jBPM is "just" a Java API. You can use it in a J2SE Java application, in a J2EE Web application or inside of an EJB Container.
I'm not sure to understand your question, but if you just want to try some application code in Eclipse, you don't need a server. You the JBoss IDE and create jBPM project.
taskInstance.setActorId directly assign the task to an actor.
AssignmentHandler may call setActorId, but the class first need to find an actorId. An example of AssignementHandler can be a Task Dispatcher which assign dynamically the tasks according to the workload of an actor set.
I don't get you for your last question.
- I don't understand how i must intergrate an assignment of actor. What is the difference between taskInstance.setActorId() and and assignmentHandler method.
It seems to me in general using AssignmentHandler is cleaner. The assignment of a task is separate from the execution of it. But I believe they have the same effect.