记一个Jackson与Hutool混用的坑

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问题出现

最近遇到一个问题，Hutool从4.1.7升级到4.6.8之后，使用feign调用出现错误：

...
Caused by: feign.codec.EncodeException: Type definition error: [simple type, class cn.hutool.json.JSONNull]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class cn.hutool.json.JSONNull and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: cn.hutool.json.JSONObject["securityGroupIds"])
    at feign.ReflectiveFeign$BuildEncodedTemplateFromArgs.resolve(ReflectiveFeign.java:349)
    at feign.ReflectiveFeign$BuildTemplateByResolvingArgs.create(ReflectiveFeign.java:213)
    at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:72)
    at feign.hystrix.HystrixInvocationHandler$1.run(HystrixInvocationHandler.java:108)
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:302)
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:298)
    at rx.internal.operators.OnSubscribeDefer.call(OnSubscribeDefer.java:46)
    ... 29 more
Caused by: org.springframework.http.converter.HttpMessageConversionException: Type definition error: [simple type, class cn.hutool.json.JSONNull]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class cn.hutool.json.JSONNull and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: cn.hutool.json.JSONObject["securityGroupIds"])
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:295)
    at org.springframework.http.converter.AbstractGenericHttpMessageConverter.writeInternal(AbstractGenericHttpMessageConverter.java:111)
    at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:226)
    at org.springframework.cloud.openfeign.support.SpringEncoder.encode(SpringEncoder.java:94)
    at feign.ReflectiveFeign$BuildEncodedTemplateFromArgs.resolve(ReflectiveFeign.java:345)
    ... 35 more
Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class cn.hutool.json.JSONNull and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: cn.hutool.json.JSONObject["securityGroupIds"])
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77)
    at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191)
    at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:312)
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:71)
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:33)
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:718)
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:639)
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:33)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
    at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1396)
    at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:913)
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:288)
    ... 39 more

问题分析

Hutool会使用JSONNull来表示空值，而SpringCloud Feign默认使用的序列化是Jackson，在远程调用的过程中使用了Map，直接传入了Hutool的JSONObject，而该Map存在空值，所以存在JSONNull，最终导致错误。

问题解决

使用DTO传递参数，总之就是不要直接把JSONObjct作为参数传递。

总结

在使用JSON序列化工具的时候，尽量不要混合使用，即使存在多个JSON工具，也不把一个JSON工具的JSON对象直接用另一个JSON工具来处理，因为每个JSON工具都有自己的对JSON的处理，包括一些优化，如果混着用就会出问题，所以，在使用JSON工具处理JSON的时候不要混着使用多种JSON

吐槽

这里吐槽下Hutool，null值处理的时候要小心，非空判断用JSONObject.isNull，不要自行处理：

String jsonStr = "{\"name\":\"seven\",\"password\":null}";

JSONObject json = new JSONObject(jsonStr);
System.out.println(json.get("password")==null);
System.out.println(json.get("password").getClass());
    
Map<String,String> map = new JSONObject(jsonStr).toBean(Map.class);
System.out.println(map.get("password")==null);
System.out.println(map.get("password").getClass());

System.out.println(json.isNull("password"));

// 结果
false
class cn.hutool.json.JSONNull
false
class cn.hutool.json.JSONNull
true