I cannot register multiple history event handler in the process engine configuration

Preconditions

• The process engine configuration does not allow to register multiple history event handler
• When ProcessEngineConfiguration#setHistoryEventHandler is called, the passed instance is set to the field historyEventHandler
• When ProcessEngineConfiguration#setHistoryEventHandler is called again at a later point, the instance present in historyEventHandler is overridden

Problem

• The Spring Boot Starter calls ProcessEngineConfiguration#setHistoryEventHandler via the process engine plugin EventPublisherPlugin
• When the user wants to register an additional history event handler, either the instance set by the Spring Boot Starter or the history event handler set by the user is overridden depending on the order of the list of process engine plugin beans

Possible Solution

• Two new attributes of type list are introduced in the process engine configuration:
  • preHistoryEventProducers
  • postHistoryEventProducers
• When a process engine is bootstrapped, a composite history event producer is constructed out of the pre list, the actual historyEventProducer and the post list
• This composite history event producer is set to the field historyEventProducer
• Like this, multiple history event producers can easily be registered