keep getting unrecognized field not marked as ignorable error while paring a json string

Question

I have a json string which is produced by a web service and I need to parse it and get values in it.

The json string looks like this:

{
  "category": {
    "Abnormal dress": [
      { "h": 44, "prob": 1.0, "w": 35, "x": 468, "y": 202 },
      { "h": 42, "prob": 0.9994152784347534, "w": 37, "x": 31, "y": 198 }
    ],
    "Safety helmet": [ { "h": 60, "prob": 1.0, "w": 45, "x": 187, "y": 163 },
      { "h": 55, "prob": 1.0, "w": 44, "x": 394, "y": 157 },
      { "h": 60, "prob": 1.0, "w": 43, "x": 322, "y": 170 },
      { "h": 58, "prob": 1.0, "w": 52, "x": 536, "y": 156 },
      { "h": 26, "prob": 1.0, "w": 19, "x": 357, "y": 192 },
      { "h": 28, "prob": 1.0, "w": 36, "x": 31, "y": 176 },
      { "h": 13, "prob": 0.9999998807907104, "w": 12, "x": 357, "y": 140 }
    ]
  },
  "file_result": "/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAIBAQEBAQIBAQECAgICAgQDAgICAgUEBAMEBgUGBgYFBgYGBwkIBgcJBwYGCAsICQoKCgoKBggLDAsKDAkKCgr/2wBDAQICAgICAgUDAwUKB9k=",
  "status": "ok"
}

I defined my object classes as below:

package murraco.util;

import java.util.List;

public class FlaskRes {
    String file_result;
    String status;
    Category category;//I have defined this field, why it cannot be recognized?
}


class Category{
    List<Target> Abnormal_dress;
    List<Target> Safety_helmet;
}

class Target{
    double h;
    double w;
    double x;//left top 
    double y;
    double prob;
}

I defined these classes because according to my knowledge, {} in json should be mapping into an Java object, and [] should be an Array in java.

Then I use below code to parse the JSON string:

import com.fasterxml.jackson.databind.ObjectMapper;
...
resStr.replaceFirst("Abnormal dress","Abnormal_dress");
resStr.replaceFirst("Safety helmet","Safety_helmet");// to handle the whitespace
FlaskRes flaskRes=mapper.readValue(resStr, FlaskRes.class);

But I got below errors:

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "category" (class murraco.util.FlaskRes), not marked as ignorable (0 known properties: ])
 at [Source: (StringReader); line: 1, column: 17] (through reference chain: murraco.util.FlaskRes["category"])
    at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:61)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnknownProperty(DeserializationContext.java:855)
    at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:1212)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1604)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownVanilla(BeanDeserializerBase.java:1582)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:299)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:156)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4524)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3466)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3434)
    at murraco.util.FlaskServiceUtil.main(FlaskServiceUtil.java:33)