Webb applications Chapter 13 Lecture 7 1 Today

Webb applications Chapter 13 Lecture 7 1

Today we will deal with l l l Web applications Servlets JSP Good design in web apps In 1999 this was cutting edge, big bubbles. 2

Client / server Client DNS helps in looking up the address Protocol (http or other) Listens on a “port” Server 3

“Web site” vs “Web app” l l l Web site • • • The first use of a web site was to distribute information, such as static HTML documents. Server side content on a web site is static. All application processing occurs on the client side. • • • Provides server side application processing. Web browser - a Web-based interface to a server-based application. B 2 B, B 2 C • • are both accessed through a Web browser retrieving content from a Web server. But the Web app requires an application server in addition to the Web server. Web applications (“Web apps”) Web apps and web sites 4

Web apps… l Is a small part of the J 2 EE specification • • l In 1999 the APIs and tools supporting server-side Java for the Web, became part of the enterprise edition. J 2 EE formalized the definition of a Web app. J 2 EE is quite big • • Includes frameworks for handling enterprise needs for • • • Secure distributed access Scalability/performance Robustness Data persistance with transactional integrity Management of distributed resources. Introduced in Chapter 14. 5

Network programming vs Web app l Pure Java distributed applications • • • l can be realized through socket programming or Remote Method Invocation (RMI) Accesses native features of the hardware and software platform. Relative hard work, since the application must handle communications layer, security and related issues. Wep apps • Has a great advantage over “network programming” • Works on a higher level. • Reuse of standard web technologies. 6

A simple schema of a webb app system l l Relationships between web browsers, web servers and application servers. The web and application server may or may not reside on the same machine. Web application code may use any resource available to the JVM. (A more detailed design proposal comes later) 7

Application servers l l Internet Information Server (Microsoft) Netscape Enterprise Server Web. Sphere Application Server (IBM) Apache Tomcat • HTTP-server • plus app-server: JSP and Java Servlets. • Not J 2 EE compliant, since J 2 EE servers must handle Enterprise Java Beans (EJB) 8

Client side vs Server side l l Java. Script, Applets, HTML • • Typically constitute the view layer of a Web app. Client side • • Typically in the View layer Server side • • Typically is the control layer Server side • • Typically is in the control or model layer Server side JSP Servlets Supporting classes 9

A few words on the HTTP protocol l l On top of TCP/IP Stateless request/response protocol A request or response always contains • Headers • Body • • GET me this RESOURCE A HTTP response may contain a HTML document in its body. • • Info about the client Instructions to the client or meta info about the body content Continuity in a conversation? The most common client request: 10

A few words on the HTTPS protocol l l Adds security With the help of Secure Sockets Layer (SSL) that sits between TCP/IP and the HTTP layer Assures confidentiality in the point-to-point communication. May also handle Authentication via certificates. 11

HTTP request l l l Has a method • • GET (request a resource) POST (Transmit information, e. g. from a HTML form) PUT (Store a resource on the server) And a few other not so common methods Has headers • Optional number of them Has body 12

URI, URL, URN ? ? ? l Uniform Resource Identifier (URI) • • • l http: //my. domain. com: 9080/my/site/my. Page. html Uniform Resource Locator (URL) • • l Generic term. Helps us “distinguish what is being identified from all other things within its scope of identification” Refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism. http: //my. domain. com: 9080/my/site/my. Page. html Uniform Resource Name (URN) • • Location of the resource within the local host or domain. /my/site/my. Page. html 13

HTML and XHMTL l l HTML • • may not be well formed Tags may not have closing tags • May not have proper nesting of tags • Is not case sensitive • Tag attributes may not be enclosed in double quotes • Any well-formed XHTML document is a well-formed XML document. • • oopsidaisy • itworx XHTML always is, has, is, and is! 14

Web app packaging l l l J 2 SE apps are packaged into JAR files Web apps are packaged into WAR files Enterprise apps are packaged into EAR files 15

WAR files l l l l Include all Web resources: HTML, XHTML, JSP files Image files into a “images” folder (by convention) Executable java code into WEB-INF/classes Put any JAR files into folder lib. Add a file web. xml into folder WEB-INF. This “deployment descriptor” is important. May contain an ordinary MANIFEST. MF file in folder META -INF. May contain additional application server-specific files. If including java source: put them in “WEB-INF/source” 16

Servlets l l l The role is to accept requests from the client and invoke application logic to fulfill the request Act as controllers in a MVC oriented Web app. Is an entry point into a Web app. 17

