Servlets: Building Your First Servlet

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ServletsBuilding Your First Servlet

• Ethan Cerami
• New York University

2
Road Map

• Generic Template for Creating Servlets
• Servlet 2.2 API
• Hello World Examples
• Outputting Text, HTML, and the current time.
• Compiling your own Servlets
• Instructions for installing/compiling servlets on
  I5
• Packaging Servlets
• HTML Utilities

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Generic Servlet Template
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Servlet Template

• First, lets take a look at a generic servlet
  template.
• The code does not actually do anything, but all
  your future servlets will follow this general
  structure.
• The most important pieces are noted in yellow.

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import java.io. import javax.servlet. import
javax.servlet.http. public class
ServletTemplate extends HttpServlet public
void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
// Use "request" to read incoming HTTP headers
// (e.g. cookies) and HTML form data (e.g. data
the user // entered and submitted).
// Use "response" to specify the HTTP response
status // code and headers (e.g. the content
type, cookies). PrintWriter out
response.getWriter() // Use "out" to send
content to browser
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Generic Template

• Import the Servlet API
• import javax.servlet.
• import javax.servlet.http.
• To create servlets, you must remember to always
  use these two import statements.

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Generic Template

• All your servlets must extend HTTPServlet.
• HTTPServlet represents the base class for
  creating Servlets within the Servlet API.
• The Full Servlet API is available at
• http//www.java.sun.com/products/servlet/2.2/javad
  oc/index.html
• Once you have extended HTTPServlet, you must
  override one or both
• doGet() to capture HTTP Get Requests
• doPost() to capture HTTP Post Requests

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doGet and doPost

• The doGet() and doPost() methods each take two
  parameters
• HTTPServletRequest encapsulates all information
  regarding the browser request.
• Form data, client host name, HTTP request
  headers.
• HTTPServletResponse encapsulate all information
  regarding the servlet response.
• HTTP Return status, outgoing cookies, HTML
  response.
• If you want the same servlet to handle both GET
  and POST, you can have doGet call doPost or vice
  versa.

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Getting an OutputStream

• The HTTPResponse object has a getWriter() method.
• This method returns a java.io.PrintWriter object
  for writing data out to the Web Browser.
• PrintWriter out response.getWriter()

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Hello World!
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Hello World!

• We are finally ready to see our first real
  servlet.
• This servlet outputs Hello World! as plain
  text, not HTML.
• Note All examples are available on the web
  site.
• Lets take a look at the code, and then see the
  servlet in action.

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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 PrintWriter
out response.getWriter()
out.println("Hello World")
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Output Stream

• Once you have an OutputStream object, you just
  call the println() method to output to the
  browser.
• Anything you print will display directly within
  the web browser.
• As we will now see, you can also output any HTML
  tags.

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Generating HTML

• To generate HTML, you need to add two steps
• Tell the browser that you are sending back HTML.
• Modify the println() statements to return valid
  HTML.

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HelloWWW.java
import java.io. import javax.servlet. import
javax.servlet.http. public class HelloWWW
extends HttpServlet public void
doGet(HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException
response.setContentType("text/html") PrintWrite
r out response.getWriter() out.println("ltHTMLgt
\n" "ltHEADgtltTITLEgtHello
WWWlt/TITLEgtlt/HEADgt\n"
"ltBODYgt\n" "ltH1gtHello
WWWlt/H1gt\n" "lt/BODYgtlt/HTMLgt")

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Generating HTML

• To return HTML, you must set the content MIME
  type to text/html
• response.setContentType("text/html")
• Remember that you must set the content type
  before you output any content.
• Once you have set the MIME type, you can return
  any HTML document you want.

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Time Servlet

• Lets try one more simple servlet
• Using the java.util.Date object, you can obtain
  the current time.
• Lets create a simple Servlet that outputs the
  current time.

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import java.io. import java.util. import
javax.servlet. import javax.servlet.http. pub
lic class TimeServlet extends HttpServlet
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException
response.setContentType("text/html") Date
now new Date() PrintWriter out
response.getWriter() out.println("ltHTMLgt\n"
"ltHEADgtltTITLEgtHello
WWWlt/TITLEgtlt/HEADgt\n"
"ltBODYgt\n" "ltH1gtHello
WWWlt/H1gt\n" "ltH2gtTime is now
"now"lt/H2gt" "lt/BODYgtlt/HTMLgt")

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Installing / Compiling Servlets
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Basic Setup

• I5 now has the Jakarta-Tomcat Servlet Engine
  installed.
• Tomcat is an open source servlet engine.
• Supports both Servlets and JSPs.
• Maintained by the same group that develops the
  Apache Web Server.

