How to implement custom validation annotations?

Create the Annotation:

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = MyValidator.class)
public @interface MyConstraint {
    String message() default "Invalid value";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

Implement the Validator:

public class MyValidator implements ConstraintValidator<MyConstraint, String> {
    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        return value != null && value.startsWith("ABC");
    }
}

Use it on DTO:

@MyConstraint
private String code;