Java Training Introduction to Servlets

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Java TrainingIntroduction to Servlets

• Written by Jeff Smith

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What is a Servlet? -1

• Servlets are server side Java classes that add
  functionality to a web server in a manner similar
  to the way applets add functionality to a
  browser.
• Servlets can be used as a replacement for CGI
  scripts (or ASP scripts) and they support the
  standard request/response protocol supported by
  web servers.
• In this request/response protocol, a client sends
  a request message to a server and the server
  responds by sending back a reply message
• Reply is usually as HTML
• Could be image, audio stream, video stream, etc.

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What is a Servlet? -2

• Servlets generally implement either the Servlet
  interface or extend the HttpServlet abstract
  class
• Most programmers extend HttpServlet class
• More on this class later
• Servlets run in a servlet container (Tomcat is a
  popular one)
• Servlets are platform independent
• Your servlets will work on an Apache web server
  running on Linux or a IIS web server running on
  Windows XP
• Servlets are generally invoked from a hyperlink
  or from the action attribute of an HTML form

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What is a Servlet? -3
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What is a Servlet? -4
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Web/Servlet Architecture
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Servlets -1

• Servlets can run on the web server platform as
  part of the same process as the web server
  itself.
• Alternatively, the servlet container can also run
  on a different machine
• Tomcat is a popular (and free) servlet and JSP
  container and can be downloaded from the JavaZone
  web page
• Apache Tomcat can also run web services
• You can use Apache Axis (SOAP implementation for
  web services) with Tomcat

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Servlets -2

• The web server communicates with servlets via a
  simple interface javax.servlet.Servlet. This
  interface defines three important methods
•   init() -- invoked on servlet startup, put
  init code hereservice() -- each client request
  invokes this methoddestroy() -- put cleanup code
  here
• The servlet container controls the lifecycle of
  the servlet objects
• initializes, invokes, and destroys each servlet
  instance.
• You can write a servlet by extending the Servlet
  class and implement these three methods

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Servlets -3

• Servlet Lifecycle
• init() -- This method is called when a servlet
  is first invoked by the servlet container. It
  could be used to write code that initializes
  files or connections to databases
• service() -- This method process requests from
  the client (for example, a hyperlink or form
  action). It returns a response (usually HTML)
  back to the client.
• The service() method provides two parameters
  ServletRequest and ServletResponse to provide
  access to the request object and response object
• destroy() -- Called just before servlet is
  destroyed by container to enable programmer to
  clean up any resources (e.g. close a db
  connection)

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Servlet Example -1
import java.io. import javax.servlet. public
class MyServlet implements Servlet private
ServletConfig config public void init
(ServletConfig config) throws
ServletException this.config config
public void destroy() // do nothing
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Servlet Example -2
public ServletConfig getServletConfig()
//optional
//method return(config) public
void service(ServletRequest req,
ServletResponse res)
throws ServletException, IOException
res.setContentType( "text/html" )
PrintWriter out res.getWriter()
out.println( "lthtmlgtMy Servlet!!!lt/htmlgt" )
out.close()
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Useful Servlet Methods -1

• getRealPath() -- method translates a relative or
  virtual path to a new path relative to the
  server's HTML documentation root location. This
  is useful if your servlet needs to access a file,
  for example.
• getServlet() -- Returns a Servlet object of a
  given name. Useful when you want to access the
  services of other servlets.
• log() -- Writes information to a servlet log
  file. The log file name and format are server
  specific.
• Note When extending the HttpServlet class, you
  access these methods via the ServletContext. For
  example
• realPath getServletContext().getRealPath(virtual
  Path)

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HTTP GET vs. POST -1

• HTTP GET sends the contents of the form fields as
  extra data appended to the URL specified in the
  form's ACTION attribute.
• For example, if you had a form with an action
  MyServlet and a single text element called search
  and you typed in "football" and clicked the
  submit button, the resulting URL might look
  likehttp//www.noaa.gov/MyServlet?searchfootba
  ll
• Note that you could bookmark this URL to quickly
  reload it in a browser.
• HTML forms that use do HTTP GETs should not
  contain too much data (1000 chars?) or they might
  overflow the web server

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HTTP GET vs. POST -2

• HTTP POST sends the contents of the form fields
  as a separate data stream to the web server
• HTTP POSTs cannot be bookmarked by the browser
• HTML forms that use do HTTP POSTs can send large
  amounts of data to the web server without any
  problems
• The data that is sent is not visible in the URL
  that appears in the browser
• For example, if you have a password field, you
  would want to do a HTTP POST so the password
  doesn't appear in the URL

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HttpServlet -1

• Instead of implementing the Servlet interface,
  most servlets extend the HttpServlet class
  because it has http support built in, making it
  easier to access the request and response
  objects.
• When you extend HttpServlet, you generally write
  code for one or both of the following methods
• doGet() -- corresponds to HTTP GET
• doPost() -- corresponds to HTTP POST

