Li Tak Sing COMPS311F. Static attributes in Servlets Since Servlets are also Java classes, you can...

transcript

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"

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"

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

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

sep/web/webapplicaiton/web/factorial.jsp

factorial.jsp<%@page contentType="text/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

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

Li Tak Sing COMPS311F. Static attributes in Servlets Since Servlets are also Java classes, you can...

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