Not sure where requests will be blocked, or will you explain "cost a lot"?

AFAIR, when a new message arrives, as it is associated with a session, it's beng processed by a IoProcessor. As an IoProcessor also holds the selector, we can't process more than one message at a time in a single IoProcessor (hopefully, we have more than one IoProcessor). So if the message is costly to process (ie, yu access a database in the middle), in a loaded system, messages can be blocked a long time before being processed, waiing for the crrent message to be completed.

This is what I meant. Hope it's clearer.

I followed the execution flows of SocketAccept and SocketConnect, IoProcessors do not process any handler in their own threads, but throw them to org.apache.mina.filter.executor.ExecutorFilter, moreover, the filter process the user handlers within a threadpool(about 16 threads). So I don't think every requests waiting on this IoProcessor will be blocked, but fail.

The IoProcessor class inherits from the AbstractPollingIoProcessor class, which contains an inner class named Processor, which is responsible for handling incoming messages. This class implements Runnable, and call the process() method, which iterates on all the selected sessions, one by one, do a read() for each one of them, and call the filterChain.fireMessageReceived(buf) method if some data has been received. If yiu don't put an executor in your chain, and if this chain processing is costly, then the whole mchansim will wait until your costly handling is finished.

Sorry, my mistake, it is 1.x in my hand

Won't be refactored in 2.0

There are ways to handle such a situation either by using an executor filter, or by dealing with costly operations on the IoHandler