Pros and cons of Java rules engines [closed]

What are the pros and cons to adopting the Java rules engines JESS and Drools?

Use a rule engine if you need to separate the business rules from the application logic. The Does Your Project Need a Rule Engine article has a good example:

For example, a typical storefront
system might involve code to calculate
a discount:

if (product.quantity > 100 && product.quantity < 500) {
  product.discount = 2;
} else if (product.quantity >= 500 && product.quantity < 2000) {
  product.discount = 5;
} else if (product.quantity >= 2000) {
  product.discount = 10;
}

A rule engine replaces the above with
code that looks like this:

ruleEngine.applyRules(product);

Up to you to decide whether putting a rule admin console in the hands of non-technical people is a good thing or not 🙂

More details in Should I use a Rules Engine?, Why use a Rule Engine?, Some Guidelines For Deciding Whether To Use A Rules Engine and on Google.

Are there any other players?

Other players include JRules, Corticon (JRules is the most famous IMO – which doesn’t mean the best).

how do they compare in other areas like ease of use, performance, level of integration with your code?

Can’t tell you precisely, I only have a little (positive) experience with Drools. But you’ll get some feedback from blog posts like JBoss Drools vs ILog JRules – an anecdotal story (be sure to read it) or Working with Drools from a JRules perspective. I’m sure you can find more of them on Google (but I would give Drools a try).