Why did parseFrom() function hang using protobuf in java socket?

Question

I just want to create echo server/client using protobuf and java.

I tested with protobuf-java-2.4.1 and jdk1.7.

I wrote echo server code like below

// create server socket and accept for client connection.
// ...
link = servSock.accept();                       
Person person = Person.parseFrom(link.getInputStream()); // blocking position
person.writeTo(link.getOutputStream());

I think it is not necessary to note Person.proto.

The client code is only send Person object using socket input stream and receive echo Person object.

// socket connect code was omitted.
Person person = Person.newBuilder().setId(1).setName("zotiger").build();
person.writeTo(echoSocket.getOutputStream());
person = Person.parseFrom(echoSocket.getInputStream());

But server was blocked in parseFrom function when the server and client both run.

I found if i use writeDelimitedTo() and parseDelimitedFrom(), then that is ok. I don't understand why the writeTo() and parseFrom() function does not working.

Why did the server blocking in there?

Is it necessary to send some end signal from client side?

Brian Roach Brian Roach · Accepted Answer · 2013-05-29T13:55:39

The reason you have to use writeDelimitedTo()/parseDelimitedFrom() is that otherwise protocol buffers may have no idea how much data it needs to read from the socket. That presents a problem (I say may because you could of course create a message with only fixed length fields that wouldn't require this ... but protocol buffers has to deal with both cases)

The writeDelimitedTo() method writes the length of the message to the OutputStream then the message itself. Its counterpart parseDelimitedFrom() reads the length, then the message.

You can use writeTo() and pasrseFrom() with streams but only if you want to write/read a single message and are closing the stream after writing. The reader will then get an EOF to indicate the end of the message (also the case when reading from a file that contains only a single message).

Why did parseFrom() function hang using protobuf in java socket?

2 Answers