Servlet was developed to execute the business logic. You can develop EJB, or Java Beans to process the business logic. So, you can reuse the same business logic in many different place. So, in servlet you can re-use the already developed business logic multiple times. Here the resuability means the re-use of business logic in the form of Java Beans/EJB etc.
JSP is developed to develop the view of the application. You are not supposed to write business processing logic in JSP.
Ujjwal B Soni <baroda, gujarat, india> <+918866008962>
"Helping hands are better than praying lips......"
The main problems with reusability have to do with how dependent the code is on the environment in which it was originally used. If you put the actual business logic in a generic JavaBean and employ that JavaBean from the servlet, the business logic would tend to be more reusable than if you'd placed it directly in the servlet. This concept is known as "separation of concerns".
An IDE is no substitute for an Intelligent Developer.