我终于写了整个验证器：

import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import javax.validation.Constraint;
import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
import javax.validation.Payload;

import org.apache.commons.beanutils.PropertyUtils;

@Target( { TYPE })
@Retention(RUNTIME)
@Constraint(validatedBy = CheckAtLeastOneNotNull.CheckAtLeastOneNotNullValidator.class)
@Documented
public @interface CheckAtLeastOneNotNull {

     String message() default "{com.xxx.constraints.checkatleastnotnull}";

        Class<?>[] groups() default {};

        Class<? extends Payload>[] payload() default {};

        String[] fieldNames();

        public static class CheckAtLeastOneNotNullValidator implements ConstraintValidator<CheckAtLeastOneNotNull, Object> {

            private String[] fieldNames;

            public void initialize(CheckAtLeastOneNotNull constraintAnnotation) {
                this.fieldNames = constraintAnnotation.fieldNames();
            }

            public boolean isValid(Object object, ConstraintValidatorContext constraintContext) {


                if (object == null)
                    return true;

                try {

                    for (String fieldName:fieldNames){
                        Object property = PropertyUtils.getProperty(object, fieldName);

                        if (property!=null) return true;
                    }

                    return false;

                } catch (Exception e) {
                   System.printStackTrace(e);   
                    return false;
                }
            }

        }

}

用法示例：

@Entity
@CheckAtLeastOneNotNull(fieldNames={"fieldA","fieldB"})
public class Reward {

    @Id
    @GeneratedValue
    private Integer id;

    private Integer fieldA;
    private Integer fieldB;

    [...] // accessors, other fields, etc.
}