drools rule: stateful vs stateless knowledge session

Both the sessions maintain a state. The difference is that a
stateful session also maintains its state between session invocations (calls to the fireAllRules method). This is useful when we need to call rules multiple times over a period of time while making iterative changes to its state.

e.g.

1. update fact and re-fire rule:

code snapshot 

StatefulKnowledgeSession s1 = getSession();
User u1 = new User();
u1.setUserId("u1");
u1.setEmptyBottle(3);
u1.setMoney(100);
s1.insert(u1);
s1.fireAllRules();
s1.dispose();

//update fact and re-fire rule
u1.setMoney(10);
u1.setEmptyBottle(80);
FactHandle fh = s1.getFactHandle(u1);
s1.update(fh, u1);
s1.fireAllRules();
s1.dispose();

2. add/delete rule and re-fire rule

code snapshot 

StatefulKnowledgeSession s1 = getSession();
User u1 = new User();

try {
s1.insert(u1);
s1.fireAllRules();

//add new rules to session and re-fire rule
KnowledgeBuilder kbuilder1 = KnowledgeBuilderFactory
.newKnowledgeBuilder();
kbuilder1.add(ResourceFactory.newClassPathResource(fileName1),
ResourceType.DRL);
s1.getKnowledgeBase().addKnowledgePackages(kbuilder1.getKnowledgePackages());

s1.fireAllRules();

} finally {
s1.dispose();
}
}