Re: Java (android) socket reconnection
On 12/9/2012 10:06 AM, artik wrote:
Hi,
Could somebody help me resolve I think small problem for You but huge for me.
How to correctly support reconnecting client sockets.
I have one thread (it's name thrd1) for controlling connection (and reconnection if is it needed) and thread (it's name thrd2) for cyclical sending data to server (it is written in Delhi).
Some strange happends when I stop server and start it again after some time.
Server receives information that my client (java-android) wants to connect several times (it depends on time - how long server doesn't respond) - but after these attempts connection back to almost normal state.
Problem is in these too many attempts in connection and in this that when time not responding of server is enough long my application crushes.
In my opinion problem is in "not-cleaning" socket after my stop method? How to do it perfectly?
Could somebody help me to resolve my huge problem?
[... code snipped; see up-thread ...]
One problem (there may be others) is that your two threads
both use the `sock' variable without synchronization. Let's
call the threads T (the "transmitter") and C (the "connector"),
and imagine the following scenario:
T: writes to `sock', gets I/O error
T: sets `sock' null in response to the error
C: sees `sock' null, sets `sock = new Socket()'
C: performs `sock.connect()'
T: sees `sock' non-null and writes to it
That's presumably what you intended, but since there's no
synchronization something like this might happen instead:
T: writes to `sock', gets I/O error
T: sets `sock' null in response to the error
C: sees `sock' null, sets `sock = new Socket()'
T: sees `sock' non-null, writes, gets error (not connected)
T: sets `sock' null in response to second error
C: calls `sock.connect()' and gets NullPointerException
The problem is that either thread can try to use `sock'
while the other is in the middle of a sequence of steps that
make `sock' temporarily unusable. You must use synchronization
to ensure that neither thread touches `sock' while the other
is manipulating it. (You must use synchronization for some
other, subtler reasons, too.)
Among the other possible problems: It seems odd that after
an I/O error you just abandon the Socket without calling close()
on it. This looks very much like a resource leak that could
eventually bring down your program even if nothing else does.
(Or maybe not: I haven't studied it deeply.)
--
Eric Sosman
esosman@comcast-dot-net.invalid