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HttpServlet -2

• The doGet() and doPost() methods take two
  parameters
• HttpServletRequest -- contains all the
  information about the request made by a web page,
  including any form data
• HttpServletResponse -- object you can use to send
  data (usually html) back to the calling web page

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HttpServlet -3

• doGet() and doPost() methods generally do the
  following
• Read the request data (html form data)
• Process the request (perhaps invoking the code in
  other user-written classes within the servlet's
  package)
• Set response object headers (what type of
  response will be sent--for example, will it be
  HTML or a JPEG image?)
• Write the response data (for example, send HTML
  back to the calling web page)

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HttpServlet Style Suggestions

• It is a good idea to keep your servlet classes as
  lean as possible
• In general, good OOP design would suggest that
  you put your "business" or "application" logic in
  a separate class and have your servlet's doPost()
  or doGet() methods invoke that code
• For example, you don't want to have a doPost()
  method with 300 lines of code in it
• Your servlet can directly return HTML, but in
  most real world (and complex) web apps, your
  servlets would not return HTML via out.println()
  statements.
• Instead, your servlets would return JSP pages
  with all the HTML formatting already in it. These
  JSP pages could be created by a tool like
  Dreamweaver

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HttpServlet Example -1

• This web page prompts the user for the background
  color, title, and message that will be passed to
  a servlet. The servlet will read this data and
  create a new web page using the supplied
  information.

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HttpServlet Example -2

• In this example, the servlet will read the
  information typed into the web form and generate
  a new html document based on the user supplied
  information
• To keep things simple, it will build the html
  string in the Java code and return it (it will
  not use JSP)
• The form action for this web page might look
  like
• ltform action"http//www.noaa.gov/DesignPageServle
  t"
• method"post"gt

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HttpServlet Example -3
import javax.servlet. import
javax.servlet.http. import java.io. import
java.util. public class DesignPageServlet
extends HttpServlet //Initialization public
void init(ServletConfig config)
throws ServletException
super.init(config)
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HttpServlet Example -4
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
String sBKCOLOR request.getParameter("BKCOLOR")
String sTITLE request.getParameter("TITLE")
String sMESSAGE request.getParameter("MESS
AGE") response.setContentType("text/html")
PrintWriter out new PrintWriter
(response.getOutputStream())
out.println("lthtmlgtltheadgtlttitlegt")
out.println(sTITLE "lt/titlegtlt/headgt")
out.println("ltbody bgColor" sBKCOLOR "gt")
out.println("lth1gt" sTITLE "lt/h1gt")
out.println("ltbrgtltbrgt" sMESSAGE)
out.println("lt/bodygtlt/htmlgt") out.close()

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Tomcat Setup -1

• Tomcat is a free servlet container that you can
  download from the web
• Download it from herehttp//tomcat.apache.org/
• After installation, the Tomcat folder will
  contain "Start Tomcat" and "Stop Tomcat"
  shortcuts
• Make sure you have the JAVA_HOME environment
  variable set so Tomcat can find your JDK. For
  example, if your JDK is installed in c\jdk1.5_07
  then
• JAVA_HOMEc\jdk1.5_07
• In Windows, you set an environment variable by
  going to control panel, System, Advanced, and
  click on the Environment Variables button

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Tomcat Setup -2

• By default, once Tomcat has loaded a servlet, it
  does NOT reload it even if you modify and
  recompile the servlet
• You must stop Tomcat and then re-start it in
  order for Tomcat to load the new version of your
  servlet
• This results in better runtime performance since
  Tomcat doesn't need to check for re-loads each
  time a web user submits a request to the servlet
• You can override this behavior and have Tomcat
  reload the servlet each time by changing the
  "reloadable" option in the Admin Tool to "true"

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Tomcat Setup -3

• Once Tomcat is running, you can go to the Tomcat
  home page (on your PC)

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Tomcat Setup -4

• To set up a new web application (or servlet) with
  Tomcat, you need to add a directory under the
  webapps dir and create a subdirectory called
  WEB-INF.
• WEB-INF needs to contain a web.xml file (a
  Tomcat/servlet configuration file)
• Make sure you are consistent in the way you use
  upper case and lower case in your directory names
  (since UNIX web servers will be picky about that
  detail!)
• The easiest thing to do is to make a copy of the
  webdav directory and give it a new name
• After the WEB-INF directory is created, create a
  subdirectory under it called classes
• Your Java package/classes will go under this
  directory
• (See next slide)

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Tomcat Setup -5
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Tomcat Setup (Oracle Tip) -6

• Tomcat looks for libraries that match .jar
• In their wisdom, Oracle's JDBC driver is named
  classes12.zip.
• In order for Tomcat to reliably load the Oracle
  driver, you need to rename it classes12.jar (and
  place it in the Tomcat/common/lib directory)

