li tak sing comps311f. static attributes in servlets since servlets are also java classes, you can...
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Li Tak Sing
COMPS311F
Static attributes in ServletsSince Servlets are also Java classes, you
can also use static attributes to store values that can be shared by all servlets.
For example, if you want a single database connect to be used by all servlets, you can use a static attribute to store the connection and it would be accessible by all servlets.
On the other hand, a normal attribute in a servlet can only be used by the servlet only.
CountVisitThis is to count the visitor to a site, not a
page.You can try the servlet at:
http://plbpc001.ouhk.edu.hk/tma/CountVisithttp://plbpc001.ouhk.edu.hk/tma/CountVisit2
The source is at:http://plbpc001.ouhk.edu.hk/~mt311f/2010-
oct/CountVisit.javahttp://plbpc001.ouhk.edu.hk/~mt311f/2010-
oct/CountVisit2.java
CountVisit.javapublic class CountVisit extends HttpServlet { public static int count=0; protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException { try { PrintWriter out=response.getWriter(); out.println("You have visit this site for "+count+"
times."); count++; } catch (Exception e) { e.printStackTrace(); } }
CountVisit2public class CountVisit2 extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException { try { PrintWriter out=response.getWriter(); out.println("You have visit this site for
"+CountVisit.count+" times."); CountVisit.count++; } catch (Exception e) { e.printStackTrace(); } }
JSPJava Server PageJSP is an alternative way to handle HTTP
requests. Contrary to Java Servlets, the basic structure of a JSP is an HTML file.
Java code is embedded in the HTML code specified by <% and %>
A simple JSP<%@page contentType="text/html"
pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD
HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html> <head> <meta http-equiv="Content-Type"
content="text/html; charset=UTF-8">
A simple JSP <title>Hello world</title> </head> <body> <H2>Hello World</H2> Time is: <% out.println(new
java.util.Date().toString());%> </body></html>
The output You can try the JSP page at:
http://plbpc001.ouhk.edu.hk/tma/jsp/hello.jspThe source is at:
http://plbpc001.ouhk.edu.hk/~mt311f/2010-sep/web/webapplicaiton/web/hello.jsp
In order to view the code, you need to right click on the browser and select view soruce.
A simple JSPIn the JSP page, Java code should be placed between <%
%>.Every JSP page has several predefined variables:
request. This variable specifies the data included in an http request. This variable takes value from the clients' browser to pass it over to the server.
response. This variable specifies the data included in the http response. It is used with cookies and also in http headers.
out. This variable specifies the output stream otherwise known as printwriter in a page context.
session. This variable specifies the data associated with httpsession object with a specific session of a user. The main purpose of this object is to use the session information to maintain multiple page requests.
Methods of JSP variables.request
Cookie[] getCookies(): get all the cookies for this request.
String getParameter(String name): get the value of the named parameter.
outThis is a PrintWriter. Therefore, you can use println() etc
to write to the final page.session
Object getAttribute(String name). Get a named attributevoid setAttribute(String name, Object obj). Set a named
attributevoid setMaxInactiveInterval(int i). Set the maximum
inactive interval before the end of the session.
Methods of JSP variables.response
void addCookie(Cookie c). Add a cookie.
