FW: [SNMP4J] Concurrency Exception

[Marco Matchefts]

I think I found the problem.  When I close the connection,
Snmp::close calls the ::onResponse callback on all outstanding
requests.  The first thing I do in the ::onResponse callback
is invoke the following function.  The javadocs mentioned we 
need to do this to prevent a memory leak...  Also if we don't
do this, I've been getting multiple callbacks.

( (Snmp) responseEvent.getSource()).cancel(responseEvent.getRequest(),
this);

So, the Snmp::cancel function modifies the pendingRequests hashtable,
which causes the iterator in the ::close fcn to go bad.  Hence, the
concurrency exception.

So, under what scenarios should we be calling the 
::cancel to prevent the memory leak?  Should we call ::cancel for
all response events except those that throw an InterruptedException?
Thanks for the advice... 
--Marco



-----Original Message-----
From: snmp4j-bounces at agentpp.org [mailto:snmp4j-bounces at agentpp.org] On
Behalf Of Marco Matchefts
Sent: Monday, February 06, 2006 2:48 PM
To: snmp4j at agentpp.org
Subject: [SNMP4J] Concurrency Exception

I have a multithreaded app.  I am currently getting a concurrency
exception inside the stack

in the Snmp::close function.  The stack trace is below.  One thread of
my app is closing the

Snmp::connection.  Another thread may have just sent a request.
Synchronization 

should prevent both from happing at the same time.  But I am getting the
following 

exception:

 

2006-02-06 14:37:02,225 WARN  DataCollectorTask sendRequest
failure:java.util.ConcurrentModificationException

2006-02-06 14:37:02,225 DEBUG DataCollectorTask sendRequest exception:

java.util.ConcurrentModificationException

        at java.util.Hashtable$Enumerator.next(Unknown Source)

        at org.snmp4j.Snmp.close(Snmp.java:430)

        at
com.vieo.management.snmp.SNMP4jSessionFactory$SessionAdapter.close(SNMP4
jSessionFactory.java:231)

        at
com.vieo.management.snmp.SNMP4jSessionFactory$SessionAdapter.send(SNMP4j
SessionFactory.java:266)

        at
com.vieo.management.snmp.DataCollectorTask.sendRequests(DataCollectorTas
k.java:169)

        at
com.vieo.management.snmp.DataCollectorTask.collectReadings(DataCollector
Task.java:122)

        at
com.vieo.management.snmp.DataCollectorTask.run(DataCollectorTask.java:30
0)

        at
com.vieo.util.TaskScheduler$TaskQueue$ScheduledTask.run(TaskScheduler.ja
va:1056)

        at com.vieo.util.ThreadPool$TaskRunner.run(ThreadPool.java:212)

        at java.lang.Thread.run(Unknown Source)

 

I see that the pendingRequests is just a plain Hashtable.  By chance is
there another thread

doing something on the pendingRequests hashtable while I am closing the
thread?

Any insight would be very helpful.  Thanks.

--Marco

 

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