https://issues.apache.org/jira/browse/AMQ-3161 - Race condition in ActiveMQ Journal Checkpoint worker thread cleanup leads to multiple running instances patch applied with thanks, one small mod, left un synced check in store(..) such that locking only occurs if thread needs a restart
activemq/trunk/activemq-core/src/main/java/org/apache/activemq/store/kahadb/MessageDatabase.java