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# Validation
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Bean Validation API for validating JavaBeans using constraint annotations and validation groups.
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## Core Interfaces
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```java { .api }
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public interface Validator {
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    <T> Set<ConstraintViolation<T>> validate(T object, Class<?>... groups);
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    <T> Set<ConstraintViolation<T>> validateProperty(T object, String propertyName, Class<?>... groups);
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    <T> Set<ConstraintViolation<T>> validateValue(Class<T> beanType, String propertyName, Object value, Class<?>... groups);
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    BeanDescriptor getConstraintsForClass(Class<?> clazz);
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    <T> T unwrap(Class<T> type);
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    ExecutableValidator forExecutables();
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}
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public interface ValidatorFactory extends AutoCloseable {
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    Validator getValidator();
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    ValidatorContext usingContext();
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    MessageInterpolator getMessageInterpolator();
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    TraversableResolver getTraversableResolver();
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    ConstraintValidatorFactory getConstraintValidatorFactory();
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    ParameterNameProvider getParameterNameProvider();
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    ClockProvider getClockProvider();
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    <T> T unwrap(Class<T> type);
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    void close();
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}
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```
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## Constraint Annotations
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```java { .api }
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@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
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@Retention(RetentionPolicy.RUNTIME)
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@Constraint(validatedBy = {})
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public @interface NotNull {
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    String message() default "{javax.validation.constraints.NotNull.message}";
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    Class<?>[] groups() default {};
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    Class<? extends Payload>[] payload() default {};
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}
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@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
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@Retention(RetentionPolicy.RUNTIME)
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@Constraint(validatedBy = {})
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public @interface NotEmpty {
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    String message() default "{javax.validation.constraints.NotEmpty.message}";
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    Class<?>[] groups() default {};
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    Class<? extends Payload>[] payload() default {};
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}
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@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
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@Retention(RetentionPolicy.RUNTIME)
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@Constraint(validatedBy = {})
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public @interface Size {
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    String message() default "{javax.validation.constraints.Size.message}";
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    Class<?>[] groups() default {};
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    Class<? extends Payload>[] payload() default {};
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    int min() default 0;
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    int max() default Integer.MAX_VALUE;
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}
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@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
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@Retention(RetentionPolicy.RUNTIME)
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@Constraint(validatedBy = {})
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public @interface Email {
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    String message() default "{javax.validation.constraints.Email.message}";
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    Class<?>[] groups() default {};
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    Class<? extends Payload>[] payload() default {};
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    String regexp() default ".*";
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    Pattern.Flag[] flags() default {};
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}
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```
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## Usage Example
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```java
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public class User {
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    @NotNull
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    @Size(min = 2, max = 50)
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    private String name;
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    @NotNull
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    @Email
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    private String email;
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    @Min(18)
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    @Max(120)
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    private Integer age;
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    // getters and setters
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}
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@Stateless
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public class UserService {
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    @Inject
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    private Validator validator;
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    public void createUser(@Valid User user) {
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        Set<ConstraintViolation<User>> violations = validator.validate(user);
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        if (!violations.isEmpty()) {
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            throw new ValidationException("User validation failed");
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        }
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        // Save user
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    }
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}
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```