Wrong behavior if event is triggered from EventDispatcher
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Key: SCXML-112
URL: https://issues.apache.org/jira/browse/SCXML-112
Project: Commons SCXML
Issue Type: Bug
Reporter: Ales Dolecek
Method SCMLExecutor#triggerEvents is synchronized which is too naive way to
ensure that events are processed in order they arrive.
Since the callback to EventDispatcher is made by thread that holds lock on
SCMLExecutor triggering event from the dispatcher allows this thread to
re-enter the event handling.
Note: This has nothing to do with multi-threading since there is only one
thread.
Simple queue might e sufficient but in multi-threaded application would make
the first thread to enter #triggerEvents process events from other threads
queued while the method is running.
Another approach might be to call EventDispatcher in separate thread,but it has
it's drawbacks too:
a) changes single-threaded application into-multithreaded which might break
assumtions made by unaware user
b) might start too many threads
c) Java does not guarantee that threads waiting for lock will get it in same
order as they arrive => might result in non-deterministic behavior of SCXML
interpretation
Possible solutions would be:
1) add queue and allow the "first" thread server all events - might be just
fine since multi-threaded applications might create dedicate thread just for
SCXML imterpretation
2) add queue and block threads after they queue event - so they "wait" until
execute their event become first in queue
3) factor out the queue event queue management and allow for "pluggable"
strategy
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