Lifeline

How to read it

UML 2.5.1 draws a lifeline as a rectangle, its "head", followed by a vertical line, which may be dashed, for the participant's lifetime. Each lifeline is one individual participant. The head names it as name : Type, and either half may be left out, so : Order is an unnamed Order. Time runs down the page: the University of Washington's course slides put it as "down = forward in time". The thin rectangle on the line is an execution specification, usually called an activation bar or activation box. The specification draws it as a thin grey or white rectangle covering the line; the Washington slides describe it as the time the object's method is on the stack, either running its own code or waiting for another object's method to finish.

Also searched as: activation bar, lifeline symbol, sequence diagram lifeline, uml lifeline.

The start and end of a lifeline

An object created during the interaction has its head lower down the page than the others, where the creation Synchronous and return messages reaches it. A large X at the bottom of a lifeline is its destruction (UML 2.5.1; Washington's slides). Scott Ambler, in The Object Primer, says he usually leaves destruction out of his own diagrams.

Nested bars

Overlapping execution specifications on one lifeline are drawn as overlapping rectangles. The Washington slides nest activation bars to show recursion, an object calling its own method.

Look-alikes

The head is a box like a Class, but a lifeline is one participant, not a class as a whole. In some course notes the name in the head is underlined, the way an object's name is in a class diagram (King Saud University's notes). A stick figure at the head of a lifeline is an Actor.