Both Enterprise JavaBeans and Servlets are server side components. What are the major differences between them? Specifically, what is the difference between a Session Bean and a Servlet?
Enterprise JavaBeans are components meant to encapsulate business logic. They do not handle presentation and have a precise set of restrictions they must obey. An EJB may not manage threads, access files with the java.io package, have a GUI, accept socket connections, or load native libraries. These restrictions are imposed because EJBs execute inside an EJB container, roughly analogous to the way servlets execute in a servlet container. Where servlets can be used to perform many arbitrary tasks, including the generation of HTML pages, EJBs are used almost entirely as business objects. A session bean represents a client for a session or transaction and an entity bean represents a persistent business object that is usually stored in a database. Unlike servlets, a single session bean may exist per client. A single servlet may serve many clients. A session bean usually mediates client access to entity beans, shielding the client from the details of managing state.
A very common model is for a servlet to act as a client of a session bean. In this multi-tier model, a user talks to a servlet or JavaServer Page through a Web browser. The servlet manages the session, keeping track of a session bean for each of its clients and forwarding transaction requests to the session bean. The session bean may then access any number of entity beans to do things like lookup customer account data. The session bean executes its business logic and returns results to the servlet which then formats a result for the Web browser.
One of the ideas behind EJBs is to componentize server-side business logic and move it closer to the database. EJBs and servlets will typically not run on the same server. A key thing to remember is that both EJBs and servlets must execute inside of a container. For servlets, this is usually a Web server interfaced with a servlet engine. For EJBs, this is an EJB server such as the reference implementation provided with the Java 2 Enterprise Edition SDK.