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Objectives Review Servlets Deployment Configuration Sessions, - - PDF document

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Objectives Review Servlets Deployment Configuration Sessions, Cookies Handling multiple requests Apr 29, 2019 Sprenkle - CS335 1 Servlets Review What application do we need to execute servlets? What class do all web

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Objectives

  • Review Servlets
  • Deployment
  • Configuration
  • Sessions, Cookies
  • Handling multiple requests

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Servlets Review

  • What application do we need to execute servlets?
  • What class do all web servlets extend?
  • What methods do servlets need to override to

handle GET and POST requests?

  • How do servlets send an HTML document/response

to the client?

  • How do servlets get data from the client?
  • Put it all together: how do you create a dynamic

web page, i.e., a web page that processes a request from a form?

  • What tricks did you learn to help you with

debugging?

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Example Servlet Flow

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Server Web Application Server Client HTTP Request, data Response HTML Form HTML Document Web Browser s u b m i t doGet

Servlet Development Discussion

  • Distributed applications are difficult to debug

and test

Ø Multiple components: Client code? Server code?

  • Suggestions

Ø Use Eclipse to help you find errors in HTML Ø Check response’s HTML source code

  • Shows you what was written to output
  • Location of error

Ø Use Eclipse’s debugger

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Client Web App Server

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More on Java-based Web Applications

  • Structure
  • Other classes
  • Initialization, customization
  • Synchronization

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Web App Directory Structure

  • projectname

projectname/

Ø HTML, CSS, and JSP files

  • projectname

projectname/WEB /WEB-INF INF

Ø Other resources, e.g., web.xml web.xml

  • projectname

projectname/WEB /WEB-INF/classes INF/classes

Ø Servlet and utility (data structures, etc) Ø Why we put our servlets in servlets package

  • projectname

projectname/WEB /WEB-INF/lib INF/lib

Ø Jar files that application depends on

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  • Different from

Eclipse code organization

  • When Eclipse deploys

the web application, it

  • rganizes it this way.
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Servlet Interface Methods

  • init(ServletConfig

init(ServletConfig config config) ØWeb app server calls once to initialize the servlet ØTypically opening DB connection, files

  • ServletConfig

ServletConfig getServletConfig getServletConfig() () ØReturns a reference to a ServletConfig ServletConfig

  • void

void service(ServletRequest service(ServletRequest, , ServletResponse ServletResponse) ØCalled to respond to a client request

  • String

String getServletInfo getServletInfo() () ØReturns a String that describes the servlet (name, version, etc.)

  • void destroy()

void destroy() ØCalled by the server to terminate a servlet ØShould close open files, close DB connections, etc.

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Servlet Life Cycle in Web Application Server

  • Web application server creates one instance of

servlet

Ø Calls init

init method of servlet created

  • As requests come in, WAS calls service

service method

  • f appropriate servlet

Ø In turn, servlet calls appropriate doMethod

doMethod

  • When web application server shuts down, calls

destroy destroy method of each servlet

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Web Application Server

SurveyServlet Parameter Servlet

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Lab 4: Refactoring SurveyServlet

  • Currently: Inefficient implementation

Ø Read, write survey data file every time request is executed

  • In init

init Ø Automatically called by server on start up Ø Open file, read/initialize votes

  • In destroy

destroy Ø Automatically called by server Ø Write file

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Servlet Servlet Data

  • ServletConfig

ServletConfig – initialization and startup

parameters for this servlet

Ø Example methods:

  • String getInitParameter

String getInitParameter(String name)

  • String

String getServletName getServletName() ()

  • ServletContext

ServletContext – servlet container

information

ØExample methods:

  • Object getAttribute

getAttribute(String name)

  • String getInitParameter

getInitParameter(String name)

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Same method name, different context

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ServletContext ServletContext

  • One ServletContext per

web application per JVM

Ø If you have both Lab3 and FirstServlets running on Tomcat, they will each have their own ServletContext