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Tomcat Set-up
Send request to port 9712
Web Browser
Tomcat Servlet Engine
Hello World Servlet
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Getting Started

• Lets take a look at todays handout.
• Handout is also available online at
• http//ecerami.com/applied_fall_2001/handouts/i5_s
  ervlets.php4
• Change to handout
• CLASSPATH"/usr/local/jakarta-tomcat-3.2.3/lib/ser
  vlet.jar."

Add . to Classpath!
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Packaging Servlets
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What is a Package?

• Package Group of related classes.
• For example
• package coreservlets
• In real web sites, multiple programmers may be
  creating multiple servlets.
• By dividing your code base into packages, it
  helps to modularize the code.
• A very common practice in the real world, and
  used throughout our textbook.

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Creating Packages

• To create your own package, you need to follow
  three steps
• Move your .java files to a subdirectory that
  matches your package name.
• For example, the text book uses the package name
  coreservlets.
• You therefore need to create a coreservlets
  directory within /public_html/tomcat/WEB-INF/clas
  ses and place your code here.

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Creating Packages

• Insert a package statement in the first line of
  your class file
• For example package coreservlets
• Compile your Java code like this
• type servlets
• type javac coreservlets/HelloWWW2.java

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package coreservlets import java.io. import
javax.servlet. import javax.servlet.http. pub
lic class HelloWWW2 extends HttpServlet
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
response.setContentType("text/html")
PrintWriter out response.getWriter()
out.println("ltHTMLgt\n"
"ltHEADgtltTITLEgtHello WWWlt/TITLEgtlt/HEADgt\n"
"ltBODYgt\n" "ltH1gtHello
WWWlt/H1gt\n" "ltH2gtThis is a
servlet within a package.lt/H2gt"
"lt/BODYgtlt/HTMLgt")
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Invoking a Packaged Servlet

• To invoke a packaged servlet, you need to specify
  the package name and the servlet name
• http//host/servlet/packageName.servletName
• For example, to access the HelloWWW2 servlet on
  ecerami.com
• http//ecerami.com/servlet/coreservlets.HelloWWW2
• To access the servlet on I5
• http//i5.nyu.edu9712/eqc3844/servlet/coreservlet
  s.HelloWWW2

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HTML Utilities
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Servlet Utilities

• Author of our text book has created a class
  called ServletUtilities.
• This class contains some simple HTML utilities
  that you can use.
• As we read further in the book, the author adds
  more utilities to this class.
• Lets first examine the ServletUtilities class,
  and then examine how to use it.

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ServletUtilities.java

• For now, let us examine just one method
  headWithTitle().
• This method outputs
• HTML DOCTYPE, used to specify which version of
  HTML we are using.
• The title of the page via the HTML ltTITLEgt tag.

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package coreservlets import javax.servlet. impo
rt javax.servlet.http. / Some simple time
savers. Note that most are static
methods. / public class ServletUtilities
public static final String DOCTYPE
"lt!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 "
"Transitional//EN\"gt" public static
String headWithTitle(String title)
return(DOCTYPE "\n" "ltHTMLgt\n"
"ltHEADgtltTITLEgt" title
"lt/TITLEgtlt/HEADgt\n") ...
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Using Servlet Utilities

• To use the Servlet Utilities class, you just need
  to call the headWithTitle() method
• ServletUtilities.headWithTitle("Hello WWW")
• If you are placing your servlet in a different
  package, you also need to import coreservlets

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package coreservlets import java.io. import
javax.servlet. import javax.servlet.http. pub
lic class HelloWWW3 extends HttpServlet
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException
response.setContentType("text/html")
PrintWriter out response.getWriter()
out.println(ServletUtilities.headWithTitle("Hello
WWW") "ltBODYgt\n"
"ltH1gtHello WWWlt/H1gt\n"
"lt/BODYgtlt/HTMLgt")
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Compiling

• To compile this servlet
• Download ServletUtilities.java and copy to your
  coreservlets directory.
• Download HelloWorldWWW3.java and copy to your
  coreservlets directory.
• Type
• javac coreservlets/HelloWWW3.java
• Then, open browser and go to
• http//i5.nyu.edu9712/NET-ID/servlet/coreservle
  ts.HelloWWW3