Servlets

This lecture covers the JEE's approach to server-side processing. It introduces the overall Tomcat architecture and focuses on the servlet container. The hands-on lab session will walk you through the installation of Tomcat and introduces you to the capabilities of its three engines, Coyote, Catalina, and Jasper.

• Tomcat = Coyote + Catalina + Jasper. It is a reference implementation of Sun's Servlet/JSP standards.
• Coyote is a connector, Catalina is a servlet container, and Jasper is a JSP compiler.
• Client uses HTTP to reach a Web Server which has a module (Apache) or a plug-in (IIS) for JK filtering.
• The web server uses NFS to serve static files, CGI to run scripts out of process, or the JK protocol to contact Coyote in process.
• You supply your servlet subclass to Catalina and it handles the request.
• Coyote today comes with an HTTP version that bypasses the JK protocol; i.e. it acts as a web server with NFS, CGI, and SSI. Hence, the whole system is self-contained and 100% Java.
• To enable MVC, servlets are used as controllers, POJO's and Beans for the business model, and JSP for the view.

• Server makes one instance of your servlet
• It invokes the init method on it. You may need to instantiate POJO's/beans or initialize in init.
• It creates a pool of threads ready to invoke your servlet's service method.
• When a client connects, a thread is (randomly) chosen and assigned to serve this request.
• There is no client-to-thread mapping: the same client may get served by the same or a different thread.
• When it is time to stop this servlet, the server invokes its destroy method then unloads it.

• No need to override service since it filters based on the request method. Override either doGet or doPost (or both)
• The request object enables you to retrieve socket data, HTTP data, the headers, the parameters, and the payload (for POST).
• Note that parameters are available as Map<String,String[]> and that for payloads you can have a raw stream or a wrapped up one.
• The response object allows you to set the HTTP response line and headers as well as the payload.

• Read Sections 6.1 through 6.5 of our textbook.
• Take an overall look at the servlet API (accessible from our Resources Page.
• Come to Prism for a hands-on lecture