JSON. toJSONString(object, SerializerFeature.WriteMapNullValue)； Second parameter action

fastjson By alibaba Open source set json processor . And others json processor （ Such as Gson,Jackson etc. ） And others Java Object serialization and deserialization , It has obvious performance advantages
Pictured ：

Where the arrow points , because sent-1 Medium value It's empty , So it didn't print out .
Second use ：JSON.toJSONString(map, SerializerFeature.WriteMapNullValue) Specify the serialization method to print out