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Tomcat Setup (mySQL Tip) -6

• External libraries to be used by Tomcat can be
  placed in the Tomcat common/lib directory
• For example, you might put mail.jar or mySQLs
  jar there

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web.xml file -1

• Tomcats web.xml contains servlet configuration
  info
• You can find it here

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web.xml file -2

• web.xml contains servlet configuration info for
  Tomcat
• You need to add a servlet entry and a servlet
  mapping entry for each servlet you want Tomcat to
  run.
• Add your entries after the ltweb-appgt tag inside
  web.xml
• Tipstart with existing web.xml and swap in text
  below
• ltservletgt
• ltservlet-namegtDesignPageServletlt/servlet-namegt
  
• ltservlet-classgtgov.noaa.DesignPageServlet
• lt/servlet-classgt
• lt/servletgt
• ltservlet-mappinggt
• ltservlet-namegtDesignPageServletlt/servlet-namegt
  
• lturl-patterngt/DesignPageServletlt/url-patterngt
• lt/servlet-mappinggt

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web.xml file -3

• Servlet mappings are how Tomcat matches a URL to
  the correct servlet on disk
• They arent required to have the same name
  (although it usually makes sense to keep things
  simple and give them the same name)
• For example, the following URL in your browser
  might invoke the DesignPageServlet on Tomcat
• http//noaa.gov8080/mywebapp/DesignPageServlet
• is invoked with this url-pattern in web.xml
• lturl-patterngt/DesignPageServletlt/url-patterngt

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Tomcat Admin Tool -1

• Tomcat comes with a web based admin tool that you
  can use to configure Tomcat
• You don't have to use the tool--you can edit the
  config files (server.xml) manually if you know
  what you are doing
• Or if you are satisfied with the Tomcat default
  values, you don't need to run the Admin Tool at
  all
• Run the Admin Tool by going to the Windows Start
  Button, Apache Tomcat program group, Tomcat
  Administration
• This will load the admin tool in a web browser
• You will be prompted for the admin login (use the
  values you specified during the install process)

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Tomcat Admin Tool -2
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Tomcat Admin Tool -3
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Servlets, Actions, and Tomcat -1

• When Tomcat is running on your local machine, you
  specify localhost8080 in your URL (e.g. form
  action).
• By default, Tomcat runs on port 8080
• You can change this to port 80 (so you never have
  to specify a port number) by using the admin tool
  or by editing the tomcat-home/conf/server.xml
  file and changing the default port from 8080 to
  just 80
• For example, the following form action would
  refer to a servlet named MyServlet that is
  running in the wms folder on Tomcat running on
  the local machine
• ltform action'http//localhost8080/wms/MyServlet'
  methodGETgt
• MyServlet would reside in /ltTomcatDirgt/webapps/wms
  /WEB-INF/classes

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Servlets, Actions, and Tomcat -2

• You could also invoke your servlet by calling it
  directly from a browser address bar

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Servlet Development Summary

• Step 1 Create your html web form
• Step 2 Create your Java Servlet
• Step 3 Create app directory under
  tomcat/webapps
• Tip--copy webdav directory and give it new name
• Step 4 Modify web.xml file with your Servlet
  mappings
• Step 5 Start or re-start tomcat
• Step 6 Test servlet from your web form,
  remembering to restart Tomcat anytime you change
  your Servlet or your web.xml file
• Alternatively, you may be able to set
  reloadabletrue

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Exercise 1

• Create the "Design your own web page" web page
  (html)
• Write your servlet class (name it
  DesignWebPageServlet)
• Download and install Tomcat (if it has not
  already been installed on your machine)
• Create a folder with your name (e.g. jeff) under
  the tomcat-dir/webapps directory (see earlier
  slide) and edit your web.xml file to configure
  the DesignWebPageServlet
• Deploy your servlet to Tomcat (place your
  package/classes under the classes directory under
  the WEB-INF directory
• Specify your form action to be a HTTP GET to your
  servlet
• Test your web application!
• Extra credit 1 rewrite your web page to do a
  HTTP POST
• Extra credit 2 write an Ant script that builds
  and deploys your servlet

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Exercise 2

• Create a guest book web application that prompts
  the user for a name, an email address, and a
  message.
• When the user submits this information to your
  servlet, the servlet should write the user's
  information to a text file on disk (or database
  table) and then return a web page containing the
  current entries in the guest book
• Extra credit Add an admin feature that allows a
  user to delete a guest book entry if they know
  the admin password
• Extra credit write an Ant script that builds and
  deploys your servlet

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Exercise 2 (continued)
Tip start out with a simple servlet, that when
invoked from the browser simply reads a guest
book text file that you created yourself. It
should read this text file and return it to the
browser (preferably formatted nicely in an HTML
table) Tip the code that reads the guest book
file could be in a class called GuestBook.java