java中的protobuf是线程安全的吗？

Question

I have the following protobuf msg defined:

message Counts {
    repeated int32 counts = 1;
}

which is shared between threads R and W as a builder:

private final Counts.Builder countsBuilder;

Thread R will only read from countsBuilder and W will only write to countsBuilder . The shared builder will be read, written-to and (at some point) built & sent over the network.

AFAIK, concurrent reads to messages are fine, but anything else must be synchronized at a higher level by the developer? So, I can't actually write and read to the shared builder at the same time?

If this is not inherently thread-safe, I'm thinking of using some kind of thread-safe Collection<Integer> which I'll use for reading/writing and will (at some point) create a brand new message right before sending it over the network. Or am I missing something?

Thanks!

EDIT 1: I'm using protobuf 2.4.1 and java 6

EDIT 2: Some terminology and spelling fixes.

Answer 1

You should be fine if you synchronize both your read and writes:

synchronized (countsBuilder) {
   // modify countsBuilder
}

But remember that you also need to make sure that there aren't any race conditions when building the message; the writer thread is not allowed to make any writes after the message has been built.

Answer 2

according to https://developers.google.com/protocol-buffers/docs/reference/cpp It's not thread-safe in C++.

And Also not in java https://developers.google.com/protocol-buffers/docs/reference/java-generated

I encountered this problem recently, And I found this. Here is what it said on C++

A note on thread-safety:

Thread-safety in the Protocol Buffer library follows a simple rule: unless explicitly noted otherwise, it is always safe to use an object from multiple threads simultaneously as long as the object is declared const in all threads (or, it is only used in ways that would be allowed if it were declared const). However, if an object is accessed in one thread in a way that would not be allowed if it were const, then it is not safe to access that object in any other thread simultaneously.

Put simply, read-only access to an object can happen in multiple threads simultaneously, but write access can only happen in a single thread at a time.

Said on java:

Note that builders are not thread-safe, so Java synchronization should be used whenever it is necessary for multiple different threads to be modifying the contents of a single builder.