<%= %> In the last example, we use the following
code write the current time:Time is: <% out.println(new
java.util.Date().toString()); %>This can be shorten as:Time is: <%= new java.util.Date().toString()
%>
So <%= xxx %> is equilvalent to <% out.println(xxx); %>
The count exampleYou can try the JSP at:
http://plbpc001.ouhk.edu.hk/tma/jsp/count.jsp
You can download the source at:http://plbpc001.ouhk.edu.hk/~mt311f/2010-
sep/web/webapplicaiton/web/count.jsp
count.jsp<%@page contentType="text/html"
pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML
4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html> <head> <meta http-equiv="Content-Type"
content="text/html; charset=UTF-8"> <title>Count</title> </head> <body>
count.jsp <H2>Count</H2> <% String count = null; Cookie cookie[] =
request.getCookies(); if (cookie != null) { for (Cookie c : cookie) { if (c.getName().equals("count")) { count = c.getValue(); } } }
count.jsp if (count == null) { out.println("You are new here."); count = "1"; } else { out.println("You have visited this
site for " + count + " times."); count =
Integer.toString(Integer.parseInt(count) + 1); } response.addCookie(new
Cookie("count", count)); %> </body></html>
expire.jspYou can try the JSP page at
http://plbpc001.ouhk.edu.hk/tma/jsp/expire.jsp
You can download the source at:http://plbpc001.ouhk.edu.hk/~mt311f/2010-
sep/web/webapplicaiton/web/expire.jsp
expire.jsp<%@page contentType="text/html"
pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML
4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><% if (request.getParameter("create") !=
null) { Cookie c = new Cookie("expire", "10"); c.setMaxAge(10); response.addCookie(c); }
expire.jsp Cookie cookie[] = request.getCookies(); int fontsize = 3; if (cookie != null) { for (Cookie c : cookie) { System.out.println("cookie:" +
c.getName() + ":" + c.getValue() + "<br>"); if (c.getName().equals("expire")) { fontsize = 10; } } }%>
expire.jsp<html> <head> <meta http-equiv="Content-Type"
content="text/html; charset=UTF-8"> <title>JSP Page</title> </head> <body> <font size='<%=fontsize%>'> The font size is <%=fontsize%>. </font>
expire.jsp <form> <input type='submit' name='create'
value='create a cookie'/> </form> Click <a href='expire.jsp'>this</a> to
revisit this page
</body></html>
SessionCountYou can try this at
http://plbpc001.ouhk.edu.hk/tma/jsp/sessioncount.jsp
The source can be downloaded at:http://plbpc001.ouhk.edu.hk/~mt311f/2010-
sep/web/webapplicaiton/web/sessioncount.jsp
sessioncount.jsp<%@page contentType="text/html"
pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML
4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html> <head> <meta http-equiv="Content-Type"
content="text/html; charset=UTF-8"> <title>Session Count</title> </head>
sessioncount.jsp <body>
<h1>Session Count</h1>
<% Integer count = (Integer) session.getAttribute("count");
if (count == null) {
out.println("You are new here.");
count = 1;
} else {
out.println("You have visited this site for " + count + " times.");
count++;
}
session.setAttribute("count", count);%>
</body>
</html>
sessionexpireYou can try this at
http://plbpc001.ouhk.edu.hk/tma/jsp/sessionexpire.jsp
The source can be downloaded at:http://plbpc001.ouhk.edu.hk/~mt311f/2010-
sep/web/webapplicaiton/web/sessionexpire.jsp
sessionexpire.jsp<%@page contentType="text/html"
pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01
Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><% if (request.getParameter("create") != null)
{ session = request.getSession(); session.setMaxInactiveInterval(10); } else { session = request.getSession(false); }%>
sessionexpire.jsp<html> <head> <meta http-equiv="Content-Type"
content="text/html; charset=UTF-8"> <title>SessionExpire</title> </head> <body> <h1>SessionExpire</h1>
sessionexpire.jsp <% int fontsize = 3; if (session != null) { fontsize = 10; }%> <font size='<%=fontsize%>'> The font size is <%=fontsize%>. </font> <form> <input type='submit' name='create'
value='create a session'/> </form> Click <a href='sessionexpire.jsp'>this</a>
to revisit this page
sessionexpire.jsp
</body></html>
JSP directivesDirectives are used to pass high-level
information to the server about how a specific page should be translated. The general format of a JSP directive looks like the following where one or more attribute values may be specified.
<%@ directive-name attribute-name1 = “value1” attribute-name2 = “value2” ... %>
The page directiveThe page direction is used to specify
properties of the JSP page.import:
<%@ page import="java.util.*, java.text.*" %>
contentType:<%@ page contentType="text/html; charset=Big5" %>
The session attribute is used to enable or disable the use of the predefined session variable. The default value is true.<%@ page session="false" %>
The page directiveThe isThreadSafe attribute indicates that the
page is thread-safe. The default is true. If this value is false, then requests for the JSP page will be handled serially. Note that if this value is true, then the programmer is responsible to sychronize access to shared variables.<%@ page isThreadSafe="false" %>
The include directive enables the contents of a separate resource to be merged into the current JSP page at the place of the directive appears.<%@ include file="/abc.html" %>
JSP declarations <%! ... %>This is to decare variables and methods for
use in the same JSP.For example, the following declare a method
that calculate the factorial of an integer:<%! private long factorial(int n) {
if (n==0)return 1;
elsereturn n*factorial(n-1);
}%>
JSP declarations <%! ... %>Then, code in the same JSP page can now
use this method.
