[rabbitmq-discuss] Issue in Java client while fetching channel

[Ben Hood]

Arnand,

On Wed, Aug 5, 2009 at 2:24 AM, Anand Ved<anand.ved at xoriant.com> wrote:
> As per the java client documentation, channel is not thread safe and I have multiple threads that would publish messages to the Q. For each publish, I create a new channel and publish the message.

Yes, this is correct - channels are not thread safe. However, this
doesn't mean you have to dispose of them every time you publish a
message, all it means is that they shouldn't be shared across threads.
One way to achieve this is to store the channel in the ThreadLocal.

> Earlier I was not closing the channel and this lead to connection.createChannel() returning null object. Now as per java client document again, it is stated that if there are no channels available or if the current channel number is currently in use, then it would return null. Hence now I have closed the channel after each publish.

On the server side, there is no maximum number of channels per se.
This is only limited by the maximum number of processes in the Erlang
VM, which is 32768 by default, but this can be raised to any number
you like.

> If you are aware of any other case where we may end up getting a null channel, then please let me know.

As it turns out, there is actually a bug in the channel number
allocation method that under some circumstances leads to a null
channel being returned. This appears to be a bug in the HotSpot
compiler (as the attached test proves on some JDKs - I was able to
reproduce it on 1.6.0_14 on 64 bit Linux, but not on OSX or Windows).
The simple solution in the Java client is instead of comparing the max
channel against Integer.MAX_VALUE, compare against Integer.MAX_VALUE -
1, and then the compiler doesn't seem to bin the loop. So I'll raise a
bug for that and push the fix out.

HTH,

Ben
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