Jackson 把 json 转为 DBObject

需求

希望通过Jackson 把 json 转为 DBObject
use jackson, convert json string to DBObject

尝试

刚开始实现这个功能的时候，没有想到这里有坑，直接写的代码，如下

public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        
        DBObject dbObject = new BasicDBObject("h", "a");
        String json = objectMapper.writeValueAsString(dbObject);
        
        System.out.println(json);
        
        DBObject readValue = objectMapper.readValue(json, DBObject.class);
        
        System.out.println(readValue);
    }

输出为

{"h":"a"}
Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.mongodb.DBObject, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information
 at [Source: {"h":"a"}; line: 1, column: 1]
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
    at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:892)
    at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:139)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3736)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2726)
    at xxx.AsyncMessageHelper.main(AsyncMessageHelper.java:91)

可以看到 序列化 这个过程是没有问题的，正常输出 {"h":"a"}

但是 反序列化出了问题，抛了异常

看异常信息，大概问题是因为 DBObject 是一个接口，所以 Jackson 在实例化的时候，不知道该用哪个实现类去实例化

网上搜了很多文章，最后找到了一种可行的方案（也许还有其它的方案）

code

import com.mongodb.BasicDBObject;
import com.mongodb.DBObject;
import com.mongodb.util.JSON;

public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        
        DBObject dbObject = new BasicDBObject("h", "a");
        String json = objectMapper.writeValueAsString(dbObject);
        
        System.out.println(json);
        
        SimpleModule module = new SimpleModule();
        module.addDeserializer(DBObject.class, new DBObjectDeserializer(DBObject.class));
        objectMapper.registerModule(module);
        
        DBObject readValue = objectMapper.readValue(json, DBObject.class);
        
        System.out.println(readValue);
    }
    
    public static class DBObjectDeserializer extends StdDeserializer<DBObject> {
        protected DBObjectDeserializer(Class<?> vc) {
            super(vc);
        }

        @Override
        public DBObject deserialize(JsonParser p, DeserializationContext ctxt)
                throws IOException, JsonProcessingException {
            String string = p.readValueAsTree().toString();
            
            return (DBObject) JSON.parse(string);
        }
        
    }

输出

{"h":"a"}
{ "h" : "a"}

原理

首先，写了个DBObjectDeserializer,实现了标准反序列化接口
在具体的反序列化方法deserialize 中，使用了 MongoDB 官方的 JSON 工具类（这个类的方法可以正常反序列化DBObject）

其次，向我们的 ObjectMapper 注册了一个 SimpleModule
这个 module 有一个反序列化实例，就是我们刚刚写的 DBObjectDeserializer

参考

https://www.baeldung.com/jackson-custom-serialization

https://stackoverflow.com/a/12261133

https://stackoverflow.com/questions/41001302/jackson-deserializer-for-one-custom-field

https://stackoverflow.com/a/16825934