Make sure objectoutputstream is open before objectinputstream

Question

If I am not mistaken the objectoutputstream must be opened before the objectinputstream is for the connection to be made correctly. My task is to send an array from one computer to another through a socket, do some calculation then send back the calculated array. So I need to have communications both ways. I am running into problems with the connection not being made because I think the timing is off. On a small scale (2 computers) everything works fine, but for 1 computer trying to connect to multiple others (7) it isn't working.
Should I give up on object streams or is there a way to coordinate their openings? Thank you.
BTW-- the error message I get at the initial receiving computer is an IOException and the getMessage() returns null. This is the same error that i got on my small scale when I had objectinputstream opening before objectoutputstream.

Answer 1

icccapital wrote:
I don't know how reliable this source is, but here is the link where I had read that.
http://frequal.com/java/OpenObjectOutputStreamFirst.html
You seem to have misunderstood.that article. It doesn't not say that you have to construct the OOS that sends before you constuct the OIS at the other end that receives. Rather, it says that, on each end when you're doing bidirectional communication (which you are), you should construct that end's OOS before constructing its OIS. If you do this:
// client
ObjectInputStream ois = new ObjectInputStream(...);
ObjectOutputStream oos = new ObjectOutputStream(...);
// server
ObjectInputStream ois = new ObjectInputStream(...);
ObjectOutputStream oos = new ObjectOutputStream(...);then both sides are stuck in the OIS line because the OIS c'tor doesn't return until it reads something, but neither one can read because the other end--both other ends--can't write because it's can't get past the OIS. It's a form of deadlock.
So, theoretically, all you should have to do is switch the order and put OOS first on just one side, but to keep things simple I'd just do it on both:
// client
ObjectOutputStream oos = new ObjectOutputStream(...);
ObjectInputStream ois = new ObjectInputStream(...);
// server
ObjectOutputStream oos = new ObjectOutputStream(...);
ObjectInputStream ois = new ObjectInputStream(...);Again, remember, you don't have to worry about controlling the timing of the client and server relative to each other. It's just within each side, you want that side's OOS first.
Now, here's one thing I'm not sure of. I looked briefly at the code for both OIS and OOS, and OIS does indeed do a read in it's c'tor, and OOS does a write. I'm not sure why this would be any better. It looks to me like we'd now have both OOS lines blocking waiting to be able to write, so, contrary to what the article is saying (and contrary to what I just said above) it seems--at least at first glance--that we should do OOS first on one end and OIS first on the other. However, if there's buffering going on, then the OOS c'tor could just write its bytes to the buffer and return, without having to wait for them to actually be delivered.