For some reasons, being at one UserTask, I need to "look ahead" in my process and I need to read the content of Activiti custom extension attributes. What would be the best approach to do that in Activiti 5.3?
Seeing 'future' steps is not that trivial. You could try to get a ReadOnlyProcessDefinition by calling the public ReadOnlyProcessDefinition getDeployedProcessDefinition(String processDefinitionId) on RespositoryServiceImpl (you would need to cast your RepositoryService)