The internal states that are used are:
DISPATCHED       - Standard request. Not in Async mode.
STARTING         - ServletRequest.startAsync() has been called from
                   Servlet.service() but service() has not exited.
STARTED          - ServletRequest.startAsync() has been called from
                   Servlet.service() and service() has exited.
READ_WRITE_OP    - Performing an asynchronous read or write.
MUST_COMPLETE    - ServletRequest.startAsync() followed by complete() have
                   been called during a single Servlet.service() method. The
                   complete() will be processed as soon as Servlet.service()
                   exits.
COMPLETE_PENDING - ServletRequest.startAsync() has been called from
                   Servlet.service() but, before service() exited, complete()
                   was called from another thread. The complete() will
                   be processed as soon as Servlet.service() exits.
COMPLETING       - The call to complete() was made once the request was in
                   the STARTED state.
TIMING_OUT       - The async request has timed out and is waiting for a call
                   to complete() or dispatch(). If that isn't made, the error
                   state will be entered.
MUST_DISPATCH    - ServletRequest.startAsync() followed by dispatch() have
                   been called during a single Servlet.service() method. The
                   dispatch() will be processed as soon as Servlet.service()
                   exits.
DISPATCH_PENDING - ServletRequest.startAsync() has been called from
                   Servlet.service() but, before service() exited, dispatch()
                   was called from another thread. The dispatch() will
                   be processed as soon as Servlet.service() exits.
DISPATCHING      - The dispatch is being processed.
MUST_ERROR       - ServletRequest.startAsync() has been called from
                   Servlet.service() but, before service() exited, an I/O
                   error occurred on another thread. The container will
                   perform the necessary error handling when
                   Servlet.service() exits.
ERROR            - Something went wrong.


The valid state transitions are:

                 post()                                        dispatched()
   |-------»------------------»---------|    |-------«-----------------------«-----|
   |                                    |    |                                     |
   |                                    |    |        post()                       |
   |               post()              \|/  \|/       dispatched()                 |
   |           |-----»----------------»DISPATCHED«-------------«-------------|     |
   |           |                          | /|\ |                            |     |
   |           |              startAsync()|  |--|timeout()                   |     |
   ^           |                          |                                  |     |
   |           |        complete()        |                  dispatch()      ^     |
   |           |   |--«---------------«-- | ---«--MUST_ERROR--»-----|        |     |
   |           |   |                      |         /|\             |        |     |
   |           ^   |                      |          |              |        |     |
   |           |   |                      |    /-----|error()       |        |     |
   |           |   |                      |   /                     |        ^     |
   |           |  \|/  ST-complete()     \|/ /   ST-dispatch()     \|/       |     |
   |    MUST_COMPLETE«--------«--------STARTING--------»---------»MUST_DISPATCH    |
   |                                    / | \                                      |
   |                                   /  |  \                                     |
   |                    OT-complete() /   |   \    OT-dispatch()                   |
   |   COMPLETE_PENDING«------«------/    |    \-------»---------»DISPATCH_PENDING |
   |        |      /|\                    |                       /|\ |            |
   |        |       |                     |                        |  |post()      |
   |        |       |OT-complete()        |           OT-dispatch()|  |            |
   |        |       |---------«-------«---|---«--\                 |  |            |
   |        |                             |       \                |  |            |
   |        |         /-------«-------«-- | --«---READ_WRITE--»----|  |            |
   |        |        / ST-complete()      |        /  /|\  \          |            |
   |        |       /                     | post()/   /     \         |            |
   |        |      /                      |      /   /       \        |            |
   |        |     /                       |     /   /         \       |            |
   |        |    /                        |    /   /           \      |            |
   |        |   /                         |   |   /             \     |            |
   |        |  /                          |   |  /  ST-dispatch()\    |            |
   |        |  |                          |   | |                 \   |            |
   |  post()|  |  timeout()         post()|   | |asyncOperation()  \  |  timeout() |
   |        |  |  |--|                    |   | |                  |  |    |--|    |
   |       \|/\|/\|/ |     complete()    \|/ \|/|   dispatch()    \|/\|/  \|/ |    |
   |--«-----COMPLETING«--------«----------STARTED--------»---------»DISPATCHING----|
           /|\  /|\                       | /|\ |                       /|\ /|\
            |    |                        |  |--|                        |   |
            |    |               timeout()|  post()                      |   |
            |    |                        |                              |   |
            |    |       complete()      \|/         dispatch()          |   |
            |    |------------«-------TIMING_OUT--------»----------------|   |
            |                                                                |
            |            complete()                     dispatch()           |
            |---------------«-----------ERROR--------------»-----------------|


Notes: * For clarity, the transitions to ERROR which are valid from every state apart from
         STARTING are not shown.
       * All transitions may happen on either the Servlet.service() thread (ST) or on any
         other thread (OT) unless explicitly marked.