Web Applications

1. Web Applications Web Programming Spring2019 Bahador Bakhshi CE & IT Department, Amirkabir University of Technology

2. Questions • Q8) How are complex/big web applications developed? • Q8.1) Design patterns & Architecture? • Q8.2) Deployment architecture? • Q8.3) What does Java EE provide? • Q8.4) Servlet? • Q8.5) JSP?

3. Outline • MVC Design Pattern • Multilayer Design • Multitier Architecture • Introduction to Java EE • Servlet • JSP • MVC by Servlet & JSP & Beans

4. Outline • MVC Design Pattern • Multilayer Design • Multitier Architecture • Introduction to Java EE • Servlet • JSP • MVC by Servlet & JSP & Beans

5. Web Application Development • (Large) Applications cannot be developed in ad-hoc manner • We need design & architecture (SW engineering) • Concept separation, Component, Relations, … • There are so many architectural patterns • ETL, MFT, EAI, TDS, … • http://en.wikipedia.org/wiki/Architectural_pattern • Two common architectures in web applications • MVC • Multi-layer (Multi-tier)

6. What is Design Pattern • A general & reusable solution to commonly occurring problems in the design & development of software • A template for how to solve a problem that has been used in many different situations • Many similar instances of it have been solved • Not a complete finished design • A pattern must be adapted to your requirements • Cannot simply translated in to

7. MVC: Model-View-Controller • Model • Does all the computational work • All communication with the model is via methods • It is input/output free, contains data, system state • Controller (of states sequences) • Handles events/requests affecting model or view • User inputs/requests go to the controller • Available commands are determined by controller (based on model) • Tells the model what to do • View • Show the results from model or controller • Examples: Calculator, Email Client, Word Processor

8. Advantages of MVC • Separation of concerns • Computation is not intermixed with I/O •  Code is cleaner and easier to understand • Flexibility • The GUI (if one is used) can be completely revamped without touching the model in any way • Reusability • The same model used for different controller & view

9. MVC Interactions?!

10. MVC Interactions (cont’d) • Controller  View • Commands to change the view's presentation of the model • Controller  Model • Commands to update the model's state • Model  View & Controller • Notifies when there has been a change in its state • View produces updated output • Controller changes the available set of commands • View requests  Model • The information that it needs to generate an output representation to the user

11. MVC Interactions (cont’d) model controller updates model and/or view view queries model model signals changes view controller controller updates model and/or view event event is passed to the controller

12. MVC for (simple) Web Applications • Model • Database tables (persistent data) • Session information (current stat data) • Parts of processing • View • (X)HTML • CSS • Controller • Client-side scripting • Part of processing by server side scripting

13. MVC? • MVC works fine for desktop application • The origin is from graphical application development 1970s • Consider large & distributed web application, e.g., online transactions, e-banking, … • View  Model interactions?!! • View is in user’s browser • Model is in back-end server • User  Controller interaction? • User don’t have direct access to controller • Complicated processing on huge data • Model cannot both hold data & process it

14. Outline • MVC Design Pattern • Multilayer Design • Multitier Architecture • Introduction to Java EE • Servlet • JSP • MVC by Servlet & JSP & Beans

15. Layering Approach • MVC Difficulties are due to triangular topology • Linearize the topology  Layering approach • Common layering in web applications • Presentation Layer • Business logic Layer • Data (management/resource) Layer • These layers are purely abstractions • Not correspond to physical distribution • All layers may or may not be on the same machine

16. Multilayer Architecture

17. Presentation Layer • User interface of the application • GUI • HTML based browser • Displays information (processing output) which are get from the business logic layer • Gets user’s requests and pass them (with required data) to the business logic layer • The user interface of web applications can be made up client side & server side codes • What is this layer in Gmail?

18. Business Logic Layer • The work that the application needs to do • Processing/Computing/Calculations • Receives requests from presentation layer • Fetches required data from data layer • Process the request & data • Output is sent back to the presentation layer • What does this layer do in Gmail?

19. Data Layer • Provides data access interface to business logic • Hide data storage details • Hide heterogeneity of underlining systems • Communicating with data store systems • Database server • Messaging system • What is this layer in Gmail?

