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Servlets CEN 4010 11/28/2018 Copyright Ege Consulting, Inc.
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Servlets overview Servlet Technology Handling requests
Session tracking Handling Cookies JDBC Servlet 11/28/2018 Copyright Ege Consulting, Inc.
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Webserver web site files html files: text, multi-media and links
Common Gateway Interface (CGI): cgi scripts: sh, csh, Perl, … Servlets: Java program that runs on webserver JavaServer pages (JSP): Contains html and Java Compiled into a servlet and run on webserver 11/28/2018 Copyright Ege Consulting, Inc.
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Servlet Supports request/response model Different types of servlets
client send request server responds (with html page) Different types of servlets GenericServlet not protocol specific HttpServlet Serves http requests 11/28/2018 Copyright Ege Consulting, Inc.
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Servlet tasks process html forms middle-tier processing
connect to sources behind firewall (e.g. DB) maintain session information between requests serve as concentration point for multiple clients 11/28/2018 Copyright Ege Consulting, Inc.
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Servlet runs on webserver
requested via URL by client requires special webserver Tomcat, IBM application server, JRun, Oracle … can start with webserver permanent: if startup effort is high can be started on client request temporary: if rarely used servlet is unaware of when it was started 11/28/2018 Copyright Ege Consulting, Inc.
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Basic modes of operation
Multi threaded Single servlet instance runs in many threads Servlet fields shared Must consider field contention Single threaded Multiple instances, each in one thread Servlet fields are not shared 11/28/2018 Copyright Ege Consulting, Inc.
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Servlet interfaces and classes
Java Servlet packages javax.servlet javax.servlet.http Part of J2EE to compile include into CLASSPATH (tomcat)... common\lib\servlet-api.jar 11/28/2018 Copyright Ege Consulting, Inc.
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HttpServlet service method
invoked for “get” and “post” method in html form 11/28/2018 Copyright Ege Consulting, Inc.
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HttpServlet class 11/28/2018 Copyright Ege Consulting, Inc.
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Servlet example: HelloWorld.java
package mypackage; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorld extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println(“<html><head><title>Hello</title></head>”); out.println(“<body>”); out.println("Hello World“); out.println(“</body></html>”); } 11/28/2018 Copyright Ege Consulting, Inc.
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Make the servlet known as web application: web.xml
<display-name>HelloWorld example</display-name> <description>Welcome to HelloWorld</description> <servlet> <servlet-name>HelloWorld</servlet-name> <servlet-class>mypackage.HelloWorld</servlet-class> </servlet> <servlet-mapping> <url-pattern>/hello</url-pattern> </servlet-mapping> </web-app> 11/28/2018 Copyright Ege Consulting, Inc.
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html example: hello.html
<head> <title>Hello World Servlet Example</title> <body> <form action=“ method=“get”> <input type=“submit”> </form> </body> </html> 11/28/2018 Copyright Ege Consulting, Inc.
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Servlet deployment on Tomcat
Create directory ...\webapps\HelloExample Copy hello.html into this directory as index.html Create subdirectory WEB-INF Create subdirectory WEB-INF\classes Compile HelloWorld.java to WEB-INF\classes\mypackage\HelloWorld.class Copy web.xml into directory: WEB-INF Open “ 11/28/2018 Copyright Ege Consulting, Inc.
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Process Form parameters
html form defines fields with name type value(s) received by servlet via request parameter HttpServletRequest class defines helper methods 11/28/2018 Copyright Ege Consulting, Inc.
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Helper methods getParameter(“name”) getParameterValues(“name”)
Returns string (maybe empty) or null if parameter does not exist getParameterValues(“name”) Returns array of strings getParameterNames() Returns enumeration of parameter names 11/28/2018 Copyright Ege Consulting, Inc.
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html example <html> <body> <head>
<title>Request Parameters Example</title> </head> <h3>Request Parameters Example</h3> Please enter <p><form action="params" method=POST> First Name: <input type=text size=20 name=firstname> <br> Last Name: <input type=text size=20 name=lastname> <br> <input type=submit> </form> </body> </html> 11/28/2018 Copyright Ege Consulting, Inc.
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doPost method String first = request.getParameter("firstname");
String last = request.getParameter("lastname"); response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<html><head><title>Answer</title></head>"); out.println("<body>Welcome " + first + " " + last); out.println("</body></html>"); 11/28/2018 Copyright Ege Consulting, Inc.
