @NotNull
    @Email
    private String email;
    
    @Min(18)
    @Max(120)
    private Integer age;
    
    // getters and setters
}

@Stateless
public class UserService {
    
    @Inject
    private Validator validator;
    
    public void createUser(@Valid User user) {
        Set<ConstraintViolation<User>> violations = validator.validate(user);
        if (!violations.isEmpty()) {
            throw new ValidationException("User validation failed");
        }
        // Save user
    }
}