20. MVC vs. Multilayer • Which one? • Complexity of application • Fairly simple  MVC • Very large & distributed  Multilayer • These architectures are not mutual exclusive • These can (usually) be combined in large applications • UI is developed by MVC • View: HTML (browser) • Controller: Server side scripting • Model: Output of Business logic

21. Outline • MVC Design Pattern • Multilayer Design • Multitier Architecture • Introduction to Java EE • Servlet • JSP • MVC by Servlet & JSP & Beans

22. Multilayer  Multi-tier • Should/Can all layers be implemented in a single physical machine (single tier)? • Yes, If application is (fairly) simple  • Web server, HTML, CSS, PHP, Processing, MySQL, … all on the same machine • No, If application is huge & complex  • Web server on machine 1 • Data base on machine 2

23. Multitier Architecture • Two-tier Architecture: • Two layers are physically separated from the third, forming two physically separated tiers • Three-tier Architecture: • The three abstract logical layers are separated into three physically distributed tiers • N-tire Architecture: • Each abstract logical layer is implemented using multiple physical tiers

24. Two-Tier 1: Fat client • Combining the presentation layer with the business logic layer in a physical tier • This separation can be seen as the traditional client-server architecture • Client = Process + View • Server = Data • This architecture effectively makes the client “fat” and the server “thin”

25. Two-Tier Fat Client Characteristics • Deployment costs are high • Many fat clients • Business logic migration or data base modification (changes in database driver, database type, …) costs are high • Changing the business logic layer involves recompiling and redeploying the client tier • Network performance suffers • Huge raw data transfer (query & response)

26. Two-Tier 2: Thin Client • client (browser) • HTTP request • Mainly for presentation of information • Serving mainly static (D)HTML pages • Server (web server & DB server) • HTTP response • Server side programming: CGI & PHP, … • Data base server: MySql • The suitable architecture for web applications • Complex computing on huge data?!!!

27. N-Tier Architecture • In N-tier deployments, presentation layer, business logic layer, and data layer are separated into respective physical tiers • 3 tier: client + server + data base • Presentation layer is implemented by parts in both client & server sides • E.g., dynamic web page using AJAX + PHP • 4 tier: Browser + Web server + Application Server + Database server • Complicated Bussing logic layer itself can be distributed multi-tier application  N-tier

28. HTML Client (browser) Typical Web Application N-tier Architecture Web Server Application Server Database Server

29. N-Tier Architecture Characteristics • Migration costs are low • Business logic application migration • Database switching • Web server switch • OS upgrade • Each tier can be upgraded independently • Communication performance suffers • Maintenance costs are high

30. Multilayer/Multitier Implementation • Many common requirements in applications • Object creation & garbage collection, Transaction Logging and audit, Security, and much more • These are not implemented by neither OS nor Application developer • They are called middleware • Application servers provide middleware services • Application components live inside application servers

31. Application Servers • Existing technologies can be classified into three broad categories • Java based platform (Java Enterprise Edition) • .NET Framework • Other web application development frameworks • PHP frameworks: Zend, … • Ruby on Rail • …

32. Outline • MVC Design Pattern • Multilayer Design • Multitier Architecture • Introduction to Java EE • Servlet • JSP • MVC by Servlet & JSP & Beans

33. What is Java EE? • Java Platforms • Java Card: Smart card version • Java ME (Micro Edition): Embedded systems, e.g. Mobile handheld • Java SE (Standard Edition): Desktop application development • Java EE (Enterprise Edition): Enterprise distributed application software development • Java EE add libraries to SE for fault-tolerant, distributed, multi-tier, … based components • Until java 5, it has been called J2EE

34. Java EE • Java EE provides technologies (libraries) for enterprise level web applications • Java EE technologies for web applications • Servlet • JavaServer Pages • JavaServer Faces • Java Enterprise Beans • Many other libraries • Remote method invocation, Security, Database connectors, XML, …

35. Java EE Components: Business Logic • EJB: Enterprise version of Java Beans • Java classes that are designed to be reusable standalone software components • All data (properties) are private • Exposed by public getter & setter method • Public method to process the data • Enterprise version: automated (provided by container) concurrency, life-time management, security, transaction, … • Simplify developing complex application • Can be used everywhere, not necessarily web application • Beyond the scope of this course • Our application are simple enough to be implemented without using EJB

