CS651

Servlets

Java's answer to processing CGI.
Most Servlets handle such HTTP requests, there are other types of servers as well, e.g. FTP servers, Mail servers, etc and you can develop servlets for them. (you will extend a different java class when creating an HTTP-based servlet compared to a non-HTTP-based servlet).
Servlets are designed to support a request/response computing model (CGI)....client sends a request (to invoke servlet), and server responds (sending servlet response).
Java Servlet API: located in javax.* packages.

javax.servlet.*;

Generic Servlet

must import the java.servlet package
must extend the GenericServlet class or HttpServlet class if
implementing http features
must override the service method in the class

    THE Hello World Servlet:

    import java.io.*;
    import java.servlet.*;
    public class HelloWorldServlet extends GenericServlet {

        public void service(ServletRequest req, ServletResponseres)
            throws ServletException, IOException{
            PrintStream out = newPrintStream(res.getOutputStream());
            out.println("Hello world!");
        }

        public String getServletInfo() {
            return "Hello World Servlet";
        }
    }

How a Servlet works:

1) Read any data sent by the user
        e.g. from an HTML form, or an applet,etc.
2) Look up HTTP request information
        e.g Browser capabilities, requesting host, cookies, etc.

3) Generate Results
        e.g. using possibly database access- JDBC, RMI, directo computation, calling other applicaitons, etc.

4) Format the results inside a document
        e.g. produce a web-page (HTML)

5) Set HTTP response parameters
        e.g. MIME type(e.g. returning HTML), cookies, compression, etc.

6) Send the document to the client

Why build web-pages dynamically?

Web-page is based on user submitted data.
- e.g. results page from search engines, order-confirmation from on-line store, etc.
Web-page is derived from data that changes frequently
- e.g. stock prices, weather report, etc.
Web-page uses information from databases or other server-side sources.
- e.g. looking at prices in a database, etc.

Advantage of Servlets over Other Languages Processing CGI

Efficient

Threads instead of OS processes.....only one servlet copy, is persistent!!
This is unlike other languages, Perl, C/C++, etc. where you invoke a new process everytime.

Convenient

High-level utilities built-in. Makes things like parsing CGI data easier.

Powerful

Because of persistence, information can be maintained from request-to-request (cache previous computations, keep open DB connections, etc).
Different servlets can more easily share information.
Servlets can talk directly to the web-server. (other languages can but, must use server-specific APIs if they are available).

Portable

basic benefit of Java (but, scripting languages like Perl are also portable)

Secure

basic benefit of Java (array-bounds checking, etc).

Important: when you use other java classes, beans, etc. from a Servlet, you need to deploy them in a package inside of a webapplication. The location will be WEB-INF/classes/directoryMatchingPackageName. Or alternatively you can place in a JAR file and put in WEB-INF/lib. This is important because you need to import them and the default package location for different Servlet containers is not standard. The default package that a Servlet is placed into is not standardized int the Servlet Container spec so, different containers do different things!!!! Be carefull and always use packages and deploy them appropriately.