Servlets

1
Servlets
2
Web Containers and server side technologies

• A container is a runtime to manage application
  components developed according to the API
  specifications, and to provide access to the J2EE
  APIs
• The container which handles web component
  technologies i.e. servlets and JSPs (Java Server
  Pages) is tomcat
• Tomcat is open source and is available from the
  Jakarta site http//jakarta.apache.org/tomcat/
• Servlets and JSPs running under tomcat are
  server side technologies which accept requests
  from an http form and return responses in the
  form of an HTML ,XHTML or XML document
• Servlets and JSPs are equivalent You could
  write an application just using servlets or just
  using JSPs or you could mix them up, well have
  a look later what is the best policy as to when
  to use servlets or JSPs

3
Servlet interface

• Architecturally all servlets must implement the
  servlet interface
• The methods of the Servlet interface are invoked
  automatically by the servlet container (usually
  tomcat)
• The interface has five methods
• void init(ServletConfig config)
• This method is called once during the servlets
  execution cycle to initialise the servlet (the
  argument is supplied by the container)
• ServletConfig getServletConfig()
• Returns an object reference to an object that
  implements interface ServletConfig (provides
  access to servlets configuration information as
  well as information about its environment (the
  container)
• String getServiceInfo()
• Returns information such as servlets author and
  version
• void service(ServletRequest request,
  ServletResponse response)
• This method is called as a result of a client
  request
• void destroy()
• Clean up method called when the servlet is
  destroyed by its container

4
Servlets lifecycle - UML state diagram
Instantiation based on a request or at conatainer
start up
Does not exist
Instantiated
Initialisation
Initialisation failed
Initialised and/or Ready for requests
Unavailable
Back to service In case of temporary
unavailability
End of Service thread
HTTP request(s) From client(s)
Temporary or permanent failure
Service
Destroyed
Time out or container shutdown
5
HttpServlet class

• Web-based servlets typically extend the
  HttpServlet class
• Provides an abstract class to be subclassed to
  create an HTTP servlet suitable for a Web site. A
  subclass of HttpServlet must override at least
  one method, usually one of these
• doGet, if the servlet supports HTTP GET requests
• doPost, for HTTP POST requests
• doPut, for HTTP PUT requests
• doDelete, for HTTP DELETE requests
• init and destroy, to manage resources that are
  held for the life of the servlet
• getServletInfo, which the servlet uses to provide
  information about itself
• There's almost no reason to override the service
  method. service handles standard HTTP requests by
  dispatching them to the handler methods for each
  HTTP request type (the doXXX methods listed
  above).

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doGet and doPost methods

• doGet(HttpServletRequest req, HttpServletResponse
  resp)
• used to retrieve the contents of a specified URL
  (an HTML or XHTML or XML page)
• The content can be generated dynamically from the
  servlet
• Can be typed into a browsers address or Location
  field and can be bookmarked
• doPost(HttpServletRequest req, HttpServletResponse
  resp)
• Used to post data from an HTML form to a server
  side form handler that processes the data (saved
  it in a data base or retreives data from the
  database.)
• Cannot be typed into a browsers address or
  Location field and cannot be bookmarked
• doPost() method does not have a restriction on
  amount of data received.
• Safer to use doPost method!
• Except if youre using a servlet to display
  information without an associated html page the
  use doGet method

7
Simple example using doGet method

• We need an HTML client
• A servlet with a doGet method
• A web.xml which relates the name of the servlet
  to the java class
• Usually this is generated for you by the ide
  (JBuilder in this case)

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The web client
lthtmlgt ltheadgt lttitlegt HelloServlet lt/titlegt lt/head
gt ltbodygt ltform action"/HelloWorld/helloservlet"
method"get"gt ltpgtpress Submit to invoke servlet
HelloServletlt/pgt ltpgtltinput type"submit"
name"Submit" value"Submit"gtlt/pgt lt/formgt lt/bodygt
lt/htmlgt

• Note the form tags
• The action attribute specifies that the servlets
  name is referred to as helloservlet and is
  running in a folder HelloWorld in tomcat

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web.xml
ltweb-appgt ltdisplay-namegtHelloWorldlt/display-name
gt ltservletgt ltservlet-namegthelloservletlt/serv
let-namegt ltservlet-classgtepda.HelloServletlt/se
rvlet-classgt lt/servletgt ltservlet-mappinggt
ltservlet-namegthelloservletlt/servlet-namegt
lturl-patterngt/helloservletlt/url-patterngt
lt/servlet-mappinggt lt/web-appgt

