||This article relies largely or entirely upon a single source. (February 2012)|
|Structural UML diagrams|
|Behavioral UML diagrams|
Activity diagrams are graphical representations of workflows of stepwise activities and actions with support for choice, iteration and concurrency. In the Unified Modeling Language, activity diagrams can be used to describe the business and operational step-by-step workflows of components in a system. An activity diagram shows the overall flow of control.
Activity diagrams are constructed from a limited number of shapes, connected with arrows. The most important shape types:
- rounded rectangles represent actions;
- diamonds represent decisions;
- bars represent the start (split) or end (join) of concurrent activities;
- a black circle represents the start (initial state) of the workflow;
- an encircled black circle represents the end (final state).
Arrows run from the start towards the end and represent the order in which activities happen.
Hence they can be regarded as a form of flowchart. Typical flowchart techniques lack constructs for expressing concurrency. However, the join and split symbols in activity diagrams only resolve this for simple cases; the meaning of the model is not clear when they are arbitrarily combined with decisions or loops.
While in UML 1.x, activity diagrams were a specialized form of state diagrams, in UML 2.x, the activity diagrams were reformalized to be based on Petri net-like semantics, increasing the scope of situations that can be modeled using activity diagrams. These changes cause many UML 1.x activity diagrams to be interpreted differently in UML 2.x
See also 
- Business Process Modeling Notation
- Control flower graph
- Data flow diagram
- Event-driven process chain
- List of UML tools
- State diagram
- Glossary of Key Terms at McGraw-hill.com. Retrieved 20 July 2008.
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