CSE4413

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====== 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.





===== Outline =====

== The Architecture ==
  * 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.

== The Life Cycle ==
  * 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.

== The API ==
  * 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.




===== To Do =====

  * 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