在使用ObjectMapper将json转对象，调用mapper.readValue(jsonStr, XwjUser.class)时，报如下错：
com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of entity.XwjUser: no suitable constructor found, can not deserialize from Object value (missing default constructor or creator, or perhaps need to add/enable type information?)
at [Source: {"id":,"message":"Hello World","sendTime":,"intList":null}; line: , column: ]
at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:)
at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:)
at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:)
at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:)
at testJson.TestObjectMapper.testObj(TestObjectMapper.java:)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
....
解决办法：在XwjUser实体类中，加入无参构造器，即可解决