If a JoinNode does not have a joinSpec, then this is equivalent to a joinSpec Expression with the Boolean operator “and.” That is, the implicit default joinSpec condition is that there is at least one token offered on each incoming ActivityEdge. Source Unified Modeling Language 2.5.1
Alternatively, the joinSpec may consist of an Expression with the name of a single Boolean operator and no operands specified. In this case, the value of the joinSpec shall be given by applying the given operator to Boolean values indicating ... Source Unified Modeling Language 2.5.1
If the joinSpec ValueSpecification is given by a textual expression, then the names of the incoming edges may be used to denote ... the value associated with an object token offered from an ObjectFlow (if any). Source Unified Modeling Language 2.5.1
If the joinSpec ValueSpecification is given by a textual expression, then the names of the incoming edges may be used to denote a Boolean value indicating the presence (true) or absence (false) of an offer from a ControlFlow ... Source Unified Modeling Language 2.5.1
joinSpec ... This evaluation shall not be interrupted by any new tokens offered during the evaluation, nor shall concurrent evaluations be started when new tokens are offered during an evaluation. The ValueSpecification shall evaluate to a Boolean value. Source Unified Modeling Language 2.5.1
Join nodes may have a joinSpec, which is a ValueSpecification that determines the condition under which the join will emit a token. If a JoinNode has a joinSpec, then this ValueSpecification is evaluated whenever a new token is offered to the JoinNode ... Source Unified Modeling Language 2.5.1
