code: @Override
      public void compress(final List<ByteBuf> source, final BsonOutput target) {
          int uncompressedSize = getUncompressedSize(source);
  
          byte[] singleByteArraySource = new byte[uncompressedSize];
          copy(source, singleByteArraySource);
  
          try {
              byte[] out = new byte[Snappy.maxCompressedLength(uncompressedSize)];
              int compressedSize = Snappy.compress(singleByteArraySource, 0, singleByteArraySource.length, out, 0);
              target.writeBytes(out, 0, compressedSize);
          } catch (IOException e) {
              throw new MongoInternalException("Unexpected IOException", e);
          }
      }
docs: be copied into a single byte array.  For that reason the compress method defined in the base class can't be used.