[Request processing failed; nested exception is org.springframework.http.converter.HttpMessageConversionException: Type definition error: [simple type, class com.example.sprbk.tqs2.Entity.user]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.example.sprbk.tqs2.Entity.user` (no Creators, like default constructor, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 1, column: 2]] with root cause

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.example.sprbk.tqs2.Entity.user` (no Creators, like default constructor, exist): cannot deserialize from Object value (no delegate- or property-based Creator)

解决，在报错类user里加一个无参构造器，并重启后台

该bug的具体作用机制有待进一步研究