On Dec 6, 2010, at 10:31 , Koert Kuipers wrote:
But that doesn't make a parseFrom() in message interface invalid,
does it?
Indeed some other information outside the raw bytes will be needed
to pick to right Message subclass. But that's fine.
Oh, sorry, I misunderstood your question, so my answer is somewhat
invalid.
One could then:
1) pick the right subclass of Message based upon some information
outside the raw bytes (in my case something stored in a protobuf
wrapper around the raw bytes)
2) call subclass.parseFrom(bytes)
now we have to jump through more hoops for step 2 (create instance
of Message subclass, newBuilderForType, mergeFrom, isInitialized,
build)
The MessageLite.Builder interface has a mergeFrom method that does
what you want. What you should do is something like:
* Get a MessageLite instance for the message type you want to parse
(eg. something like MyMessageType.getDefaultInstance(), or
MessageLite.getDefaultInstanceForType())
* Hold on to that MessageLite instance in some sort of registry.
(HashMap<Integer, MessageLite>?)
* When you get a message, look at the protobuf wrapper to determine
the type.
* Look up the "prototype" MessageLite instance in your registry.
* Call prototypeInstance.newBuilderForType().mergeFrom(bytes).build()
This only creates a single instance of the message each time.
The .build() method will automatically check that the message is
initialized, so you don't need to call isInitialized (although you may
want to catch the exception it could throw?).
This Builder pattern is used so that the Message objects are
immutable. This means they can be passed between threads without
requiring any synchronization. See:
http://code.google.com/apis/protocolbuffers/docs/javatutorial.html#builders
Hope this helps,
Evan
--
Evan Jones
http://evanjones.ca/
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