Mongo convert Document to DBObject

You are kind of micro-optimizing here.

However, since both classes are implementations of Map, you can just do:

Document document = new Document();
BasicDBObject basicDBObject = new BasicDBObject(document);

Internally this does a Map#putAll operation that puts all entries of the Document map into the BasicDbObject map.

I know this is an old question and there is an accepted answer however it is not correct.

The proposed answer only does a shallow conversion between Document and DBOject. If your Json object contains nested objects or lists they will not be converted properly.

I got around this problem by serialising to JSON string. It is not efficient at all but might be enough in most cases, and at least it is correct:

public final class BsonConverter {
    public static Document toDocument(DBObject dbObject) {
        return Document.parse(dbObject.toString());
    }
    public static DBObject toDBObject(Document document) {
        return BasicDBObject.parse(document.toJson());
    }
}