Transcript Java and Object Orientation

Introduction to Server-Side
Web Development
Introduction to Server-Side Web
Development
Introduction to Server-Side Web Development with
Servlets and JSP; basic concepts and syntax
18th February 2005
Bogdan L. Vrusias
[email protected]
Introduction to Server-Side
Web Development
Introduction
• Most web sites want to display dynamic content based on
user’s requests and expectations.
• Most contents, such as images, text, and banner ads, are
easiest to build with HTML editors.
• We need to mix the "static" content of HTML files with
"directives" for accessing or generating dynamic content.
• Introducing:
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Introduction to Server-Side
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Contents
• History of JavaServer Pages (JSP)
• Fundamentals of server-side programming.
• Comparison to other server-side languages
• Fundamentals of JavaServer Pages / Servlets.
• Servlets
• JavaServer Pages
• Tomcat
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Introduction to Server-Side
Web Development
History: The Beginning
• 1991… beginning of WWW… beginning of HTML
• No dynamic pages… the Webmaster
• Too many static HTML pages had to be created for each
website.
• Need for change
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Introduction to Server-Side
Web Development
History: First approach - CGI
• Solution… Common Gateway Interface (CGI) technology
created.
• Dynamic pages…using Perl and C
• Problem… these languages are not the simplest
• Need for change
• Java had the answer… Servlets
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Introduction to Server-Side
Web Development
History: Servlets and JSP
• Servlets are pure Java objects that generate HTML by writing it to a
stream.
• The files do not contain any HTML tags; they simply contain Java
code which write out HTML (as strings) to produce the page that is
sent to the user's browser.
• …but it was quite painful to develop any large scale application with a
bunch of servlets running around doing println() statements.
• This is why Sun invented JSPs in the first place: to compete with the
ease-of-development of Microsoft's ASP files, which could combine
HTML tags and code in a single file.
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Introduction to Server-Side
Web Development
Definition
• JSP provide server-side scripting support for generating
web pages with combined static and dynamic content.
• JSP is a fast way to serve web pages that display
dynamically generated content.
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Introduction to Server-Side
Web Development
A dynamic Web Page
• Static web page is a page whose content consists of some HTML that
was predetermined and was typed directly into a text editor and saved
as ".htm" or ".html" file.
• Dynamic web page is a page whose content is generated at runtime
dynamically.
1. Author writes
instructions
5. HTML stream returned to browser
6. Browser processes HTML
and displays page.
3. Web server
locates instructions
file
4. Web server
processes
instructions to
create HTML
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2. Client request web page
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Introduction to Server-Side
Web Development
Requests and Responses
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Introduction to Server-Side
Web Development
Other Server-Side programming languages
• CGI
– Perl, C, C++
• ColdFusion
– Tags with encapsulated functionality
• ASP (Active Server Pages) and ASP .NET
– VBasic, JavaScript, C#
– ADO
• PHP (Personal Home Pages)
– Syntax similar to C and Perl
– Open-source, cross-platform
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Introduction to Server-Side
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JSP vs .NET and ASP
• Similarities
– Create interactive pages with web-based applications
– Separates programming logic from page design through the use of
component technology
– Faster and Easier alternative to older, more cryptic technologies
such as CGI
• Differences (?)
– JSP: Platform and server Independent
• JSP can run on any web server
• Supported by large number of tools
– ASP: Relies on Microsoft platforms and servers
• Restricted to MS Windows based platforms because of ActiveX
Controls (that is to change now?)
• Can be ported to different platforms using third party products, but
ActiveX must be present on the platform.
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Introduction to Server-Side
Web Development
Why Use JSPs?
• Write once, run anywhere (JAVA).
– JSPs are entirely platform independent, both in dynamic Web
pages and underlying server components.
• Emphasize components.
– JSPs encourage the use of reusable, cross-platform server
components called JavaBeans. Therefore saves development time,
while giving you the power and flexibility of the Java
programming language.
• Provide a front door to the Enterprise’s Java platform.
– JSPs are an integral part of the Java Platform for the Enterprise,
which brings Java technology to enterprise computing. You can
develop enterprise-wide or middle-tier server applications, using a
JSP web site as a front end.