A JSP that calculate the factorialYou can try the page at
http://plbpc001.ouhk.edu.hk/tma/jsp/factorial.jsp
The source can be downloaded at:http://plbpc001.ouhk.edu.hk/~mt311f/2010-
sep/web/webapplicaiton/web/factorial.jsp
factorial.jsp<%@page contentType="text/html"
pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML
4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<%! private int factorial(int n) { if (n == 0) { return 1; } else { return n * factorial(n - 1); } }%>
factorial.jsp<html> <head> <meta http-equiv="Content-Type"
content="text/html; charset=UTF-8"> <title>Factorial</title> </head> <body> <h1>Factorial</h1>
factorial.jsp <% String nva = request.getParameter("n"); int n = 0; if (nva != null) { n =
factorial(Integer.parseInt(nva.trim())); %> The factorial of <%=nva%> is <%=n%>.<br> <% } %>
factorial.jsp <form> <input type="text" name="n"
size="10"> <input type="submit"
name="submit" value="submit"> </form> </body></html>
Database connection for a JSP If you want to have one database connection for all request to
a page, the connection should be created as a JSP declaration.<%! private java.sql.Connection con = con(); private java.sql.Connection con() { java.sql.Connection re = null; try { Class.forName("com.mysql.jdbc.Driver"); re =
java.sql.DriverManager.getConnection("jdbc:mysql://127.0.0.1/jdatabase", "user", "password");
} catch (Exception e) { e.printStackTrace(); } return null; }%>
JavabeansJSP declarations can be used to instantiate
objects shared by requests for the same JSP page. But there are times that different JSP pages may want to share objects. JavaBeans are software components that support more flexible sharing. To facilitate sharing, JavaBeans must be coded according to certain specific coding convention.
JavaBeansA JavaBean must have a constructor with
no parameters. To use a JavaBean, you will need to place a <jsp:useBean> tag in a JSP page.
There are two ways to use a JavaBean:<jsp:useBean id = "count" scope =
"application" class = "chapter40.Count"> </jsp:useBean>
<jsp:useBean id="count" scope="application" class="chapter40.Count" />
Invoking the methods of a JavaBean<% count.increaseCount(); %>
JavaBean Lets create a JavaBean called javabean.CountVisit. Note
that a JavaBean has to be in a package.
package javabean;
public class CountVisit { public int count=0; synchronized public void addCount() { count++; } synchronized public int count() { return count; }}
A JSP page that uses javabean.CountVisitYou can try the page at
http://plbpc001.ouhk.edu.hk/tma/jsp/javabeanCount.jsp
The source can be downloaded at:http://plbpc001.ouhk.edu.hk/~mt311f/2010-
sep/web/webapplicaiton/web/javabeanCount.jsp
javabeanCount.jsp<%@page contentType="text/html"
pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01
Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><jsp:useBean id="count" scope="application"
class="javabean.CountVisit"/><html> <head> <meta http-equiv="Content-Type"
content="text/html; charset=UTF-8"> <title>Using Javabean to count</title> </head>
javabeanCount.jsp<body> <h1>Using Javabean to count</h1> This page has been visied <
%=count.count()%> times. <% count.addCount(); %> </body></html>
Scopes of JavaBeansTwo distinct instantiations can co-exist independently
with the same bean as long as they have different scopes.
The legitimate scopes from high to low are application, session, page and request.
When the scope is application, different end-users will share the same instantiation of the bean. Once created, the bean continues to live until the Web server restarts.
When the scope is session, each end-user will have his or her own bean. A user does not share the bean with another user. A session bean will live until the session expires based on the value of MaxInactiveInterval or after session.invalidate( ) is called whichever happens first.
Scopes of JavaBeansWhen the scope is page, no two JSP pages
can share the same instantiation of a bean. The difference between the page and request scopes is very subtle and does not concern us in this course.
Comparision of session, JSP declarations and JavaBeanValues in a session are accessible by one user
only and can be used in different JSP pages.Values stores in JSP declarations are
accessible by all users but is limited to a single page.
When a Javabean's scope is page or request, it is like JSP declaration. It is shared by all users but is limited to a single JSP page.
When a Javabean's scope is session, it is like a session value and is shared by a single user and can be used in different JSP pages.
Comparision of session, JSP declarations and JavaBeanWhen a Javabean's scope is application, it
is shared by all users and can be used in different JSP pages. It is like static attributes in servlet.
javabeanCount2This is exactly the same as javabeanCount.The two pages share the same JavaBean
called count.So the two pages have the same counter.You can try the page at
http://plbpc001.ouhk.edu.hk/tma/jsp/javabeanCount2.jsp