JAP Hybrid 9

Web Applications and Servlets

• A web application is accessible from the web
• Web applications use components like Servlets, JSPs, and Filters
• Web applications use elements, such as HTML, CSS, and JavaScript
• Web components run on the Web Server and respond to HTTP requests
• Web applications act as server-like components
• Web applications create HTML and XML data for the presentation layer
• Web applications dynamically process requests/responses

CGI vs Servlets

• CGI (Common Gateway Interface) scripting was a common server-side programming language
• CGI had scaling issues, process limits, and language dependencies (C, C++, Perl)
• Java Servlets are a more robust and scalable alternative to CGI

Servlet Details

• A Servlet is a Java class extending a server's capabilities
• Servlets extend web server applications, acting like Java Applets for servers instead of browsers
• Servlets have better performance, portability due to Java, robustness, and security, compared to CGI
• A web container creates threads to handle Servlet requests
• Threads share memory, are lightweight, and have low communication costs

Servlet API

• The Servlet API has interfaces/classes like Servlet, GenericServlet, HttpServlet, ServletRequest, and ServletResponse
• The Servlet API provides many interfaces/classes with documentation
• The Servlet API improves server capabilities and responds to incoming requests
• Servlets use protocols like Get or Post to return responses to requests
• A web application resides in a WEB-APP folder and is managed in a specific instance between init() and destroy() methods

Web Container

• To create a servlet, a Servlet environment and container must be used
• Apache Tomcat is a simple and common container

Apache Tomcat

• Apache Tomcat is an open-source Jakarta solutions implementation
• Jakarta solutions include Servlet, Server Pages, Expression Language, WebSocket, Annotations, and Authentication specifications
• Apache Tomcat is developed openly and released under Apache License version 2
• The project wants collaboration from developers globally to develop Tomcat

Basic Steps for Tomcat

• Set the CATALINA_HOME global variable after downloading Tomcat
• Look for the lib folder and servlet-api.jar file
• Create a folder inside the webapps folder to create a servlet
• Make a "hello" directory for a simple "Hello World" servlet demo inside the web application folder
• Each servlet needs a structure given by the WEB-INF folder
• This folder should contain classes (bytecode) and web.xml (defines the web application)

Web.xml file

• Web applications are defined in the web.xml file
• Web.xml is defined inside the <web-app> </web-app> tags.
• The content of web.xml identifies the class and name for referencing it in the application

Servlet Invocation

• The servlet can be invoked as “/Hello” (in the application folder) and run the code from the class
• Servlets are Java classes that service HTTP requests and use the javax.servlet.Servlet interface
• Web application developers create servlets that extend javax.servlet.http.HttpServlet
• The previously mentioned class is an abstract class implementing the Servlet interface, specialized for HTTP requests
• Classes from Servlet libraries are required (package javax.servlet):
• ServletException to handle exceptions
• HttpServletRequest for requests from clients
• HttpServletResponse for responses from the server

Demonstration Code

• Code can be used to demonstrate a "hello" application

Running Steps

• Start the Tomcat server using <Tomcat-installationdirectory>\bin\startup.bat on Windows
• Or use <Tomcat-installationdirectory>/bin/startup.sh on Linux
• Open http://localhost:8080/Hello in a browser
• This would display the string code within the web application rendering an HTML page

Deployment Diagrams

• Deployment diagrams show the physical hardware where software is deployed
• They portray a system's static deployment view
• They involve nodes and their relationships showing how software is deployed
• They map the software architecture to the physical system architecture, where the software will be executed as a node
• These diagrams utilize communication paths to visualize relationships between nodes

Purpose of Deployment Diagrams

• They show how software is installed on hardware components
• Depict the interaction between software and hardware during execution
• Complement component diagrams by illustrating how components are deployed on hardware

Focus of Deployment Diagram

• They emphasize hardware topology instead of logical components
• The purposes of deployment diagrams are to:
• Envision the system's hardware topology
• Show the hardware components where software components are installed
• Describe node processing at runtime

Deployment Diagram Elements

• Deployment diagrams use these notations:
• Component
• Artifact
• Interface
• Node

Drawing Deployment Diagrams

• Deployment diagrams portray the deployment view and help visualize the system's topology
• They use nodes, which represent physical hardware, to execute artifacts
• Artifact instances can be deployed on node instances
• Deployment diagrams handle:
• High performance
• Scalability
• Maintainability
• Portability
• Understandability

Essential Elements

• Nodes and artifacts are essential for identifying relationships in deployment diagrams
• It becomes easier to develop the diagram if nodes, artifacts, and their relationships are known

iTunes Example

• iTunes setup is downloadable from the website and installable on a computer
• Post-installation, iTunes interconnects with the Apple iTunes store
• Users purchase/download media and cache it in a library

Device Updates

• Devices like Apple iPod Touch and iPhone update their media libraries from a computer via USB
• Devices can download media directly from the Apple iTunes store using Wi-Fi, 3G, or EDGE

Usage of Deployment Diagrams

• Network engineers and system administrators use deployment diagrams
• They represent software deployment on the hardware system
• They show the interaction of software with hardware for execution
• Hardware must be high-quality for efficient software operation and accurate results

Designing Software

• Software applications are complex now and are standalone, distributed or web-based
• Therefore, efficient software design is necessary
• Deployment diagrams model:
• Network and hardware topology
• Distributed networks and systems
• Forwarding and reverse engineering processes
• Hardware details for client/server systems
• Embedded systems