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Introduction to Server-Side
Web Development
Advantages of Java
• According to java.sun.com:
– More mature, powerful, and scalable than Basic-based scripting
languages
– Java helps developers protect against system crashes, ASP and NT
systems are susceptible to crashing
– Java helps with memory management by providing protection
against memory leaks
• Well supported, large collection of APIs
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Introduction to Server-Side
Web Development
JSP Model Architectures
BROWSER
BROWSER
request
response
request
response
JSP / Servlet Container
JSP / Servlet Container
JSP PAGE
JSP
(VIEW)
SERVLET
(CONTROLLER)
JAVABEAN
JAVABEAN
Data Tier
Data Tier
DATA
DATA
DATA
Model 1
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DATA
Model 2
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JSP Model Architectures
• Model 1
–
–
–
–
Simple and fast
Maintainability problems
Reusability Problems
Security Problems
• Model 2 (also called Model-View-Controller)
–
–
–
–
Requires more initial effort and technical knowledge
Maintainability solutions vs. Model 1
Security solutions vs. Model 1
Reusability and extensibility solutions vs. Model 1
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Introduction to Server-Side
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What are Servlets?
• Servlets are objects that reside on a Java enabled web
server, that work on the request-response principle.
• Servlets are to the server-side what applets are to the client
side
• Servlets do not have a GUI associated with them
• Servlets are faster and cleaner than CGI scripts
• Servlets use a standard API
• Servlets provide all the advantages of Java
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Introduction to Server-Side
Web Development
Servlets Characteristics
• Security
– Servlets are called within the server context so they have all the security
of the server itself.
– They’re hidden from view and transmission.
• Robustness
– Rich application programming interfaces, APIs, are available making it
easier to build sophisticated applications (JDBC, EJB, etc.).
• Performance
– Servlets run in the same context as the application server and can be
preloaded or loaded on demand.
– They’re multi-threaded to scale with multiprocessors and heterogeneous
systems
• Portability
– Write-once, run-anywhere capability
– They are able to exploit re-useable Java components called JavaBeans.
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Introduction to Server-Side
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Servlet Code Sample
import javax.servlet.*;
import java.io.*;
import java.util.*;
public class SimpleServlet extends GenericServlet {
public void service(ServletRequest request, ServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
printWriter pw = response.getWriter();
pw.println("<html>\n<body>");
for(int counter = 1; counter <= 10; counter++) {
pw.println(counter + " Mississippi,<br>");
}
pw.println("<b>Ready or not, here I come!</b>");
pw.println("</body>\n<html>");
pw.close();
}
}
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Introduction to Server-Side
Web Development
Servlet Lifecycle
1.
When a server loads a servlet, it runs the servlet's init() method.
Even though most servlets are run in multi-threaded servers, there are
no concurrency issues during servlet initialization.
2.
After the server loads and initializes the servlet, the servlet is able to
handle client requests. It processes them in its service() method.
3.
Servlets run until they are removed from the service, by calling the
servlet's destroy() method.
•
NOTE: Servlets can run multiple service methods at a time. It is
important, therefore, that service methods be written in a thread-safe
manner. If, for some reason, a server should not run multiple service
methods concurrently, the servlet should implement the
SingleThreadModel interface.
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Introduction to Server-Side
Web Development
Servlet Lifecycle Diagram
Uninstantiated
Instantiated
init()
Initialised
Servicing
Requests
service()
doGet()
Process
Requests
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destroy()
Garbage
Collection
doPost()
Process
Requests
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Introduction to Server-Side
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Servlet Architecture Overview
• The central abstraction in the Servlet API is the Servlet interface.
All servlets implement this interface, either directly or, more
commonly, by extending a class that implements it such as
HttpServlet.
• When a servlet accepts a call from a client, it receives two objects: one
is a ServletRequest and the other is a ServletResponse.
– The ServletRequest class encapsulates the communication from the
client to the server
– The ServletResponse class encapsulates the communication from the
servlet back to the client.
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Introduction to Server-Side
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ServletRequest
• The ServletRequest interface allows the servlet access to
information such as:
– the names of the parameters passed in by the client
– the protocol (scheme) being used by the client
– the names of the remote host that made the request and the server that
received it.
• It also provides the servlet with access to the input stream,
ServletInputStream, through which the servlet gets data from
clients that are using application protocols such as the http POST
method.
CLIENT
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SERVER
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Introduction to Server-Side
Web Development
ServletResponse
• The ServletResponse interface gives the servlet methods for
replying to the client. It allows the servlet to:
– set the content length and mime type of the reply
– provides an output stream, ServletOutputStream, and a "writer"
through which the servlet can send the reply data.