• Note for each servlet you need two tags
• ltservletgt
• ltservlet-mappinggt

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HelloServlet.java
package epda import javax.servlet. import
javax.servlet.http. import java.io. public
class HelloServlet extends HttpServlet
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException
response.setContentType(text/html)
PrintWriter out response.getWriter()
out.println("lthtmlgt") out.println("ltheadgtlttit
legtHelloServletlt/titlegtlt/headgt")
out.println("ltbody bgcolor\"lightblue\"gt")
out.println("ltpgtHello World.lt/pgt")
out.println("lt/bodygtlt/htmlgt")
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Running in Tomcat

• You need to type the following URL in a web
  browser
• http//localhost8080/HelloWorld/helloservlet.html
  
• When you hit the submit button Hello World is
  displayed and the URL becomes
• http//localhost8080/HelloWorld/helloservlet?Subm
  itSubmit
• If you changed the doGet mrthod to a doPost
  method and submit the form the URL becomes
• http//localhost8080/HelloWorld/helloservlet
• You cant see the input parameters
• If you bookmark it then you cant retrieve the URL

12
Adding in request parameters

• If you wanted to add in the name of the user in
  the html page and display it in the servlets, we
  would add in
• UserIDltinput type "Text" name "requsername"
  gt
• in the html file
• and
• out.println("ltpgtfrom"request.getParameter("requs
  ername"))
• in the servlet
• Sometimes you may want to hide the request
  parameter from the user (visible when you choose
  view source from within the browser)
• You can use a hidden parameter
• ltinput type "hidden" name "action" value
  "entryform"gt
• This will be hidden from the user, you can use it
  in the servlet by
• String action request.getParameter("action")

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Initialising Servlets

• Common in real-life servlets
• E.g. initialising database connection pools
• Use SevletConfig.getInitParameters to read
  intialisation parameters
• Call getServletconfig to obtain the ServletConfig
  object
• Set init parameters in web.xml
• It is common to use init method even when you
  dont read init parameters
• E.g. to set up data structures that dont change
  during the life cycle of the servlet, to load
  information from disk e.t.c.

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Example of Initialising parameters
ltweb-appgt ltdisplay-namegtHelloWorldlt/display-name
gt ltservletgt ltinit-paramgtltparam-namegtdefaultuser
namelt/param-namegt ltparam-valuegtHarrylt/param-valu
egt lt/init-paramgt ltservlet-namegthelloservletlt/
servlet-namegt ltservlet-classgtepda.HelloServlet
lt/servlet-classgt lt/servletgt
ltservlet-mappinggt ltservlet-namegthelloservletlt/
servlet-namegt lturl-patterngt/helloservletlt/url-
patterngt lt/servlet-mappinggt lt/web-appgt
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Example of Initialising parameters
public void doPost(HttpServletRequest
request,HttpServletResponse response) throws
ServletException, IOException
response.setContentType("text/html")
PrintWriter out response.getWriter()
ServletConfig config getServletConfig()
out.println("lthtmlgt") out.println("ltheadgtlttit
legtHelloServletlt/titlegtlt/headgt")
out.println("ltbody bgcolor\"lightblue\"gt")
out.println("ltpgtHello World.lt/pgt")
out.println("ltpgtfrom "request.getParameter("requs
ername")) out.println("ltpgtand from
"config.getInitParameter("defaultusername")
"the default user") out.println("lt/bodygtlt/htm
lgt")
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Request Headers

• Every Request has headers associated with it e.g.
  whether the request is a post or get request, the
  protocol associated with the request e.t.c. The
  request object has methods associated to display
  these headers . Below is a servlet that prints
  these headers out for a particular request
• (from corewebprogramming.com)

public class ShowRequestHeaders extends
HttpServlet public void doGet(HttpServletReque
st request, HttpServletResponse response) throws
ServletException, IOException
response.setContentType("text/html")
PrintWriter out response.getWriter()
String title "Servlet Example Showing Request
Headers"
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Request Headers
out.println( "ltheadgtlttitlegtServlet
ExampleShowing Request Headerslt/titlegtlt/headgt"
"ltbody bgcolor\"B0C4DE \"gt\n"
"lth1 aligncentergt" title "lt/h1gt\n"
"ltBgtRequest Method lt/Bgt"
request.getMethod() "ltbrgt\n"
"ltBgtRequest URI lt/Bgt"
request.getRequestURI() "ltbrgt\n"
"ltBgtRequest Protocol lt/Bgt"
request.getProtocol() "ltBRgtltBRgt\n"
"lttable border1 aligncentergt\n" "lttr
bgcolor6495ed\"gt\n" "ltthgtHeader
NameltthgtHeader Value") Enumeration
headerNames request.getHeaderNames() while
(headerNames.hasMoreElements()) String
headerName (String) headerNames.nextElement()
out.println("lttrgtlttdgt" headerName)
out.println(" lttdgt" request.getHeader(header
Name)) out.println("lt/tablegt\nlt/bodygtlt/
htmlgt")
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Request Headers
19
Statelessness and Sessions

