As Thomas Kilian described here, normal behavior of nodes of Activity diagrams with tokens is:
A node becomes active when at all of its incoming
InformationFlow connectors
a token has arrived. When the node finalizes it sends single tokens along all its outgoing InformationFlow connectors.
But also he added:
There are special nodes like fork and merge which behave a bit different
I know that behavior of "merge" node differs because it becomes immediately active by receiving first token and accepts one among several alternate flows. But what is the difference between behavior of fork/join nodes with tokens with normal behavior?
Merge
- and DecisionNode
s look the same, but are different elements: .
In a diagram you can only distinguish both by looking at the incoming and outgoing InformationFlow
s. The first has multiple incoming and one outgoing while the second has the opposite relation. A MergeNode
accepts any incoming token and forwards it directly to its single outgoing InformationFlow
. So unlike Action
s it will not wait for all tokens. The DecisionNode
in contrast accepts only a single token and lets it pass to only one of its outgoing InformationFlow
s. It is the modeler's responsibility to set guards in a way that only one evaluates to true. If there are more (or even unguarded) InformationFlow
s the token will take any arbitrary free route.
Fork
and Join
are also two different elements which look the same: (or vertically).
You can also distinguish them by the number of in-/outgoing InformationFlow
s. Fork
has one in and multiple out and Join
vice versa. A Fork
will send as many tokens as it has outgoing InformationFlow
s once a token arrives at its single incoming InformationFlow
. The Join
will (like Action
s) wait for tokens arrive at all of its incoming InformationFlow
s. Only then it will emerge a single token at its single outgoing InformationFlow
.
So while Merge
- and DecisionNode
s control the flow of a single token (execution path) Fork
and Join
are used to start and synchronize parallel execution paths.
来源:https://stackoverflow.com/questions/52181253/difference-between-behavior-of-fork-join-nodes-toward-tokens-with-other-nodes-of