Java Server Pages technology. Similar to Microsoft 's Active Server Pages, it injects Java logic into HTML page templates. Enterprise JavaBeans, which define a standard component architecture for deployment of Java-based business logic. Application servers create and maintain user sessions. Network services also need to know which logged-in user can see which Web page.
Application servers can define and enforce access-control scenarios.
High-performance database connectivity. An application server can create a pool of database connections, then reuse them to ensure speedy access to data. A network service can't fail. When a service runs as multiple instances, there's no single point of failure. A replicated service isn't just highly available, it's also scalable.