在日常开发中经常会用到String类型的数据当作数值进行映射，势必会做出数值范围的校验，可以通过自定义注解的办法简化代码实现，减少冗余代码。 

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = StrRangeValidator.class)
public @interface StrRange {/*** 错误提示* @return*/String message() default "value is not in given range";/*** 最小值* @return*/double min() default Double.MIN_VALUE;/*** 最大值* @return*/double max() default Double.MAX_VALUE;/*** 是否包含边界* @return*/boolean closeMin() default true;/*** 是否包含边界* @return*/boolean closeMax() default true;/*** 是否可空* @return*/boolean nullable() default true;Class<?>[] groups() default { };Class<? extends Payload>[] payload() default { };
}

这里的groups，payload是必须的。其他方法是根据需要设定的参数：

1. 允许null值跳过校验

2. 边界值开区间、闭区间

3. 自定义errorMessage

validatedBy 是核心的验证逻辑：

public class StrRangeValidator implements ConstraintValidator<StrRange, String> {private boolean nullable;private BigDecimal min;private BigDecimal max;private boolean closeMin;private boolean closeMax;@Overridepublic void initialize(StrRange constraintAnnotation) {nullable = constraintAnnotation.nullable();min = new BigDecimal(String.valueOf(constraintAnnotation.min()));max = new BigDecimal(String.valueOf(constraintAnnotation.max()));closeMin = constraintAnnotation.closeMin();closeMax = constraintAnnotation.closeMax();}@Overridepublic boolean isValid(String s, ConstraintValidatorContext constraintValidatorContext) {if (s == null && nullable){return true;}try {BigDecimal val = new BigDecimal(s);boolean checkMin = closeMin ? min.compareTo(val) <= 0 : min.compareTo(val) < 0;boolean checkMax = closeMax ? val.compareTo(max) <= 0 : val.compareTo(max) < 0;return checkMin && checkMax;} catch (Exception ex) {return false;}}
}

String到枚举值的反向解析和验证也是比较常见的问题，也可以通过自定义注解的方式简化此类解析判断。

再来一个枚举验证：

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = ValidEnumValidator.class)
public @interface ValidEnum {/*** 错误提示* @return*/String message() default "invalid enum value";/*** 目标类型* @return*/Class<?> target();/*** 是否可空* @return*/boolean nullable() default true;Class<?>[] groups() default { };Class<? extends Payload>[] payload() default { };
}

public class ValidEnumValidator implements ConstraintValidator<ValidEnum, String> {private Class<?> clazz;private boolean nullable;@Overridepublic void initialize(ValidEnum constraintAnnotation) {nullable = constraintAnnotation.nullable();clazz = constraintAnnotation.target();}@Overridepublic boolean isValid(String s, ConstraintValidatorContext constraintValidatorContext) {if (!clazz.isEnum()) {return false;}if (s == null && nullable) {return true;}try {Method method = clazz.getDeclaredMethod("of", String.class);return method.invoke(null, s) != null;} catch (Exception e) {return false;}}
}

注意，枚举需要保持类型一致：String，都存在这样的of方法

@AllArgsConstructor
@Getter
public enum EAccountAuthTypeEnum {OPEN_ACCOUNT("1", "开户"),;private final String code;private final String msg;public static EAccountAuthTypeEnum of(String code) {return Arrays.stream(values()).filter(ele -> ele.getCode().equals(code)).findFirst().orElse(null);}
}