springboot 接口返回json中null转换成空数组或空字符串（递归+反射实现）

本来想加一个Jackson的配置类修改ObjectMapper实现把null转空字符串或空数组，但是加上之后发现没效果，找不到问题在哪里，不知道是不是我使用@RestControllerAdvice全局返回处理类的问题，所以就自己写了一个工具类方法处理，就在全局返回处理类里面调用

找到配置不生效的问题在哪里了，见springboot中添加Jackson配置类不生效

全局返回处理类是用kotlin写的，用来封装统一响应实体和处理全局异常的，用java也是一样的，语法换成java就行。当然，这不是这篇的博文重点，重点是处理null的方法

package com.gt.gxjhpt.configuration

import cn.dev33.satoken.exception.NotLoginException
import cn.dev33.satoken.exception.NotPermissionException
import cn.dev33.satoken.exception.NotRoleException
import cn.hutool.json.JSONUtil
import com.gt.gxjhpt.entity.RestfulResp
import com.gt.gxjhpt.enumeration.RespCodeEnum
import com.gt.gxjhpt.exception.MyException
import com.gt.gxjhpt.utils.MyUtils
import org.springframework.core.MethodParameter
import org.springframework.http.HttpStatus
import org.springframework.http.MediaType
import org.springframework.http.converter.HttpMessageConverter
import org.springframework.http.server.ServerHttpRequest
import org.springframework.http.server.ServerHttpResponse
import org.springframework.http.server.ServletServerHttpResponse
import org.springframework.validation.BindException
import org.springframework.web.bind.MethodArgumentNotValidException
import org.springframework.web.bind.annotation.ExceptionHandler
import org.springframework.web.bind.annotation.ResponseStatus
import org.springframework.web.bind.annotation.RestControllerAdvice
import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice
import javax.validation.ConstraintViolationException

@RestControllerAdvice
class GlobalResponseBodyAdvice : ResponseBodyAdvice {

    override fun supports(returnType: MethodParameter, converterType: Class>): Boolean {
        return true
    }

    override fun beforeBodyWrite(
        body: Any?,
        returnType: MethodParameter,
        selectedContentType: MediaType,
        selectedConverterType: Class>,
        request: ServerHttpRequest,
        response: ServerHttpResponse
    ): Any? {
        return when (body) {

            is RestfulResp<*> -> body

            is String -> JSONUtil.toJsonStr(RestfulResp().success(body))

            null -> RestfulResp().success()

            else -> {
                val httpResponse = response as ServletServerHttpResponse
                if (httpResponse.servletResponse.status.equals(200)) {
                    // 设置null值为空字符串或空数组
                    MyUtils.setNullValue(body)
                    return RestfulResp().success(body)
                } else {
                    return body
                }
            }
        }
    }

    //其他参数异常
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    @ExceptionHandler(value = [BindException::class])
    fun handleBindException(): RestfulResp<*>? {
        return RestfulResp().enumResp(RespCodeEnum.PARAM_ERR)
    }

    //parameter异常
    //为了安全，就不将报错信息返回到前端，只返回粗略信息
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    @ExceptionHandler(value = [ConstraintViolationException::class])
    fun handleValidationException(): RestfulResp<*>? {
        return RestfulResp().enumResp(RespCodeEnum.PARAM_ERR)
    }

    //bean异常
    //为了安全，就不将报错信息返回到前端，只返回粗略信息
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    @ExceptionHandler(value = [MethodArgumentNotValidException::class])
    fun handleMethodArgumentNotValidException(ex: MethodArgumentNotValidException?): RestfulResp<*>? {
        return RestfulResp().enumResp(RespCodeEnum.PARAM_ERR)
    }

    //我的自定义异常
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    @ExceptionHandler(
        MyException::class
    )
    fun myException(e: MyException): RestfulResp<*>? {
        e.printStackTrace()

        return if (e.msg != null) {
            RestfulResp().error(e.msg, e.code)
        } else if (e.respCodeEnum != null) {
            RestfulResp().enumResp(e.respCodeEnum)
        } else {
            RestfulResp().unknown()
        }
    }

    //运行时异常
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    @ExceptionHandler(
        RuntimeException::class
    )
    fun runtimeException(e: RuntimeException): RestfulResp<*>? {
        e.printStackTrace()
        return RestfulResp().unknown(e.message)
    }

    //空指针异常
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    @ExceptionHandler(
        NullPointerException::class
    )
    fun nullPointerException(e: NullPointerException): RestfulResp<*>? {
        e.printStackTrace()
        return RestfulResp().unknown("空指针-->>" + e.stackTrace[0].toString())
    }

    //未知异常
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    @ExceptionHandler(
        Exception::class
    )
    fun exception(e: Exception): RestfulResp<*>? {
        e.printStackTrace()
        return RestfulResp().unknown(e.message)
    }

    // 未提供token
    @ResponseStatus(HttpStatus.UNAUTHORIZED)
    @ExceptionHandler(value = [NotLoginException::class])
    fun exception(e: NotLoginException): RestfulResp<*>? {
        e.printStackTrace()
        return RestfulResp().unknown(e.message)
    }

    // 无角色
    @ResponseStatus(HttpStatus.FORBIDDEN)
    @ExceptionHandler(
        NotRoleException::class
    )
    fun exception(e: NotRoleException): RestfulResp<*>? {
        e.printStackTrace()
        return RestfulResp().unknown(e.message)
    }

    // 无权限
    @ResponseStatus(HttpStatus.FORBIDDEN)
    @ExceptionHandler(
        NotPermissionException::class
    )
    fun exception(e: NotPermissionException): RestfulResp<*>? {
        e.printStackTrace()
        return RestfulResp().unknown(e.message)
    }
}

处理null的方法，不需要继续递归的类型，要写在第一个else if条件里面，工具类使用hutool

public static void setNullValue(Object body) throws IllegalAccessException, InstantiationException {

        Class aClass = body.getClass();

        if (Collection.class.isAssignableFrom(aClass)) {
            for (Object it : Convert.toList(body)) {
                MyUtils.setNullValue(it);
            }
        } else if (String.class.isAssignableFrom(aClass) || NumberUtil.isNumber(body.toString())
                || BooleanUtil.isBoolean(aClass) || CharUtil.isChar(body) || Date.class.isAssignableFrom(aClass)) {

        } else if (Map.class.isAssignableFrom(aClass)) {
            Map objectMap = Convert.toMap(String.class, Object.class, body);
            objectMap.forEach((k, v) -> {
                try {
                    if (v == null) {
                        v = "";
                    } else {
                        MyUtils.setNullValue(v);
                    }
                } catch (IllegalAccessException | InstantiationException e) {
                    e.printStackTrace();
                }
                objectMap.put(k, v);
            });

        } else { // 自定义响应对象

            List fields = CollUtil.toList(aClass.getDeclaredFields());

            // 父类属性
            Class superclass = aClass.getSuperclass();
            while (superclass != null && superclass != Object.class) {
                fields.addAll(CollUtil.toList(superclass.getDeclaredFields()));
                superclass = superclass.getSuperclass();
            }

            for (Field field : fields) {

                // 取消权限检查
                field.setAccessible(true);

                Object value = field.get(body);
                if (value == null) {
                    if (Collection.class.isAssignableFrom(field.getType())) {
                        field.set(body, new ArrayList<>());
                    } else if (String.class.isAssignableFrom(field.getType())) {
                        field.set(body, "");
                    }
                } else {
                    MyUtils.setNullValue(value);
                }
            }
        }

    }