Java Servlets

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Java Servlets
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Java Servlets

• SUNs answer to CGI
• Run within a Java web server
• We configure Apache to talk to Tomcat
• Steps
• Create instance of your class and load
• Launch thread to call specific methods
• One instance of class, many threads

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My First 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 PrintWriter
out response.getWriter() out.println(Hello
World)
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Notes

• Default return mime type is text/plain
• Can also implement doPost
• Same arguments
• What if you want to answer to both?
• Running the servlet
• Compile
• You will need the java 2 enterprise edition jar
  in your classpath
• Put class into WEB-INF/classes
• Invoke ? http//host/context/servlet/HelloWorld

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Another Example
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.setCon
tentType(text/html) PrintWriter out
response.getWriter() out.println(ltHTMLgt\n
ltHEADgtlttitlegtHello WWWlt/titlegtlt/HEADgt\n
ltbodygt\n lth1gtHello
World!lt/h1gt\n lt/bodygtlt/htmlgt)
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The HttpServletRequest Object

• String getParameter(String key)
• String getParameterValues(String key)
• Enumeration getParameterNames()

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The HttpServletResponse Object

• void setContentType(String type)
• PrintWriter getWriter()
• ServletOutputStream getOutputStream()
• Used to send binary data back to client
• void setHeader(String name, String value)
• Other methods as well

8
Performance Issues

• Strings are immutable
• New string object is created for each string
• Also for each concatenation of 2 strings
• ? a lot of object creation and cleanup
• Use a single StringBuffer object
• Use append() method to add text to it
• Use flush method to push output to client

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Organizational Issues

• WEB-INF/classes can get crowded
• Use java packages to clean things up
• package mypackage //place at top of file
• Put classes in WEB-INF/classes/mypackage
• Invoke http//host/context/servlet/mypackage.Hello
  World

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Headers and CGI variables

• Request object has methods to access
• String value getHeader(String name)
• Others
• PrintEnv code
• http//clotho/snell/TestApps/TestGetPostServlet.h
  tml

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Cookies

• Use the Cookie class
• Constructor
• Cookie(String name, String value)
• Cookie Methods
• String getDomain() or void setDomain(String dom)
• String getName() or void setName(String name)
• String getValue() or void setValue(String value)
• String getPath() or void setPath(String path)
• boolean getSecure() or void setSecure(boolean
  flag)
• int getMaxAge() or void setMaxAge(int seconds)

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Cookies

• Create a cookie
• Construct it
• Set values
• Add the cookie to the response before content
  type
• response.addCookie(Cookie theCookie)
• Get Cookies from request
• Cookie cookies request.getCookies()

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Sessions

• High level API
• HttpSession object
• Built on top of cookies or URL-rewriting
• You dont have to worry which one
• Convenient place to store information
• Arbitrary objects
• Associated with each session

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The Session API

• Get the HttpSession object
• HttpSession session request.getSession(true)
• true automatically creates one if one doesnt
  exist
• Get and Set information using HttpSession methods
• Object getAttribute(String name)
• void setAttribute(String name, Object value)
• void removeAttribute(String name)
• String getAttributeNames()
• String getID()
• boolean getId()
• long getCreationTime() and long
  getLastAccessedTime()
• int getMaxInactiveInterval() and void
  setMaxInactiveInterval(int sec)
• void invalidate()

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Example Toy Shop

• http//clotho/snell/TestApps/ToyShop.html

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The Servlet Life Cycle

• Initialization
• void init() or
• void init(ServletConfig config)
• Be sure to call super.init(config) on first line
• doXxx methods are called (doGet, doPut, etc.)
• Consider if you need single thread
• public class YourServlet extends HttpServlet
  implements SingleThreadModel
• Destruction
• void destroy()
• dont rely on this. The server could crash