When to call Workflow in a Custom Java Application

I have a Custom Java application which needs workflow to create tasks and each task is completed by several folks(groups). I need to be able to show what tasks are open for a user when he/she logs into custom Java App. When user clicks on task and completes, it should propagate to next task and assign to next group. Basically want to separate UI logic from Work flow logic.

We do not want to use a BPM solution as they come with their own UI and using just workflow piece from BPM does not add any value to use BPM tool and it is expensive.

Questions: If i buy a separate workflow engine which provides Java API 1) Where should I have the logic of when to call Workflow engine ? If Custom Java App has the logic to call workflow engine then, it defeats the purpose of separating workflow logic. 2) Does Custom Java App constantly call Workflow engine at every event and workflow only reacts to certain events ?

Thank You shiva_r99