• Subclasses of ServletResponse give the servlet more protocolspecific capabilities. For example, HttpServletResponse
contains methods that allow the servlet to manipulate HTTP-specific
header information.
CLIENT
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SERVER
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Introduction to Server-Side
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Interacting with Clients I
• Servlet writers who are developing HTTP servlets that
specialize the HttpServlet class should override the
method or methods designed to handle the HTTP
interactions that their servlet will handle.
• The candidate methods include:
–
–
–
–
doGet, for handling GET, conditional GET and HEAD requests
doPost, for handling POST requests
doPut, for handling PUT requests
doDelete, for handling DELETE requests
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Introduction to Server-Side
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Interacting with Clients II
• An HttpServletRequest object provides access to HTTP header
data, such as any cookies found in the request and the HTTP method
with which the request was made. It allows the you to obtain the
arguments that the client sent as part of the request:
– For any HTTP method, you can use the getParameter() or
getParameterValues() method, which will return the value(s) of a
named parameter. The method getParameterNames() provides the
names of the parameters.
• For responding to the client, an HttpServletResponse object
provides ways of returning the response data to the user. You can use
the writer returned by the getWriter() method to return text data
to the user.
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Introduction to Server-Side
Web Development
Reading Form Data Example - GET
<form action="/servlet/aservlet" method="GET">
<input type="HIDDEN" name="key1" value="abc"/>
<input type="HIDDEN" name="key2" value="def"/>
<input type="SUBMIT" value="Go...."/>
</form>
When submitted, the browser would execute the following URL:
http://www.somewhere.com/servlet/aservlet?key1=x&
key2=y
Alternatively
<a href="/servlet/aservlet?key1=x&key2=y">
LINK</a>
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Introduction to Server-Side
Web Development
Reading Form Data Example - POST
The HTML
<form action="/servlet/MyNameServlet" method="POST">
Please Enter your name
<input type="TEXT" NAME="yourname"/>
<input type="SUBMIT" VALUE="Submit"/>
</form>
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Reading Form Data Example - POST
The Servlet
...
public class AServlet extends HttpServlet {
public void doPost(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException
{
String name=req.getParameter("yourname");
String reply="<HTML>\n"+
"<HEAD><TITLE>My Name Servlet Response</TITLE></HEAD>\n"+
"<BODY>\n<CENTER><BR><B>\n"+
"Hello "+name+"\n"+
"</B></CENTER>\n</BODY>\n</HTML>";
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println(reply);
out.close();
}
}
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Introduction to Server-Side
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Demo
• Some examples will now be demonstrated using the
Tomcat server…
http://host/servlet/ServletName
http://host/servlet/PackageName.ServletName
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Introduction to Server-Side
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How Servlets relate to JSPs
• JSPs and servlets are two different ways to accomplish the
same goal: generating dynamic HTML pages using Java
code. One puts Java code in your HTML, and one puts
HTML in your Java code.
• Functionally, they are equivalent. In fact, under the covers,
the web server takes a JSP and converts it to the
corresponding servlet and dynamically compiles it.
• BUT servlets have the following deficiencies:
– It is hard to write and maintain the HTML
– You cannot use standard HTML tools
– The HTML is inaccessible to non-Java developers
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Introduction to Server-Side
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JSP Code Sample
<%@ page language="java" %>
<html>
<body>
<% for (int counter = 1; counter <= 10; counter++) {%>
<%= counter %> Mississippi,<BR>
<% }%>
<b>Ready or not, here I come!</b>
</body>
</html>
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Introduction to Server-Side
Web Development
JSP Syntax
• Scriptlets
• Expressions
• Declarations
• Comments
• Directives
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Web Development
Scriptlets
<%
// Java Code
%>
e.g.
<% for (int i = 1; i <= 10; i++) {%>
<%= i %> Mississippi,<BR>
<% }%>
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Expressions
An expression prints out a string value on the page's buffer
output:
<%= counter %>
<%= new java.util.Date() %>
Using scriplets this would look like:
<% out.print(counter) %>
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Declarations
<%!
// this integer can be used anywhere in this JSP page
private int myVariable = -1;
// this function can be called from anywhere in this JSP page
public boolean isPositive() {
return ( myVariable > 0 );
}
%>
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Web Development
Comments
The comments are not printed on page's buffer, therefore are not viewed by the
client.
