Application Architectures for Interactive Web Applications: Java JSP and Model/View/Controller Techniques

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Application Architectures for Interactive Web
Applications Java JSP and Model/View/Controller
Techniques

• Jeff Schmitt
• November 16, 2001

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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.

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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.

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Tiered Systems
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Servlet Lifecycle
service()
init()
destroy()
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Servlet Sequence Diagram
Client
Webserver
Servlet
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Servlet Multithreading
Client 1
Servlet
Client 2
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ServletsAdvantages Disadvantages

• 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

• Scalable
• Persistent
• Simple
• Flexible
• Support
• Stable API
• Run in separate memory space from Server

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XML Notation

• Begin and end within same tag ltsomething/gt
• Begin and end as two separate tags ltsomethinggt
  lt/somethinggt
• Document Type Definitionlt?xml version"1.0"
  encoding"ISO-8859-1"?gt
• lt!DOCTYPE web-app PUBLIC "-//Sun Microsystems,
  Inc.//DTD Web Application 2.2//EN""http//java.su
  n.com/j2ee/dtds/web-app_2_2.dtd"gt

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Deployment Descriptor web.xml
lt?xml version"1.0" encoding"ISO-8859-1"?gt lt!DOCT
YPE web-app PUBLIC "-//Sun Microsystems,
Inc.//DTD Web Application 2.2//EN""http//java.su
n.com/j2ee/dtds/web-app_2_2.dtd"gt ltweb-appgt ltservl
etgtltservlet-namegt SnoopServlet
lt/servlet-namegtltservlet-classgt test.SnoopServlet
lt/servlet-classgtltload-on-startupgt -454677
lt/load-on-startupgt lt/servletgt ltservlet-mappinggt
ltservlet-namegt SnoopServlet lt/servlet-namegtlturl-p
atterngt SnoopServlet lt/url-patterngt lt/servlet-mapp
inggt ltservlet-mappinggtltservlet-namegt
SnoopServlet lt/servlet-namegtlturl-patterngt
/snoop/ lt/url-patterngt lt/servlet-mappinggt
lttaglibgtlttaglib-urigthttp//jakarta.apache.org/t
aglibs/dbtagslt/taglib-urigtlttaglib-locationgt/WEB-I
NF/dbtags.tldlt/taglib-locationgt lt/taglibgt
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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)

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JSP Lifecycle
Client
Webserver
Servlet
JSP
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JSP scripting elements

• JSP Commentslt-- jsp comment --gt
• Declarations placed in static part of
  servletlt! String filePath"/users"gt
• Expressions evaluate to a stringlt
  request.getParameter("NAME")gt
• Scriptlets any Java statementslt (java
  statements) gt
• Template Text inserted into responseltTABLE
  BORDER1gtltTRgtltTHgtNameltTDgtltINPUT NAME"NAME"
  size50 MAXLENGTH50gt

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JSP Default Objects
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JSP Object Scope
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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 tagslt_at_
  taglib uri"jakarta-taglib/dbtags" prefix"sql"
  gt
• Custom tags save in Tag Library File .tld
• Many useful Tag Libraries exist for the common
  tasks such as database access, email, file access

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DBTags

• From Apache Jakarta project
• Allow use of JDBC with Java details hidden
• Easy to use
• Invisible exception handling

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ltsqlconnectiongt

• establish connection to server

lt-- open a database connection --gt
ltsqlconnection id"conn1"gt lt-- required
--gt ltsqlurlgtjdbcmysql//localhost/testlt/
sqlurlgt lt-- optional --gt
ltsqldrivergtorg.gjt.mm.mysql.Driverlt/sqldrivergt
lt-- optional --gt ltsqluserIdgtrootlt/sqlu
serIdgt lt-- optional --gt
ltsqlpasswordgtnotVerySecurelt/sqlpasswordgt
lt/sqlconnectiongt
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Statement and Query

• The "escapeSql" tag can be used inside a SQL
  query to SQL-escape your input values if they
  contain single quotes.

lt-- insert a row into the database
--gt ltsqlstatement id"stmt1" conn"conn1"gt
lt-- set the SQL query --gt ltsqlquerygt
insert into test_books (id, name) values (3,
'ltsqlescapeSqlgt ltrequest.getParameter("book_
title")gtlt/sqlescapeSqlgt') lt/sqlquerygt lt--
execute the query --gt ltsqlexecute/gt
lt/sqlstatementgt
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ltsqlresultsetgt

• lttablegt
• ltsqlstatement id"stmt1" conn"conn1"gt
• ltsqlquerygt
• select id, name, description from test_books
  order by 1
• lt/sqlquerygt
• lt-- loop through the rows of your query --gt
• ltsqlresultSet id"rset2"gt
• lttrgt lttdgtltsqlgetColumn position"1"/gtlt/tdgt
• lttdgtltsqlgetColumn position"2"/gtlt/tdgt
• lttdgtltsqlgetColumn position"3"/gt
• lt-- print out a comment if the book has no
  description --gt
• ltsqlwasNullgtno descriptionlt/sqlwasNullgtlt/td
  gt
• lt/trgt
• lt/sqlresultSetgt
• lt/sqlstatementgt
• lt/tablegt

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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

ltjspinclude page"header.jsp"/gt ltjspforward
page"html.jsp/gt
ltjspforward page"html.jsp"gt ltjspparam
name"page" value"guestview"/gt lt/jspforwardgt
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JSP Forward
Client
Webserver
Servlet 1
JSP
Servlet 2
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Classical Model-View-Controller Architecture
Model Queries
View
Model
Change Notification
User Events
Select a View
Model Changes
Controller
Example A Clock Application
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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

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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 Cache
Freemarker
ModelRoot
Resulting web page
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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

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Freemarker RootHash
rootHash
NAME AFF1 AFF2 EMAIL
Joe
Towson U
guestbook
COSC
joe_at_towson
NAME AFF1 AFF2 EMAIL
NAME AFF1 AFF2 EMAIL
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Freemarker Template Scripting

• Value Substitution from rootHashrequest.NAME
• Freemarker Commentsltcommentgt . . . lt/commentgt
• List iterationltloop guestbook as entrygt . . .
  lt/listgt
• Conditionalltifgt . . . ltelsegt . . . lt/ifgt
• Text inclusion from another template
  fileltinclude "footer.html"gt

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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

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Guestbook Java Web Application
html.jsp?pageguestbook
jspforward
action or link
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Guestbook Demo
http//www.mycgiserver.com/fontanini/introfm/html
.jsp?pageguestbook
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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

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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?