Internet

1
Internet Communications

• Server-side programming for
• the WWW (lecture 7)
• Java Servlets
• JSP

2
Servlets

• Java Servlets are a more recent alternative to
  CGI programming.
• Java servlets are programs that run on the Web
  server and build web pages on the fly (much
  like a CGI program).
• Java servlets typically take input from forms on
  Web pages and write out a new Web page in
  response.
• They should not be confused with Java Applets -
  which run on the client.

3
Servlets vs CGI

• Java servlets have following advantages over CGI
• More efficient -
• using CGI a new process is started for each
  request.
• Using servlets the java program stays running and
  new requests are handled by a new thread.
• More powerful -
• For example, Java servlets can handle sessions
  and maintain information about the user across
  requests.
• More secure

4
Writing and Running Servlets

• How you set up servlets depends very much on
  local setup.
• We will look briefly at an example servlet for
  handling HTTP requests, and how it is set up.
• First write your servlet a Java program which
  uses special servlet library classes.
• Example below writes Hello World!

5
Hello World Servlet

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class HelloWorld extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse
  response)
• throws ServletException, IOException
  
• response.setContentType("text/plain")
• PrintWriter out response.getWriter()
• out.println(Hello World")
• out.close()

Standard stuff
Set file type to plain Get hold of stream for
writing back to web page, and write Hello World
6
Explanation

• Standard servlet mantra involves
• importing standard servlet classes
• creating a subclass of HttpServlet
• creating a doGet method which deals with
  requests to that Servlet.
• This must have a request and a response argument,
  which provide handles to input from the page, and
  the output stream to the page.
• Setting the content type to be text/plain or
  text/html
• Creating a PrintWriter such that writing to this
  will write back a Web page to the client.

7
Servlets at MACS

• Now set it up You need a special server running
  to handle the servlet requests, e.g., TOMCAT
  server.
• We then compile the servlet code, and place
  servlets in the appropriate directory.
• We can now run it using a URL like the following
  
• http//www2.macs.hw.ac.uk8080/examples/servlet/He
  lloWorld

8
Dealing with Form Input

• It is easy to extend our servlet to handle input
  from a form.
• There is a special getParameter method that we
  can use to get hold of the values entered given
  the name of the input element.
• E.g. given ltinput typetext nameage /gt
• use request.getParameter(age)

9
Servlets Summary

• Servlets provide many useful facilities for
  creating dynamic web pages.
• They are efficient, as once running the program
  stays running dealing with all requests. CGI
  programs are started afresh with each request.
• Moderately easy to program once you master the
  mantra.
• But much more than has been described here!

10
Java Server Pages (JSP)

• Java Servlets can be awkward to use.
• The Java programs often consist mostly of
  statements to write out HTML (with just a few
  dynamic bits to do calculations, database access
  etc).
• It may be difficult to write servlets to produce
  attractive well styled web pages.
• JSP allows you to mix standard static HTML pages
  with dynamically generated HTML.
• Hybrid of HTML and Java Servlets

11
JSP - Example

• Following example illustrates how we can get some
  servlet Java code embeded in the HTML.

lthtmlgt ltheadgt lt/headgt ltbodygt lth1gt Todays date
lt/h1gt lt new java.util.Date()
gt lt/bodygt lt/htmlgt
Bit of JSP
12
JSP

• JSP avoids having to write servlets consisting
  mostly of println statements to write out HTML.
• JSP pages are automatically converted into
  servlet code on first access then stored as
  servlets on the server.
• JSP not to be confused with JavaScript, which is
  handled on the client side.
• To use them effectively you need to understand
  servlets.

13
To Use JSP

• JSP files end in .jsp and are stored in special
  location.
• Code is embedded in lt gt brackets.
• Some special variants of this for special
  purposes.
• Especially useful is lt . gt which evaluates an
  expression and sticks it in the output.
• Some useful variables are automatically
  available.
• In particular the servlet response parameter
  which contains data from forms etc.
• And the out variable which is a printwriter
  sending output back to web browser to be
  incorporated in web page.

14
More JSP Examples
lthtmlgt ltheadgtlt/headgt ltbodygt lth1gt One plus one
lt/h1gt 1 1 lt 11 gt lt/bodygt lt/htmlgt
Evaluates the expression 11 and puts the
result in the output.
15
More JSP Examples
lthtmlgt ltheadgtlt/headgt ltbodygt lth1gt And the title
is lt/h1gt lt request.getParameter("title")
gt lt/bodygt lt/htmlgt
16
Other Features of JSP

• JSP includes built in features that allow
• database access
• cookie handling
• session management
• mail
• etc etc

17
Other server side languages

• Other server side languages include ASP, PHP. All
  allow us to mix HTML with code. The code may LOOK
  like JavaScript, but is interpreted on SERVER,
  not client.
• ASPlthtmlgt ltbodygt lt response.write("Hello
  World!") gt lt/bodygt lt/htmlgt
• PHP ltbodygt lt?php echo Hello World ?gt

18
Summary

• JSP provides convenient form of server side
  scripting, allowing mix of HTML and java.
• Converted on first use to a servlet.
• Good for fairly simple things. Can get convoluted
  for more complex things.
• Really need to understand servlets to do complex
  JSP applications.
• Many other languages with similar syntax.