We have just downloaded the latest JGroups 2.6.3.GA version for some initial multicast testing. There is no expirience with the API on our side. The initial review of the Message class shows us huge memory requirements of each message instance. The HashMap for headers and the ReentrantReadWriteLock for headers access seems to be a bit to much overhead for messages with the data (buf) lenght of few bytes.

Despite using the heavy RRWLock there is an unprotected read access to the headers map possible. The method getHeaders returns the headers as unmodifiableMap instead of a copy created within the read lock. An iteration over the returned unmodifiableMap might throw ConcurrentModificationException as the same headers map object might be modified using the putHeader method.