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Keep track of session http is a stateless protocol
but there is need to maintain session information username, password shopping cart … possible: use hidden field(s) better: HttpSession 11/28/2018 Copyright Ege Consulting, Inc.
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Keep track of session session can remember values for attributes across servlet invocations methods: session = request.getSession(true); session.setAttribute(“name”, object); session.getAttribute(“name”); 11/28/2018 Copyright Ege Consulting, Inc.
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getSession session = request.getSession(true);
retrieves current session from request “true” parameter causes new session to be created if it does not exist session remains alive until: it times out (reaches time maximum) explicit cancellation 11/28/2018 Copyright Ege Consulting, Inc.
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Attributes getAttribute(“name”) setAttribute(“name”, value)
value can be any object removeAttribute(“name”) getAttributeNames() 11/28/2018 Copyright Ege Consulting, Inc.
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HttpSession methods getID() isNew() getCreationTime()
getLastAccessedTime() invalidate() setMaxInactiveInterval() getMaxInactiveInterval() 11/28/2018 Copyright Ege Consulting, Inc.
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SessionServlet: doPost (1/3)
HttpSession session = request.getSession(true); String heading; if (session.isNew()) { heading = "Welcome, Newcomer"; } else { heading = "Hello Again"; } 11/28/2018 Copyright Ege Consulting, Inc.
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SessionServlet: doPost (2/3)
Integer accessCount = (Integer)session.getAttribute("accessCount"); if (accessCount != null) { accessCount = new Integer(accessCount.intValue() + 1); } else { accessCount = new Integer(0); } session.setAttribute("accessCount", accessCount); 11/28/2018 Copyright Ege Consulting, Inc.
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SessionServlet: doPost (3/3)
response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<html><head><title>Answer</title></head>"); out.println("<body> " + heading + "<br>"); out.println("Number of previous accesses " + accessCount ); out.println("</body></html>"); 11/28/2018 Copyright Ege Consulting, Inc.
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Handle Cookies allow to store information in the browser
can be retrieved by servlet 2 types temporary cookies lasts as long as the browser instance permanent cookies must have expiration date can be deleted Note: browsers can disable cookies 11/28/2018 Copyright Ege Consulting, Inc.
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To establish a Cookie create instance of class Cookie
Cookie mine = new Cookie(“name”, “value”); permanent has max age mine.setMaxAge(seconds); can have domain and path mine.setDomain(“.aul.fiu.edu”); mine.setPath(“/catalog”); 11/28/2018 Copyright Ege Consulting, Inc.
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To access Cookies through request parameter find your cookie
Cookie all[] = request.getCookies(); find your cookie for (int i=0; i<all.length; i++) { out.println(“Cookie name: “ + all[i].getName()); out.println(“ value: “ + all[i].getValue()); } 11/28/2018 Copyright Ege Consulting, Inc.
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To save Cookie through response header Example:
response.addCookie(singleCookie); Example: Cookie mine = new Cookie(“user”, “ege”); mine.setMaxAge(60 * 60 * 24 * 365); response.addCookie(mine); 11/28/2018 Copyright Ege Consulting, Inc.
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Access database servlet may access database using JDBC load driver
get connection formulate statement execute statement process result set 11/28/2018 Copyright Ege Consulting, Inc.
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Example: database access
Class.forName("com.sybase.jdbc.SybDriver"); conn = DriverManager.getConnection("jdbc:sybase:Tds:ocelot.aul.fiu.edu:7100", “student", “student"); response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<html><head><title>Customers</title></head><body>"); String s1 = "select * from Customers;"; Statement s = conn.createStatement(); ResultSet result = s.executeQuery(s1); out.println("<table border=1><tr><th>ID<th>name<th>city<th>street</tr>"); while (result.next()) { out.println("<tr><td>" + result.getInt(1)); out.println("<td>" + result.getString(2)); out.println("<td>" + result.getString(3)); out.println("<td>" + result.getString(4) + "</tr>"); } out.println("</table></body></html>"); 11/28/2018 Copyright Ege Consulting, Inc.
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Performance considerations
new connection is created for every request to servlet other choices place connection into session 11/28/2018 Copyright Ege Consulting, Inc.
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