When upgrading a project’s Mongo driver from 2.13.0 to 3.4.2, we got the following exception:

org.bson.codecs.configuration.CodecConfigurationException: can't find a codec for class java.math.BigDecimal.

Well it seems that Mongo doesn’t support BigDecimal any longer. There we made our own BigdDecimal Codec. If you search for the ”can't find a codec for class” error, you inevitably end up at this StackOverflow entry. However that is about saving nested, custom objects into Mongo. If you search for a while longer you will end up in the Mongo docs that has a simple example with Integer. Seems easy to adopt that for BigDecimal.

But when we did that, we got the same error. it seemed like Mongo was just ignoring our call to the CodecRegistry! The problem was what is described here – we were using a BasicDBObject and not a Document so the Mongo driver wasn’t hitting the CodecRegistry.

I made a Gist overview of how we got it working. The key was adding the Transformer via the static class with this line:  BSON.addEncodingHook(BigDecimal.class, new BigDecimalTransformer()); . We needed both that Transformer and the Codec in the registry for it to work.

I hope that this post can save someone else a few hours/days of head-scratching.