How to construct a Builder from a FieldDescriptor with JavaType.MESSAGE

[Jon Emerson]

Hi There,

I'm writing a utility to convert data stored in MongoDB back to protocol buffer format.  My strategy is to look for each field in the protocol buffer definition, and if there's a matching field from my MongoDB object, then pull that data and set it on my protocol buffer builder.  This works great, except for embedded messages.  I can't figure out how to create a Builder for an embedded message.

Here's my first try:

          switch (fieldDescriptor.getJavaType()) {

            ...

            case MESSAGE:

              Class<T> type = (Class<T>) fieldDescriptor.getDefaultValue().getClass(); // getDefaultValue throws.

              messageBuilder.addRepeatedField(fieldDescriptor,

                  fromDBObject((BasicBSONObject) object.get(fieldName), type));

              break;

But the noted line throws with the error, "java.lang.UnsupportedOperationException: FieldDescriptor.getDefaultValue() called on an embedded message field."

Here's my second try:

          switch (fieldDescriptor.getJavaType()) {

            ...

            case MESSAGE:

              Message.Builder embeddedMessageBuilder = messageBuilder.getFieldBuilder(fieldDescriptor); // getFieldBuilder throws.

              Class<T> type = (Class<T>) embeddedMessageBuilder.getDefaultInstanceForType().getClass();

              messageBuilder.addRepeatedField(fieldDescriptor,

                  fromDBObject((BasicBSONObject) object.get(fieldName), type));

              break;

Here, the noted line throws with the error, "java.lang.UnsupportedOperationException: getFieldBuilder() called on a non-Message type."

I'm not sure what else to try.  My question boils down to a very simple one: Given a FieldDescriptor that describes an embedded repeated MESSAGE field, how can you create a Builder for it?

Thanks!

jon

[Feng Xiao]

As it's a repeated field, you should use "getRepeatedFieldBuilder".

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[Jon Emerson]

Ah, thanks for the response!  That wasn't it though - Calling .getRepeatedFieldBuilder(FieldDescriptor, int) against a Builder I'm just starting to populate will inevitably give only IndexOutOfBounds exceptions... Since nothing's set in the repeated field yet!

But your response did give give me a huge hint: LOTS of new functionality for working with repeated field builders was added in the November 2014 release to Github.  I was still using the release from July.  So I didn't see .newBuilderForField(FieldDescriptor)... that was what I needed!  After upgrading to the latest branch I was up and going in no time.

Here's my final (working!) code:

            case MESSAGE:

              Message.Builder embeddedMessageBuilder =

                  messageBuilder.newBuilderForField(fieldDescriptor);

              Class<T> type = (Class<T>) embeddedMessageBuilder.getDefaultInstanceForType().getClass();

              for (T t : fromDBList((BasicBSONList) object.get(fieldName), type)) {

                messageBuilder.addRepeatedField(fieldDescriptor, t);

              }

              break;

It's still a bit janky that I construct a Builder to get its DefaultInstance to then get its Class (I do this because I call my methods recursively, and the top-most method needs a hint as to the top-most Object's class)... But, hey, it works for now.  If there's more elegant ways to get a Class from a repeated FieldDescriptor, I'd love to hear it.

Thanks!!