Servlets

CGI Model

Pieces:

Clients: web browsers ie IE, Netscape
Web server: Apache, Netscape Enterprise, IIS
CGI Protocol: Specifying what a request/responce looks like
Handler programs: Any executable residing on the web server

Interaction:

Client makes a request by specifying a URL+additional info. Basically a method call, the method and arguments.
The web server (specified in the URL) receives the request.
The web server identifies the request as a CGI request
The web server locates the program corresponding to the request.
The web server starts up the handling program (heavy weight process creation!!)
The web server feeds the request parameters to the handler (through stdin or environment variables).
The handler executes
Output of the handler is sent via stdout back to the webserver for rerouting back to the requesting web browser. Output is typically a web page.
The handler terminates.

Servlet Model

Pieces:

Clients: web browsers ie IE, Netscape
Web server: Apache, Netscape Enterprise, IIS
CGI Protocol: Specifying what a request/responce looks like
Servlet container: Running JVM, hooked into the web server, loads and maintains servlets, session info, object store
Servlet: A Java class used to handle a specific request.

Interaction:

Client makes a request by specifying a URL+additional info. Basically a method call, the method and arguments.
The web server (specified in the URL) receives the request.
The web server identifies the request as a servlet request
The web server passes the request to the servlet container.
The servlet container locates the specified servlet (Java code, loaded and running in the container JVM)
The servlet container feeds the request parameters to the servlet (through HttpServletRequest, HttpServletResponse).
The servlet executes in a separate thread
The servlet can store/retrieve objects (possibly session scoped) from the servlet container.
Output of the servlet is sent via a PrintWriter back to the requesting web browser.
The servlet continues to be available in the servlet container.

Servlets vs CGI

Servlets...

are generally much faster than CGI scripts because a different process model is used.
process model allows resource sharing (ie database connection sharing). Container is always available.
use a standard API that is supported by many web servers.
have all the advantages of the Java programming language, including ease of development and platform independence.
can access the large set of APIs available for the Java platform.
have access to CGI support through the Servlet API
session awareness, ability to store and retrieve session scoped objects from servlet container.

Servlet API (javax.Servlet)

Some of what follows is from the Servlet 2.4 specification

Servlet Container: Is a part of a web server or application server that
- provides the network services over which requests and responses are set
- decodes MIME based requests
- formats MIME based responses
- contains and manages servlets through their lifecycle
Servlet Interface
- is the central abstraction of the servlet API
- All servlets either implement this interface, or extend a class that implements this interface
- GenericServlet and HttpServlet, two classes in the servlet API that implement the Servlet interface
HttpServlet
- Web developers extend HttpServlet to implement their servlets.
- Servlets are initialized once, handle multiple requests and can be destroyed.
- Most often override doGet() and doPost() with code to handle http GET and POST requests.
ServletContext
- interface defines a servlet’s view of the web application within which the servlet is running.
- the Container Provider is responsible for providing an implementation of the ServletContext interface in the servlet container.
- The ServletContext is rooted at a known path within the webserver
- Using the ServletContext object, a servlet can
  - log events
  - obtain URL references to resources
  - set and store attributes that other servlets in the context can access.
- Example: all servlets associated with this lecture are part of one application the csc309 servlet context. References to servlets within that application are via the URL http://127.0.0.1:8080/csc309
- 1 instance of ServletContext/web application/container
- Provides access to Initialization Parameters
  - Defined in the deployment descriptor
  - Methods: getInitParameter, getInitParameterNames()
- Context Attributes
  - Context level shared objects. Can be bound into the context and accessed by any servlet that is part of the application.
  - Methods: setAttribute(), getAttribute(), getAttributeNames(), removeAttributes()
  - More than 1 thread may be accessing a shared object.
  - Application developer responsible for synchronizing access to shared objects
- Resources
  - Gives servlet access to local documents associated with the application. (i.e. static web pages, images)
  - Methods: getResource(), getResourceAsStream(), getResourcePaths()
HttpServletResponse
- encapsulates all information to be returned from the server to the client
- In the HTTP protocol, this information is transmitted from the server to the client either by HTTP headers or the message body of the request.
- Headers: Can set/manipulate http headers
  - primitive manipulation: setHeader(), addHeader()
  - convenience methods: setContentType(), sendRedirect(), sendError()
- getWriter() - Obtain a PrintWriter, used to return character data to the client
Examples: Non-interactive web pages
- Hello World Under the URL http://127.0.0.1:8080/csc309/servlet/HelloWorld HelloWorld.java
- Flow between many pages (note the different kinds of hrefs):
  - Hello1 Hello1.java
  - Hello2 Hello2.java
- Returning different content: ImageFileServlet ImageFileServlet.java
  How is this different from an img tag? The servlet can create the image on the fly. JPEGServlet JPEGServlet.java
HttpServletRequest
- encapsulates all information from the client request
- In the HTTP protocol, this information is transmitted from the client to the server in the HTTP headers and the message body of the request.
- Manipulate 'form' variables: getParameter(), getParameterNames(), getParameterValues()
- getCookies() returns all cookies associated with the request
- getSession() returns the Session instance associated with the request (see below)
Examples: Using forms and form variables
- CGILibExample CGILibExample.java.
  Note: I override both the doGet() and doPost() methods since the original CGILibExample used both the post and get methods at the same time.
Sessions
The Hypertext Transfer Protocol (HTTP) is by design a stateless protocol. To build effective web applications, requests from a particular client must be associated with each other. Many strategies for session tracking have evolved over time, but all are difficult or troublesome for the programmer to use directly.
The Servlet API specification defines a simple HttpSession interface that allows a servlet container to use any of several approaches to track a user’s session without involving the Application Developer in the nuances of any one approach.
Session Tracking mechanisms
- A session token is passed back and forth between the client and servlet container. The token allows the container to associate a session with a particular session.
- The servlet container adapts to whether the client (browser) is accepting cookies.
- Cookies: If the client accepts cookies, sessions will identified by a token stored on the browser as a cookie.
- URL Rewriting: If the client is not accepting cookies, session token is communicated via the URL. Use the HttpServletResponse encodeURL method.

HttpSession

One instance for each active session

HttpSession session = request.getSession(); 
// session=current session or newly created one, if none exists

Object o = session.getAttribute("name"); 
// o=the object associated with "name" in this session

session.setAttribute("name",o); 
// o is the object associated with attribute "name" for this session

session.invalidate(); 
// Invalidates this session then unbinds any objects bound to it.

// NOTE: More than one thread may try to access a session resource.
// The application author is responsible for synchronizing access to
// session resources.

Example: SessionExample SessionExample.java.
Note: Run the above with cookies off and on, notice the URLs. Shut down and restart your browser between accepting and rejecting cookies.
Example:
- Stat Example
- Java code Stat.java, ReportStat.java, UpdateStat.java
- Note: This uses a ServletContextListener for the purposes of initializing the context. See web.xml in this context as well as StatStartupListener.java Note: Synchronization is not complete, that is, it is possible for threads to interfere on shared objects. You should fix this.
- html blank.html, index.html, statMenu.html

All examples

NEW: the following download has the contexts directories stripped to a minimum. That is, I removed the servlets and jsp subdirectories from each context. You should view these as minimal contexts.

You can download webapps.tar.gz (containing all examples used in this lecture).

References

Servlet 2.4 Specification
We did not talk about ServletContextListeners. These are what you use to initialize the context (for example). See the specification (pg 80). If you want to use this to initialize a connection pool, you will need to modify web.xml to hookup your Listener. See the classes directory as well as web.xml in the stat context (to tell the container (Tomcat) to send events to the specified listener).