Together, servlets and JSP provide a powerful platform for generating web pages dynamically. Servlets and JSP, which are collectively called web components, can access resources like JDBC and enterprise beans. Because web components can access databases using JDBC, they can provide a powerful platform for e-commerce by allowing an enterprise to expose its business systems to the Web through an HTML interface. HTML has several advantages over more conventional client applications, in Java or any other language. The most important advantages have to do with distribution and firewalls. Conventional clients need to be distributed and installed on client machines, which is their biggest limitation: they require additional work for deployment and maintenance. Applets, which are dynamically downloaded, can be used to eliminate the headache of installation, but applets have other limitations, such as sandbox restrictions and heavyweight downloads. In contrast, HTML is extremely lightweight, does not require prior installation, and does not suffer from security restrictions. In addition, HTML interfaces can be modified and enhanced at their source without having to update the clients.
Firewalls present another significant problem in e-commerce. HTTP, the protocol over which web pages are requested and delivered, can pass through most firewalls without a problem, but other protocols such as IIOP or JRMP cannot. This restriction has proven to be a significant barrier to the success of distributed object systems that must support access from anonymous clients, because it means that distributed object applications generally cannot be created for a client base that may have arbitrary firewall configurations. HTTP does not have this limitation, since practically all firewalls allow HTTP to pass unhindered.
The problems with distribution and firewalls have led the EJB industry to adopt, in large part, an architecture based on the collaborative use of web components (servlets/JSP) and Enterprise JavaBeans. While web components provide the presentation logic for generating web pages, EJB provides a robust transactional middle tier for business logic. Web components access enterprise beans using the same API as application clients. Each technology is doing what it does best: servlets and JSP are excellent components for generating dynamic HTML, while EJB is an excellent platform for transactional business logic. Figure 17-2 illustrates how this architecture works.
This web component-EJB architecture is so widely accepted that it begs the question, “Should there be a united platform?” This is the question the J2EE specification is designed to answer. The J2EE specification defines a single application server platform that focuses on the interaction between servlets, JSP, and EJB. J2EE is important because it provides a specification for the interaction of web components with enterprise beans, making solutions more portable across vendors that support both component models.