How a web app processes requests l l 1. HTTP request 2. Activation of Servlets that are loaded into a Web container (JVM). Servlets may validate user input. 3. invocation of supporting classes in the model layer. 4. A HTML page is assembled and sent back to client. 18

The Servlet API l l Javax. servlet. http. Http. Servlet • Base class to be extended • • • Init(): may be overrided, but is often not. service(): May be overrided, but is often not do. Xxx(): Is often overrided Most common methods do. Get(), do. Post(), do. Put(). Either service() or do. Xxx() must be overrided. You should get familiar with the javax. servlet. http package • • • l • Http. Servlet class Http. Servlet. Request Http. Servlet. Response Read chapter 13. 19

My first servlet package examples. servlets; import java. io. *; import java. util. Date; import javax. servlet. *; import javax. servlet. http. *; public class Today. Servlet extends Http. Servlet { public void do. Get( Http. Servlet. Request request, Http. Servlet. Response response ) throws IOException, Servlet. Exception { response. set. Content. Type("text/HTML"); response. set. Header("Pragma", "no cache"); response. set. Header("Expires", "-1"); Print. Writer out = response. get. Writer(); out. println("<HTML>"); out. println("<head>"); out. println("<title>Today</title>"); out. println("</head>"); out. println("<body>"); out. println("<h 1>The current date and Time is: </h 1>"); Date today = new Date(); out. println("" + today + ""); out. println("</body>"); out. println("</HTML>"); out. flush(); } } 20

Another example public class Login. Servlet extends Http. Servlet { public void do. Post(Http. Servlet. Request req, Http. Servlet. Response resp) throws Servlet. Exception, IOException { Http. Session session = req. get. Session(false); if ( session != null ) { session. invalidate(); } Club. Member skier = null; try { skier = get. Member. Data(req); confirm. Login( req, resp, skier ); } catch( Bad. Data. Exception e ) { req. set. Attribute( "message", e. get. Message() ); Request. Dispatcher rd = get. Servlet. Context(). get. Request. Dispatcher("/badinput. jsp”); rd. forward( req, resp ); } catch( Exception e ) { get. Servlet. Context(). set. Attribute( "exception", e ); Request. Dispatcher rd = get. Servlet. Context(). get. Request. Dispatcher("/error. Report. jsp"); rd. forward( req, resp ); } return; } 21

Easy to l l Use session objects Forward to a JSP page (View) for output back to client • see previous slide l The request object • Contains data from the “form” • You may add more data to it before forwarding to a JSP page. 22

How to store data on the server side l l l Http. Session object • • Duration: client session. Automatically handled through cookies. Http. Servlet. Request object • Duration: Request-response cycle The Servlet object • Is available to all servlets. Should not be client or request specific. • get. Servlet. Context. set. Attribute() • Don’t use the Servlet fields. Not thread safe. Many threads (requests) may invoke methods in the same servlet object. 23

Remember l You should design all conversations so that each request-response could be the last exchange in the conversation 24

Java Server Pages l l Everything you can do with a servlet, you can do with a JSP page. Why both? • Clumsy • Team of people with different skills • Cleaner design (MVC). 25

MVC design with JSP l l l JSP presents info and results (View layer) Servlet gets user input and invokes application logic in the control and/or model layer Servlet forwards results to a JSP page the creates a view and sends it as a response back to the client 26

JSP example code <!DOCTYPE HTML PUBLIC "-//W 3 C//DTD HTML 4. 01 Transitional//EN"> <HTML> <HEAD> <META HTTP-EQUIV="Pragma" CONTENT="no-cache"> <META HTTP-EQUIV="Expires" CONTENT="-1"> <TITLE>Bad. Input. Message. jsp</TITLE> </HEAD> <BODY> <CENTER> <IMG SRC="images/Ski. Club. gif" ALT="Near Hills Ski Club " > </CENTER> <h 2>Unable to complete operation</h 2> There is a problem with the informaton you entered: <TABLE Border=1 BGCOLOR=#0000 FF> <TR><TD> <%= request. get. Attribute( "message" ) %></TD></TR> </TABLE> Use the BACK button to try again. </BODY </HTML> 27

JSP tags l <%= … %> • Expression inserts the value of the Java expression l <% … %> l See figure 13 -13 and the web. • Execute embedded java code. 28

Example of flow of a HTTP request and response l Servlet recieves HTTP request • If GET, forward to JSP that displays a form • If POST, retrieve user input and forward to model layer classes for processing l l Servlet forwards results to the JSP The JSP produces output. 29