Jeff Schmitt November 16, 2001
description
Transcript of Jeff Schmitt November 16, 2001
Application Architectures for Interactive Web Applications:
Java JSP and Model/View/Controller
Techniques
Jeff Schmitt
November 16, 2001
ABSTRACT
This talk is the report of my sabbatical leave during Spring 2001 in
which I explored some new techniques in using the Java language to
develop interactive web applications.
Java Server Pages technology (JSP) facilitates the development of
interactive websites with dynamically generated content. JSP programs
contain ordinary HTML tags mixed with Java code. Custom tag libraries
and Template processing are techniques which allow the Java code to be
isolated from the HTML. This application architecture is
based on the principles of Model/View/Controller.
Web Application Components• Client -- typically a Web Browser
• HTTP -- request, response protocol between client and server
• HTML files -- static web content
• Webserver -- serves HTML files and
– looks for certain request patterns, then forwards request to servlet engine, for example: /servlet/*
• Servlet Engine -- supports execution of servlets
• Servlet -- handles request from client, generates response to client
• JSP Engine -- supports the compilation of files into Servlets
• Web Application -- a collection of HTML files, Servlets and JSP pages to implement an information system on the web.
Tiered SystemsSingle Tier -- Mainframe
LAN
Two Tier- Client-Server
INTERNET
Database
ServerApplication
Server
Web
Server
N-Tier -- Web Application
Servlet Lifecycle
init() destroy()
service()
Servlet Sequence Diagram
Client Webserver Servlet
request() init()
destroy()
service()
Process requestresponseresponse
Servlet Multithreading
Client 1 Servlet
request
Client 2
request
response
response
ServletsAdvantages Disadvantages• Scalable• Persistent• Simple• Flexible• Support• Stable API• Run in separate
memory space from Server
• Servlet mapping(servlet is not a web page)
• Hard for Web Designer to change the HTML part of the code
• HTML (presentation) and Java code (business logic) mixed together
XML Notation
• Begin and end within same tag:<something/>
• Begin and end as two separate tags:<something>
…</something>
• Document Type Definition<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN""http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
JSP -- Java Server Pages
• Similar to servlets, but they resemble HTML pages
• Placed in web folders mixed with HTML files, acts like an HTML file
• Compiled automatically to a servlet when first referenced or when the file is changed by programmer.
• Intended to help separate the presentation (HTML response) from the logic (processing logic).
• Similar to Microsoft's Application Server Pages (ASP)
JSP Lifecycle
Client Webserver Servlet
request
service()
init()
Process requestresponse
response
JSP
request JSP to ServletCompile Servletservlet
JSP scripting elements• JSP Comments
<%-- jsp comment --%>
• Declarations: placed in static part of servlet<%! String filePath="/users";%>
• Expressions: evaluate to a string<%= request.getParameter("NAME")%>
• Scriptlets: any Java statements<% (java statements) %>
• Template Text: inserted into response<TABLE BORDER=1><TR><TH>Name:<TD><INPUT NAME="NAME" size=50 MAXLENGTH=50>
JSP Default ObjectsObject Class Scope Commentsrequest HttpServletReq
uestRequest The client request
response HttpServletResponse
Page The servlet response
out JspWriter page An object for writing to the output stream
session HttpSession Session Created to track same user as they visit and return to pages in web application
application ServletContext Application Allows servlets to communicate with each other
JSP Object Scope
Scope Meaning
Page Available to the handling page only
Request Available to the handling page and to any servlelt or JSP that control is forwarded to or included
Session Available to any servlet or JSP within the same session (across repeat visits to web application)
Application Available to any servlet or JSP within the same web application
Custom JSP tags
• Assemble reusable JSP code for easy use• An XML-like front end to access Java
programming• Designed to bridge the intersection between java
programmers and web designers• Tag library holds definitions of custom tags
<%@ taglib uri="jakarta-taglib/dbtags" prefix="sql" %>
• Custom tags save in Tag Library File .tld• Many useful Tag Libraries exist for the common
tasks such as database access, email, file access
DBTags
• From Apache Jakarta project• Allow use of JDBC with Java details hidden• Easy to use • Invisible exception handling
<sql:connection>
• establish connection to server
<%-- open a database connection --%> <sql:connection id="conn1"> <%-- required --%> <sql:url>jdbc:mysql://localhost/test</sql:url>
<%-- optional --%> <sql:driver>org.gjt.mm.mysql.Driver</sql:driver>
<%-- optional --%> <sql:userId>root</sql:userId>
<%-- optional --%> <sql:password>notVerySecure</sql:password>
</sql:connection>
Statement and Query
• The "escapeSql" tag can be used inside a SQL query to SQL-escape your input values if they contain single quotes.
