When you write an Externalizer class, RiverMarshaller (RM) does:

writeExternalizerClass(objClass, externalizer);

And inside it:

ClassTable.Writer classTableWriter = classTable.getClassWriter(objClass);
if (classTableWriter != null)

{ write(Protocol.ID_PREDEFINED_EXTERNALIZER_CLASS); classCache.put(objClass, classSeq++); writeClassTableData(objClass, classTableWriter); }

else

{ write(Protocol.ID_EXTERNALIZER_CLASS); writeString(classResolver.getClassName(objClass)); classCache.put(objClass, classSeq++); doAnnotateClass(objClass); writeObject(externalizer); }

Now, if I provide my own writer, it'll only write Protocol.ID_PREDEFINED_EXTERNALIZER_CLASS and writeClassTableData()

On the reading side, in RiverUnmarshaller (RU), the following happens:

case Protocol.ID_PREDEFINED_EXTERNALIZER_CLASS: {
final int idx = classCache.size();
classCache.add(null);
final Class<?> type = readClassTableClass();
final Externalizer externalizer = (Externalizer) readObject();

And this means that RU expects the Externalize in the wire but RM didn't write it! This fails.