How do I check on the size of the queue? I know it's not JMS standard, but most JMS implementations have a way to do this.

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You can view the queue depth using the MBeans in ActiveMQ 4.x. Use any JMX management console to see the statistics. See How can I monitor ActiveMQ.

You can also browse the contents of a queue using the JMS QueueBrowser.

Or you can access statistics programmatically