ProtocolEncoderOutputImpl uses ConcurrentLinkedQueue and at first look it seems to be thread-safe. But really concurrent execution of flush method isn't thread-safe (and write-mergeAll also).
E.g. in RTMP several channels multiplexed in single connection. According protocol specification it's possible to write to different channels concurrently. But it doesn't work with MINA.
I've synchronized channel writing, but it doesn't prevent concurrent run of flushing (in 2.0.4 it's done directly in ProtocolCodecFilter.filterWrite, but ProtocolEncoderOutputImpl.flush has the same problem).
Here the fragment of flushing code:
Suppose original packets sequence is A, B, ...
Concurrent run of flushing may proceed as following:
thread-1: Object encodedMessage = bufferQueue.poll(); // gets A packet
thread-2: Object encodedMessage = bufferQueue.poll(); // gets B packet
thread-2: nextFilter.filterWrite(...); // writes B packet
thread-1: nextFilter.filterWrite(...); // writes A packet
so, resulting sequence will B, A
It's quite confusing result especially when documentation doesn't contain any explanation about such behavior.