Java Servlet Lab Envrion Setting of Java Servlet Servlet Engine searches file from dir “/usr/jt/webapps”. To facilitate access to local files, make the link: ln -s /home/lec/jia/www/java/js /usr/jt/webapps/jia “/usr/jt/webapps/jia /” point to “~jia/www/java/js/” Following the configuration of servlet engine: 1. The servlet engine looks servlet .class file in a dir: /usr/jt/webapps/ e.g., http://jserv.cs.cityu.edu.hk:8080/jia/ servlet/XXX is: ~jia/www/java/js/WEB-INF/classes/XXX “servlet” ≡ “/WEB-INF/classes” 2. All servlet .class files MUST be in dir: ~jia/www/java/js/WEB-INF/classes/ The corresponding URL will be: http://jserv.cs.cityu.edu.hk:8080/jia/servlet/GetServlet 3. All .html files MUST be in dir: ~jia/www/java/js/html/ The corresponding URL will be: http://jserv.cs.cityu.edu.hk:8080/jia/html/test.html 1
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Java Servlet
Lab Envrion Setting of Java Servlet
Servlet Engine searches file from dir “/usr/jt/webapps”.To facilitate access to local files, make the link:ln -s /home/lec/jia/www/java/js /usr/jt/webapps/jia“/usr/jt/webapps/jia/” point to “~jia/www/java/js/”
Following the configuration of servlet engine:1. The servlet engine looks servlet .class file in a dir:
/usr/jt/webapps/e.g., http://jserv.cs.cityu.edu.hk:8080/jia/servlet/XXX is:
• Servlet runs on the web site. To enable servlets, it
needs to install a servlet engine (e.g., Jserv, or Tomcat), which is a module of a web-server and runsinside the web-server process.
• Servlet generates HTML pages, sent to browser for display, a similar technology as ASP.
• Servlet replaces CGI program to link browser-
clients to back-end servers.
• Servlet can support many protocols of request /response type, such as HTTP, URL, FTP, SMTP.That’s, any requests coming in the form of these
protocols can be processed by a servlet (Servlet APIdefines the support of these protocols).
• The most common protocol that Servlet supports isHTTP.
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Java Servlet
Servlet Life Cycle
Servlets run on the web server platform as part of the
web-server process. The web server initializes, invokesand destroys servlet instances. It invokes servlets via Servlet interface, consisting of three methods:
• init(). It is called only once, when the servlet is firstloaded. It is guaranteed to finish before any other calls are made to the servlet.
• service(). Each request from a client results in asingle call to this method. It receives (from the webserver) a ServletRequest object & a ServletResponseobject to get request / send reply to clients.
• destroy(). It is called to allow your servlet to cleanup any resources. Most of the time, it can be an
empty method.
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Java Servlet
A Simple Servlet
N.B. run by http://jserv...hk:8080/jia/servlet/GenericServlet
import javax.servlet.*;public class GenericServlet implements Servlet {
private ServletConfig config;
public void init (ServletConfig config)throws ServletException {this.config = config;
}
public void destroy() {} // do nothingpublic ServletConfig getServletConfig() {
return config;}public String getServletInfo() {
return “A Simple Servlet”; }
public void service (ServletRequest req,ServletResponse res) throws ServletException, IOException {
Most of programs extend class HttpServlet , andimplement methods doPost () and doGet ().
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Java Servlet
HttpServletRequest / HttpServletResponse
• Interfaces HttpServletRequest and
HttpServletResponse extend interfaces ServletRequest and ServletResponse.
• The methods of HttpServlet receives two parameters: a HttpServletRequest object, whichcontains the request from the client, and a
HttpServletResponse object, which contains theresponse to the client.
• When a client makes a HttpServlet call, the webserver creates a HttpServletRequest object and a HttpServletResponse object, and passes them to theservlet’s service() method, which passes them further
to doGet () or doPost () methods.
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Java Servlet
HttpServletRequest
Some important methods of HttpServletRequest :
String getParameter (String name)returns the value of the named parameter
Enumeration getParameterNames()returns names of all the parameters sent to servlet
String[] getParameterValues( String name)returns an array of values of the named parameter
Cookie[] getCookies()
HttpSession getSession( Boolean create)
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Java Servlet
HttpServletResponse
Some important methods of HttpServletResponse :
void setContentType (String type)specifies the MIME type of the response to the
browser, e.g., “text/html”.
PrintWriter getWriter()
obtains a character-based output stream.