<%-- insert a row into the database --%><sql:statement id="stmt1" conn="conn1">
<%-- set the SQL query --%> <sql:query> insert into test_books (id, name) values (3, '<sql:escapeSql> <%=request.getParameter("book_title")%></sql:escapeSql>')</sql:query>
<%-- execute the query --%> <sql:execute/>
</sql:statement>
<sql:resultset>
<table>
<sql:statement id="stmt1" conn="conn1">
<sql:query>
select id, name, description from test_books order by 1
</sql:query>
<%-- loop through the rows of your query --%>
<sql:resultSet id="rset2">
<tr> <td><sql:getColumn position="1"/></td>
<td><sql:getColumn position="2"/></td>
<td><sql:getColumn position="3"/>
<%-- print out a comment if the book has no description --%>
<sql:wasNull>[no description]</sql:wasNull></td>
</tr>
</sql:resultSet>
</sql:statement>
</table>
JSP Action Elements
• Control Tags for request-time include
• Optional parameters for Control tags
• Allow JSP programs to implement the Controller aspect of the Model-View-Controller architecture
<jsp:include page="header.jsp"/>
<jsp:forward page="html.jsp/>
<jsp:forward page="html.jsp">
<jsp:param name="page" value="guestview"/>
</jsp:forward>
JSP Forward
Client Webserver Servlet 1
request
service()
Process request
responseresponse
JSP
request JSP to ServletCompile Servletservlet
Servlet 2
request JSP to ServletCompile Servletservlet
Process request
service()
Classical Model-View-Controller Architecture
View
Controller
Model
Example: A Clock Application
Change Notification
User Events
Select a ViewModel Changes
Model Queries
Problems with JSP
• Although intended to help separate the presentation logic (VIEW) from the business logic (MODEL), JSP does not go far enough
• JSP programs are often a mix of HTML and business logic, which creates a problem if the design of the page (VIEW) needs to be changed
• HTML changes need to be made by web designers who are not necessarily programmers
• Solutions to this problem include WebMacro, Struts, FreeMarker, Velocity
FreeMarker Template Processing
• Makes substitutions of text in a template based on a set of identifiers in a table ModelRoot)
• Works with any form of text files including HTML, XML. and Data files
Template
CacheFreemarkerModelRoot
Resulting
web page
FreeMarker Data Structures
• SimpleHash -- a random access table of words.Each word is associated with a value which can be any of the three Freemarker structures
• SimpleList -- a sequential access list of any of the three Freemarker structures
• SimpleScalar -- a String of text
ModelRoot -- the root of the structuremust be a SimpleHash
Freemarker RootHashrootHash
guestbook
NAME
AFF1
AFF2
NAME
AFF1
AFF2
NAME
AFF1
AFF2
Joe
Towson U
COSC
joe@towson
Freemarker Template Scripting
• Value Substitution from rootHash${request.NAME}
• Freemarker Comments<comment> . . . </comment>
• List iteration<loop guestbook as entry> . . . </list>
• Conditional<if> . . . <else> . . . </if>
• Text inclusion from another template file<include "footer.html">
FreeMarker Template Cache
• Manages the folder where templates are stored• Loads and compiles templates into tokenized form
for efficiency• Recognizes changes to templates while application
is running
Guestbook: Java Web Applicationhtml.jsp?page=guestbook
html.jsp?page=login.jsp
html.jsp?page=guestsign html.jsp?page=guestview
guestview.jspguestsign.jsp
guestview.jsp
checklogin.jsp
guestview.jsp
delete?id=34
jsp:forward action
or link
Guestbook Demo
http://www.mycgiserver.com/~fontanini/introfm/html.jsp?page=guestbook
I18N• Internationalization• Language and Cultural differences• Dates Times• Browser sends header: Accept-language: en-us• First two characters choose template file:
– guestbook_en.html– guestbook_it.html
• Example: http://www.mycgiserver.com/~fontanini/fm/guestbook.jsp
Conclusion
• JSP is Java that looks like HTML• Custom Tags make programming “easy”• Separation of business logic and presentation is
desirable for larger “Enterprise” projects• JSP alone does not achieve this separation• EJB, and other technologies are complicated• Template processing provides the desired
separation, rather easily and elegantly.• Thank you for attending. Any questions?