36. Java EE Components: Presentation • Client side • Client can use HTML, Java Applet, Java Application, … • Server side • Servlets are special classes to realize request-response model (get, post,… of HTTP) • External server side code • JSP is a developer-friendly wrapper over servlet classes • Embed server side code • Faces & Faceletssimilar to JSP but uses custom tags which can be converted to anything • Beyond the scope of this course

37. Java EE Multi-tier Architecture

38. Java EE Containers • Requirements from different aspects • All java codes must run a virtual machine • To run Servlet & JSP we need something like PHP runtime environment • Application server (middleware) is need in business logic • These different terms are named container in Java EE terminology • The runtime support for Java EE applications components • Provides a view of the underlying Java EE API • Security, Logging, Remote Method Invocation

39. Java EE Container Architecture Tomcat, JRun, … GlassFish, BEA, … Browser Web Container Applet Container EJB Container Java Servlets Applet EJBs JSP Application Client Container JSF App.Client JRE Databases and Other Resources

40. Outline • MVC Design Pattern • Multilayer Design • Multitier Architecture • Introduction to Java EE • Servlet • Introduction • JSP • MVC by Servlet & JSP & Beans

41. What is Servlet?! • A Java application that runs on the web server in response to HTTP requests (GET, …) • A normal java application + • Read/Write access to HTTP headers • Access to HTML Forms • Sessions, Cookies, … • Similar to CGI, output is sent to client • Servlet is used to generate dynamic content to return to browser: HTML, XML, … • Each servlet runs as separated thread inside servlet container

42. The Servlet Model HTTP Get/Post Browser Java Enabled Web Server Servlet container Servlet Servlet HTML XML Resources JavaBeans Database

43. What does Servlet Container do? • Maintains mapping between URLs & Servlet Classes • Processes HTTP request and extracts information (e.g. headers, cookies, session, …) and provides them through implicit classes for servlets • Loads servlet when its corresponding URL is requested for the first time • Runs servlet each time the URL is requested • A thread per request • Captures the output and sends back them to client • HTML page is produced by print statements out.println("<html>"); …

44. Why Servlet: Servlets vs. CGI Scripts • Servlets do what CGI scripts do; so, why should I learn/use it? Because, it is better approach • More efficient • No new process per request (as CGI), with servlets, servlet is loaded at first time, container stays running and handles each request with a lightweight Java thread • More Convenient • Servlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting HTTP headers, handling cookies, tracking sessions, …

45. Why Servlet: Servlets vs. CGI Scripts • More Portable • Servlets are written in the Java programming language and follow a standard API. Servlets are supported directly or by a plugin on virtually every major web server • More Secure • CGI programs are often executed by general-purpose operating system shells, however, servlets run in container which impose more security enforcement (sand boxing) • More powerful • It is java & its container provide mechanisms to use other java application, e.g. EJB  multi-layer architecture

46. Why Servlet: Its Role in Java EE

47. How does Servlet Work? Assumption: servlet is already loaded in RAM • There is a mapping between URL & Servlet classes, e.g. /1/2/3  abc.class • The “web.xml” file (we see later) • When there is a request for URL (/1/2/3/), container calls servicemethod of the class (abc.class) • Multiple concurrent requests  Multiple threads running the servicemethod • The service method calls do??? method according to the type of requests • E.g., doGetfor GET method

48. How does Servlet Work? (cont’d) • The method accesses to the request information • HTTP headers, form data, cookies, sessions, … • Through objects which are passed to the method by container • The method processes the request • Normal java application, DB access, using EJB, … • The method generates output • Setting response headers, setting cookies, updating session, printing HTML to output stream, … • Using the objects which are provided by container

49. Servlet Life Cycle • Creation: Upon the first request for its URL • Single instance created • init() method called • init()is NOT called again on further requests • Service: On each request for it • Spawns a new thread and calls service() • service() checks HTTP request type and calls appropriate do??? (Get, Post, Put...) • TRACE & OPTION is implemented, HEAD is use header of doGET • Destroy: Removing from memory because of e.g., servlet idle, … • destroy() allows you to close DB connections, wrap up, etc... • Don't count on destroy to write persistent state

50. Outline • MVC Design Pattern • Multilayer Design • Multitier Architecture • Introduction to Java EE • Servlet • Implementation • JSP • MVC by Servlet & JSP & Beans