MongoDB Java Driver: Convert BsonDocument to Document and back

Question

I am using the MongoDB Java Driver for my project to access my database from Java.

I usually use Document as it's quite easy to use and all methods in MongoDBCollection, such as find() work with it and return Document instances.

However, in some cases I want to use the equivalent BsonDocument which is more verbose but offers type-safety by implementing Map<String,BsonValue>, which Document does not have because it implements Map<String,Object>.

I am able to convert a Document into a BsonDocument with this:

BsonDocument bsonDoc = document.toBsonDocument(BsonDocument.class, MongoClient.getDefaultCodecRegistry());

The problem here is, that all methods in MongoDBCollection, like insertOne() only take Document instances, so I can not use these.

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For me, it looks like there are 2 ways to solve this problem:

If the BsonDocument created by toBsonDocument() is in some way backed by the orginal Document, I could use the original Document instance even when I made modifications to the BsonDocument, because the original Document would reflect these changes, right? Does it work this way or is the BsonDocument just a copy?

The second way would be to convert from BsonDocument back to Document. Is this in some way possible?

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Thanks in advance!

Answer 1

Assuming you have an instance of CodecRegistry, you can convert BsonDocument to Document and vice-versa using this adapter:

public class DocumentAdapter {
    private final CodecRegistry registry;
    private final Codec<Document> codec;

    public DocumentAdapter(CodecRegistry registry) {
        this.registry = registry;
        this.codec = registry.get(Document.class);
    }

    public Document fromBson(BsonDocument bson) {
        return codec.decode(bson.asBsonReader(), DecoderContext.builder().build());
    }

    public BsonDocument toBson(Document document) {
        return document.toBsonDocument(BsonDocument.class, registry);
    }
}

Answer 2

public static Document bsonToDocument(BsonDocument bsonDocument) {
    DocumentCodec codec = new DocumentCodec();
    DecoderContext decoderContext = DecoderContext.builder().build();
    return codec.decode(new BsonDocumentReader(bsonDocument), decoderContext);
}