Process composition is supported in jBPM by means of the process-state. The process state is a state that is associated with another process definition. When graph execution arrives in the process state, a new process instance of the sub-process is created and it is associated with the path of execution that arrived in the process state. The path of execution of the super process will wait till the sub process instance has ended. When the sub process instance ends, the path of execution of the super process will leave the process state and continue graph execution in the super process.

<process-definition name="hire">
  <start-state>
    <transition to="initial interview" />
  </start-state>
  <process-state name="initial interview">
    <sub-process name="interview" />
    <variable name="a" access="read,write" mapped-name="aa" />
    <variable name="b" access="read" mapped-name="bb" />
    <transition to="..." />
  </process-state>
  ...
</process-definition>

This 'hire' process contains a process-state that spawns an 'interview' process. When execution arrives in the 'first interview', a new execution (=process instance) of the 'interview' process is created. If no explicit version is specified, the latest version of the sub process as known when deploying the 'hire' process is used. To make jBPM instantiate a specific version the optional version attribute can be specified. To postpone binding the specified or latest version until actually creating the sub process, the optional binding attribute should be set to late. Then variable 'a' from the hire process is copied into variable 'aa' from the interview process. The same way, hire variable 'b' is copied into interview variable 'bb'. When the interview process finishes, only variable 'aa' from the interview process is copied back into the 'a' variable of the hire process.

In general, When a subprocess is started, all variables with read access are read from the super process and fed into the newly created sub process before the signal is given to leave the start state. When the sub process instances is finished, all the variables with write access will be copied from the sub process to the super process. The mapped-name attribute of the variable element allows you to specify the variable name that should be used in the sub process.