Efficient POJO mapping to/from Java Mongo DBObject using Jackson

Question

Although similar to Convert DBObject to a POJO using MongoDB Java Driver my question is different in that I am specifically interested in using Jackson for mapping.

I have an object which I want to convert to a Mongo DBObject instance. I want to use the Jackson JSON framework to do the job.

One way to do so is:

DBObject dbo = (DBObject)JSON.parse(m_objectMapper.writeValueAsString(entity));

However, according to https://github.com/FasterXML/jackson-docs/wiki/Presentation:-Jackson-Performance this is the worst way to go. So, I am looking for an alternative. Ideally, I would like to be able to hook into the JSON generation pipeline and populate a DBObject instance on the fly. This is possible, because the target in my case is a BasicDBObject instance, which implements the Map interface. So, it should fit into the pipeline easily.

Now, I know I can convert an object to Map using the ObjectMapper.convertValue function and then recursively convert the map to a BasicDBObject instance using the map constructor of the BasicDBObject type. But, I want to know if I can eliminate the intermediate map and create the BasicDBObject directly.

Note, that because a BasicDBObject is essentially a map, the opposite conversion, namely from a scalar DBObject to a POJO is trivial and should be quite efficient:

DBObject dbo = getDBO();
Class clazz = getObjectClass();
Object pojo = m_objectMapper.convertValue(dbo, clazz);

Lastly, my POJO do not have any JSON annotations and I would like it to keep this way.

Answer 1

You can probably use Mixin annotations to annotate your POJO and the BasicDBObject (or DBObject), so annotations is not a problem. Since BasicDBOject is a map, you can use @JsonAnySetter on the put method.

m_objectMapper.addMixInAnnotations(YourMixIn.class, BasicDBObject.class);

public interface YourMixIn.class {
    @JsonAnySetter
    void put(String key, Object value);
}

This is all I can come up with since I have zero experience with MongoDB Object.

Update: MixIn are basically a Jackson mechanism to add annotation to a class without modifying said class. This is a perfect fit when you don't have control over the class you want to marshal (like when it's from an external jar) or when you don't want to clutter your classes with annotation.

In your case here, you said that BasicDBObject implements the Map interface, so that class has the method put, as defined by the map interface. By adding @JsonAnySetter to that method, you tell Jackson that whenever he finds a property that he doesn't know after introspection of the class to use the method to insert the property to the object. The key is the name of the property and the value is, well, the value of the property.

All this combined makes the intermediate map go away, since Jackson will directly convert to the BasicDBOject because it now knows how to deserialize that class from Json. With that configuration, you can do:

DBObject dbo = m_objectMapper.convertValue(pojo, BasicDBObject.class);

Note that I haven't tested this because I don't work with MongoDB, so there might be some loose ends. However, I have used the same mechanism for similar use cases without any problem. YMMV depending on the classes.