Java support for WWW

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Java support for WWW

• Babak Esfandiari (sources Qusay Mahmoud, Roger
  Impey, textbook)

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Java support

• The URL class
• Applets
• Servlets and JSPs

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The URL class

• A facility to retrieve objects from the network
• Decodes the object based on its extension
• For example, a .gif file will generate an Image
  object
• Can be extended to any object type that you want
• The objects must obviously be addressable by a
  URL
• So far, support for http and file
• Can be extended to support ftp and others

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URL class example

• URL url new URL(http//java.sun.com/index.html)
  
• InputStream in url.openStream()
• Or
• URL url new URL(http//java.sun.com/duke.gif)
• Image im (Image) url.getContent()
• An HTTP connection is made
• See full URL doc

5
Java Applets

• Client-side
• See code sample in course web site

6
Servlets

• What is a servlet?
• Servlets vs. CGI scripts
• How do they work?
• Example

7
What is a Servlet?

• A server-side technology
• Designed to overcome some limitations of existing
  technologies (e.g. CGI is stateless)
• Characteristics
• A light-weight task that can be executed as a
  thread
• A servlet can remain in memory (a CGI script
  terminates when it finished)
• Advantages
• A servlet can service multiple client requests
• Can handle multiple clients without
  reloading/reinitialization

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Servlets.

• Servlets are written in Java
• Can be used to
• Generate dynamic contents
• Talk to databases
• Work with cookies
• Session management

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How do they work?
HTTP
Receive Request
Engine
Create Thread
Servlet
Gen Response
Send Response
10
Servlet Framework

• The package javax.servlet
• At the top level there are three interfaces
• ServletConfig, Servlet, Serializable
• The servlet interface
• init()
• service()
• destroy()
• getServiceConfig()
• getServiceInfo()

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The GenericServlet

• It is an abstract class that implements Servlet
• public abstract GenericServlet implements
  Servlet, ServletConfig, Serializable
• void init()
• abstract void service()
• void destroy()
• ServletConfig getServletConfig()
• String getServiceInfo()
• void log()

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The HttpServlet

• It is an abstract class that extends the
  GenericServlet abstract class
• It provides a convenient framework for handling
  the HTTP protocol
• These two classes (GenericServlet and
  HttpServlet) ease the task of writing servlets
• In the simplest case, you need to provide
  implementation for service()

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Life cycle of a servlet

• Servlet is loaded into memory by server init()
• Servlet processes client requests service()
• Servlet is remove by server destroy()
• service() is responsible for handling incoming
  client requests
• public void service(ServiceRequest request,
  ServletResponse response) throws
  ServletException, IOException
• Delegates HTTP requests doGet() doPost()

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Retrieving Parameters

• Use
• public String getParameter(String name)
• public String getParameterValues(String name)
• So if you have a parameter name username in a
  form then to retrieve the value use
• String name request.getParameter(username)

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Example

• Consider the following form
• ltform action"RequestParamExample" methodPOSTgt
• First Name ltinput typetext size20
  namefirstnamegt
• ltbrgt
• Last Name ltinput typetext size20
  namelastnamegt
• ltbrgt
• ltinput typesubmitgt
• lt/formgt

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Example.

• In a browser, this would look like
• When Submit Query is clicked we have the
  output
• First Name Qusay
• Last Name Mahmoud

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Example

• The servlet
• public class RequestParams extends HttpServlet
• public doPost(HttpServletRequest re,
  HttpServletResponse response)
• PrintWriter out response.getWriter()
• out.println("lthtmlgtltbodygtltheadgtlttitlegttest
  lt/titlegtlt/head")
• out.println("ltbodygt")
• String firstName req.getParameter("first
  name")
• String lastName req.getParameter("lastna
  me")
• out.println("First Name " firstName
  "ltbrgt")
• out.println("Last Name " lastName)
• out.println("lt/bodygtlt/htmlgt")

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Servlets for WAP

• It is possible to use Servlets to generate WML
• PrintWriter out response.getWriter()
• out.println("lt?xml version\"1.0\"?gt")
• out.println("lt!DOCTYPE wml etc")
• out.println("ltwmlgt")
• out.println("ltcard title\"MobileDate\"gt")
• out.println(" ltp align\"center\"gt")
• out.println("Date and Time Serviceltbr/gt")
• out.println("Date is " new java.util.Date())
• etc      

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What else?

• Programming cookies and keeping track of sessions
  is easy with Servlets.APIs provided for this
• Explore Cookies and Session Management on your
  own!
• Something to think about handheld devices do not
  support cookies, so how do you keep track of
  sessions??

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JSP

• Server-side technology
• Enables you to embed Java code within an HTML
  document
• JSP documents have the extension .jsp
• When an HTTP request is received, the compilation
  engine converts the JSP document into a Java
  Servlet then the servlet will be loaded
• Java code is embedded between lt and gt

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Example

• // file hello.jsp
• lthtmlgtltheadgtlttitlegtexamplelt/titlegtlt/headgt
• ltbodygt
• lt String visitor request.getParameter(user)
• if (visitor null) visitor there
• gt
• Hello, lt visitor gt!
• lt/bodygt
• lt/htmlgt

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Example

• Request parameters are passed into JSP pages
  using normal HTTP parameter mechanisms (using GET
  and POST)
• Therefore, for the hello.jsp
• If invoked with http//host/hello.jsp it will
  print Hello, World!
• If invoked with http//host/hello.jsp?userMary,
  it will print Hello, Mary!

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JSP for WAP

• lt?xml versiongt
• lt!DOCTYPEetcgt
• lt response.setContentTyp("text/vnd.wap.wml")
• gt
• ltwmlgt ltcard title"MobileDate"
• Date and Time Serviceltbr/gt
• Date is lt new java.util.Date() gt
• lt/cardgt
• lt/wmlgt
• Or you can use the PAGE directive

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What else?

• Additional characters may appear after the
  initial lt, such as !, , or _at_ to further
  prescribe the meaning of the tag
• lt! Double radius 3.5 gt
• lt 2 Math.PI radius gt
• lt_at_ include filenotice.html gt
• It is possible to create re-usable components
  with JSP using Java Beans
• Explore on your own!

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HTTP Servers

• If you dont have access to an HTTP server, Id
  recommend that you install Jakarta-Tomcat for
  your personal use .so you explore Servlets and
  JSP further.
• http//jakarta.apache.org/tomcat