ServletOutputStream getOutputStream()obtains a byte-based output stream.
void addCookie ( Cookie cookie)
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Java Servlet
HTML file for doGet ServletN.B. URL: http://jserv.cs.cityu.edu.hk:8080/jia/html/GetServlet.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><HTML><!-- Fig. 29.6: HTTPGetServlet.html -->
<HEAD><TITLE>Servlet HTTP GET Example</TITLE></HEAD>
•Servlet can be used with Applet as a replacement of CGI programs. Applet starts a servlet by POST (or GET), in the same way as starting a CGI program.
•Applet passes parameters to servlet in the sameformat as HTML form, e.g., “name=X+Jia&addr=CityU
&ID=5012344”, so servlet can use getParameter to getthe parameter value.
• The Applet processes the results from the servlet inthe same way as from a CGI program.
• HTTP server must be in version 1.1 or above, i.e.,it uses protocol:
POST /jia/servlet/CoffeeServlet HTTP/1.1
See a sample in java/servlet/CoffeeApplet.java
AppletHTTPserver
Servlet DBMSsystem
JDBC
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Java Servlet
An Applet connecting to a Servlet
// source code: java/js/WEB-INF/classes/CoffeeApplet.java// move CoffeeApplet.class and test.html to /js/html
// URL: http://jserv.cs.cityu.edu.hk:8080/jia /html/test.html public class CoffeeApplet extends Applet implements Runnable {
public synchronized void start() {if (worker == null) {
Thread worker = new Thread(this);worker.start(); }
}
public void run() {Socket s; String sdata = “type=select_COFFEES”;DataInputStream in; PrintStream out;s = new Socket("jserv.cs.cityu.edu.hk",8080);in = new DataInputStream(s.getInputStream());out = new PrintStream(s.getOutputStream());
String line = in.readLine();while (! line.equals("START_DATA"))
line = in.readLine();while (! line.equals("END")) {
results.addElement(line);line = in.readLine(); }
out.close(); in.close();}
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Java Servlet
The Servlet Paired with Applet in JDBC// code: java/js/WEB-INF/classes/CoffeeServlet.java
• The servlet working with an applet is similar to theCGI program in the 3-tier structure of JDBC.
•The servlet gets request from applet via“getParameter” method on param “request” (see thesample program for details). Note that the string
posted from applet must have the parameter in theright format!
• The servlet sends reply back to applet via “println”method on param “response”, which will be read bythe applet. Note that since applet cannot processHTML statements easily, the servlet needs to prepare
replies to the applet in a sequence of string lines(same as the CGI prog), one for each row of data.
• The applet working with the servlet here is almostthe same as the applet in the 3-tier structure of JDBC.
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Java Servlet
Servlet paired with the Applet
public class CoffeeServlet extends HttpServlet {
public void doPost( HttpServletRequest request,HttpServletResponse response )
throws ServletException, IOException {try {
response.setContentType("text/html");PrintWriter out = response.getWriter();
// make JDBC connection and executeQuery// process ResultSet to a line of string "results"return(results.toString());
}}
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Java Servlet
Servlet Cookies
HTTP server is stateless, i.e., it does not keep any infoabout the clients. Some applications require that theresponse to a client’s request depends on the data sentfrom the client earlier. Cookie is for this purpose.
• Cookies are extracted from the client’s data sent tothe servlet. They are sent back to the client to store at
the client side.
• A cookie is associated with an age. When the ageexpires, it will be removed.
• The cookies are included in a header of the requestand sent to the servlet together with a request. The
servlet can analyse the cookies and reponse to therequest accordingly.
• HTTPSession is a similar technology as the Cookiefor the same purpose.
Browser Servlet
1. generate cookiesfrom client-requst
2. send cookies to browser
for storage
3. include cookies inthe next client-request
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Java Servlet
Methods on Cookies
Cookies are strings, defined in
javax.servlet.http.Cookie.
Construct a Cookie:Cookie cookie = new Cookie( name, value);
Set an age for a cookie:cookie. setMaxAge (seconds);
Add a cookie to a response to the client:response.addCookie (cookie);
Get cookies from a client’s request:Cookie cookies[];
private final static String isbn[] = { "0-13-226119-7","0-13-5289-6", "0-13-0125-5", "0-13-45695-5"};
public void doPost( HttpServletRequest request,HttpServletResponse response )
throws ServletException, IOException{
String language = request.getParameter( "lang" );
Cookie c = new Cookie( language, getISBN( language ) );c.setMaxAge( 120 ); // seconds until cookie removedresponse.addCookie( c ); // must precede getWriter
// send HTML page to clientoutput.println( "<HTML><HEAD><TITLE>" );output.println( "Cookies" );output.println( "</TITLE></HEAD><BODY>" );output.println( "<P>Welcome to Cookies!<BR>" );output.println( "<P>"+lang+" is a great language." );output.println( "</BODY></HTML>" );
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