Question

The MongoDB Java driver documentation of the packet org.bson mentions various “Lazy” versions of other classes. Unfortunately the Javadocs of these classes can barely be called documentation.

What is their purpose and how does their behavior differ from the normal versions?

Answer 1

Under normal operation, the driver creates and consumes documents using the DBObject Map-like interface. When inserting documents, it iterates over the map to convert it the corresponding BSON representation. When querying, it creates new documents by putting key-value pairs into the map.

But there are times when you want to work with raw BSON and not pay the cost of all this serialization and deserialization. That’s what the lazy DBObject implementations are for. Instead of treating them as a map, a custom encoder instead writes the bytes directly to the BSON stream. Similarly, a custom decoder writes the raw bytes directly into the lazy DBObject.

In this context, the meaning of the term lazy is that, since the lazy equivalents still have to implement the DBObject interface, they do so by “lazily” interpreting the raw BSON byte array that they contain.

One last note: the lazy DBObject classes are very likely not going to be included in the upcoming 3.0 release of the driver, as the entire serialization is changing in a way that is not compatibile with lazy DBObjects. There will be equivalent functionality, but not API compatible.