<%-comments over many
lines of code
--%>
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Web Development
Directives
• Here is what directives look like in a general form:
<%@ directive attribute="someValue"
attribute="anotherValue" ... %>
• There are three directives:
<%@ page ... %> specifies information that affects the page
e.g. <%@ page import="java.util.Date, java.text.*" %>
<%@ include ... %> includes a file at the location of the include directive
(parsed)
e.g. <%@ include file="/copyright.html" %>
<%@ taglib ... %> allows the use of custom tags in the page
e.g. <%@ taglib uri="/WEB-INF/tlds/myTagLib.tld"
prefix="myPrefix" %>
<myPrefix:displayImage file="logo.jpg" />
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Web Development
Page Directive
• <%@ page import="package.class" %>
• <%@ page errorPage="Relative URL" %>
• <%@ page isErrorPage="true" %>
• <%@ page contentType="application/vnd.ms-excel"
%>
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Include Directive
• Use the include directive to include a file in the main JSP
document at the time the document is translated into a
servlet.
• What is included is the actual content of the respective file.
<%@ include file="header.jsp" %>
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JSP Action Elements
• There are three different types of action elements:
– Standard
•
•
•
•
•
<jsp:include>
<jsp:forward>
<jsp:useBean>
<jsp:plugin>
<jsp:getProperty> and <jsp:setProperty>
– JSTL (JavaServer Pages Standard Tag Library)
• It provides tag libraries in the subject of Internationalization and
formatting, XML, SQL, and the core tags.
– Custom
• Tags constructed by the developer.
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Introduction to Server-Side
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Include Action Element
• The jsp:include action lets you include the output (not the not the
actual code) of a page at request time:
<jsp:include page="header.jsp" />
<jsp:include page="header.jsp" %>
<jsp:param name="title" value="Welcome"/>
</jsp:include>
NOTE: Is it similar to <%@ include file="header.jsp"%> ???
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Forward Action Element
• The jsp:forward action lets you forward the output to another page:
<jsp:forward page="main.jsp" />
NOTE: Not recommended… !!! Better use the forward method of the
RequestDispatcher object.
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Implicit Objects and the JSP Environment
• The scriptlets and expressions written in a JSP page do not stand alone
as a complete program – they need an environment in which to
operate.
• The JSP container provides this environment and makes it accessible to
the page author through what are called implicit objects:
–
–
–
–
–
–
–
–
–
request (HttpServletRequest)
response (HttpServletResponce)
application (ServletContext)
session (HttpSession)
out (JspWriter)
pageContext
config
page
exception
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Introduction to Server-Side
Web Development
Basic request Methods
• String getParameter(String name)
• Enumeration getParameterNames()
• String getMethod() (e.g "POST", "GET")
• Cookie[] getCookies()
• String getHeader(String name)
• Enumeration getHeaderNames()
• HttpSession getSession(boolean create)
• ...
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Basic response Methods
• void sendRedirect()
• void addCookie(Cookie cookie)
• void setHeader(String name)
• void addHeader(String name)
• void sendError(int sc [, String msg])
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Web Development
Basic session Methods
• Object getAttribute(String name)
• void setAttribute(String name, Object value)
• Enumeration getAttributeNames()
• int setMaxInactiveInterval()
• int getMaxInactiveInterval()
• void invalidate()
• void isNew()
• void logout()
• String getId()
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Basic application Methods
• String getRealPath(String path)
• Object getAttribute(String name)
• void setAttribute(String name , Object value)
• void removeAttribute(String name)
• Enumeration getAttributeNames()
• void log(String msg)
• URL getResource(String path)
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Basic out Methods
• void print(String value)
• void println(String value)
• void newLine()
• void clear()
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Web Development
Tomcat
• Tomcat is the official JavaServer Pages Web server.
• Tomcat is the official reference implementation of the Java
Servlet and JavaServer Pages technologies. Developed
under the Apache license in an open and participatory
environment.
• Other popular Web servers are:
–
–
–
–
–
Apache
Macromedia JRun
Jetty
Internet Information Server (IIS)
Jigsaw
http://jakarta.apache.org/tomcat/
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Introduction to Server-Side
Web Development
Exploring Tomcat
• Please follow the demonstration on exploring Apache
Tomcat.
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Introduction to Server-Side
Web Development
Closing
• Questions???
• Remarks???
• Comments!!!
• Evaluation!
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