• HTTP is a stateless protocol
• Different requests from the client are treated
  completely differently with no relationship
  between the different requests
• A protocol is stateful if the response to a given
  request may depend not only on the current
  request but also on t he outcome of previous
  requests
• Stateful is important
• On line banking you only need to register once
  each request e.g. looking at your balances,
  transferring money from one account to another
  doesnt require you to re-register every time
• On line shopping with a shopping cart.
• Session
• The server should be able to identify that a
  series of requests from a single client form a
  single working session
• State
• The server should be able to remember information
  related to previous requests in a single session

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Session Tracking with the Java Servlet API

• The javax.servlet.http.HttpSession interface
  encapsulates the notion of a session
• Provides the getSession() method which you can
  use to access the HttpSession object associated
  with the client making the request
• The HttpSession object is an implicit object and
  is created for each client
• The web container maintains this object for the
  duration of the client session
• Each session consume memory on the server side,
  so its unwise to keep sessions open forever
• Most e-commerce sites limit this interval to
  under 30 mins
• You can specify the time in an a deployment
  desriptor element ltsession-configgt (the web.xml
  file)

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Methods for Managing State

• setAttribute() method
• public setAttribute(String name, Object
  attribute)
•   Binds an object to this session, using the name
  specified
• getAttribute() method
• public Object getAttribute(String name)
• This method returns the attribute bound with the
  specified name in this session, or null if no
  object is bound under the name
• getAttributeNames() method
• public Enumeration getAttributeNames(String name)
• Returns an Enumeration of String objects
  containing the names of all the objects bound to
  this session.
• removeAttribute() method
• public void removeAttribute(String name)
• Removes the object bound with the specified name
  from this session.

22
Using the state management methods

• String un request.getParameter("username")
• request.getSession().setAttribute("user", un)
• String un(String)request.getSession().getAttribut
  e("user")

23
Request Dispatching

• Allows a servlet or a JSP page to dispatch a
  request to another servlet or JSP page or even an
  HTML page which will then be responsible for any
  further processing and for generating the
  response.
• Has an interface called javax.servlet.RequestDispa
  tcher for this purpose
• Has two methods which allow delegation of the
  request-response processing to another resource
  after the calling servlet has finished any
  preliminary processing
• public void forward(ServletRequest req,
  ServletResponse resp) throws ServletException,
  java.io.IOException)
• Allows the request to be forwarded to another
  servlet or JSP page which then takes over the
  responsibility of producing the response
• public void include(ServletRequest req,
  ServletResponse resp) throws ServletException,
  java.io.IOException)
• Allows the inclusion of the content produced by
  another ersource in the calling sevlets response

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Examples of Request Dispatching
RequestDispatcher rd request.getRequestDispatche
r("oracleconnection") rd.include(request,
response) if ( ( (String) request.getSession().ge
tAttribute("status")). equalsIgnoreCase("O
K")) response.sendRedirect("controlservlet.htm
l") else rd request.getRequestDispatcher(
"viewerror") rd.forward(request, response)
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Examples of Request Dispatching

• oracleconnection is the name of a servlet
  OracleConnection.java
• Specified in web.xml
• ltservletgt
• ltservlet-namegtoracleconnectionlt/servlet-namegt
• ltservlet-classgtia.OracleConnectionlt/servlet-cl
  assgt
• lt/servletgt
• ltservlet-mappinggt
• ltservlet-namegtoracleconnectionlt/servlet-namegt
• lturl-patterngt/oracleconnectionlt/url-patterngt
• lt/servlet-mappinggt
• Status is the name of an attribute that
  OracleConnection set to have the value OK or
  fail
• public void sendRedirect(java.lang.String location
  ) throws java.io.IOException
• Sends a temporary redirect response to the client
  using the specified redirect location URL. This
  method can accept relative URLs the servlet
  container must convert the relative URL to an
  absolute URL before sending the response to the
  client. If the location is relative without a
  leading '/' the container interprets it as
  relative to the current request URI. If the
  location is relative with a leading '/' the
  container interprets it as relative to the
  servlet container root. (source java.sun.com)
• viewerror is the name of a Servlet specified in
  web.xml.