Solution for missing space fields when fastjson parses generic json strings

The scenario is as follows:

There is a public interface that returns a json string, but the type of object represented by the json string is uncertain, either ClassA or ClassB.

public class ClassA{
   private String key;
   private String value;
   ...
}

public class ClassB{
   private String host;
   private int id;
   private String user;
   private String db;
   ...
}

//情况1：返回ClassA的json字符串
str = {"key":"user", "value":"ming"}
//情况2：返回ClassB的json字符串串
str = {"host":"localhost", "id":1, "user":"ming", "db":null}

In this case, we generally parse the result into a JSONObject object:

JSONObject obj = JSON.praseObject(str);

But using the above method, parse the above JSONObject and pass it to other interfaces (assuming A).
A The received JSONObject may lose some field information, such as the db field in the str above. When printing, the db field is missing.

Therefore, the solution to this situation:

HashMap<String, Object> result = JSON.parseObject(str, 
            new TypeReference<HashMap<String, Object>>(){});
JSONObject obj = JSON.praseObject(JSON.toJSONString(result, 
            SerializerFeature.WriteMapNullValue));

At this time, the parsed JSONObject object is passed to other interfaces. At this time, the object information is complete,
that is, the fields will not be missing.