  • Share state among multiple clients

Ø Allow multiple users to interact in, e.g., chat rooms,

  • nline meeting, reservation systems
  • Info about servlet’s environment

Ø E.g., server’s name

  • log(): method to write to a log file
  • Context attributes

Ø getAttribute, setAttribute, removeAttribute

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web.xml web.xml File

  • Describes how to deploy the web application
  • XML file

Ø Used for data Ø Marked up with elements Ø Same rules as XHTML: close most recently opened tag, attributes in quotes

  • DTD: Document Type Definition

Ø Define elements that can be in a particular XML document Ø Includes specification of attributes, nesting

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<tag attr="value"> Content </tag>

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Annotations

  • In Servlets 3.x, we can easily configure a web

application using annotations

Ø Don’t need to directly update web.xml Ø Provide defaults, can be overridden in web.xml

  • Example:

Ø Means the URL pattern “/SurveyServlet” maps to this servlet (servlets.SurveyServlet)

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@WebServlet("/SurveyServlet") public class SurveyServlet extends HttpServlet {

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Add init parameters

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Another Annotation Example

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@WebServlet( urlPatterns = { "/SurveyServlet" }, initParams = { @WebInitParam(name = "surveyFile", value = "survey.dat") }) public class SurveyServlet extends HttpServlet {

Default values Can override these in the web.xml

Why would we want to be able to

  • verride these values in a separate (text) file?

Why web.xml overriding?

  • Can modify behavior of application without

modifying the Java code and recompiling

Ø May not have access to source code

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web.xml web.xml File

  • Top-level: <webapp

webapp>

  • <servlet

servlet> element describes a servlet

  • <servlet

<servlet-mapping> mapping> element maps URLs to servlets

Ø May want to have shorthands, aliases Ø Restrict users’ direct access to servlets

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web.xml web.xml File: Subelements of <servlet> <servlet>

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<servlet servlet-name> name>

canonical name of the deployed servlet

<servlet servlet-class> class>

fully qualified class name of the servlet

<init <init-param param>

  • ptional parameter containing a name-value

pair that is passed to the servlet on initialization. Contains elements, <param-name> and <param-value>, which contain the name and value, respectively, to be passed to the servlet.

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Example of Configuring web.xml

  • Configure SurveyServlet to use a given file
  • Add the following to web.xml file:
  • Note that <init-param> is a child of

<servlet>, which means your web.xml file

would look like what?

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<init-param> <param-name>surveyFile</paramname> <param-value>survey.dat</param-value> </init-param>

Note about init-params in web.xml

  • If you set init-param in web.xml, you need to

annotate the servlet with its name

  • You can have multiple configurations for the

same [servlet] class

Ø Using the name lets the application server know that the annotations and the web.xml configurations are both part of the same configuration

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Using Init Parameter

  • Configure SurveyServlet to use a given file

Ø Either in annotation or web.xml

  • Modify init method to call HttpServlet

HttpServlet’s getInitParameter getInitParameter method

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// calls HttpServlet method, i.e., this’s method filename = getInitParameter("surveyFile"); // open file …

MAINTAINING STATE ACROSS REQUESTS

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Maintaining State

  • If you have multiple pages, how can you save or

accumulate data?

Ø Example scenario: buying a book

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Login Form Which Book Credit Card Verify Server Server Server Server

D1 D1 D1, D2 D1, D2 D1, D2, D3 D1, D2, D3 D1, D2, D3, D4

Maintaining State

  • If you have multiple pages, how can you save or

accumulate data?

Ø Hidden fields (type=hidden type=hidden) Ø Cookies Ø Sessions

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Login Form Which Book Credit Card Verify Server Server Server Server

D1 D1 D1, D2 D1, D2 D1, D2, D3 D1, D2, D3 D1, D2, D3, D4

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Hidden Fields

  • Data is coming from client
  • Users can see the hidden fields

Ø View HTML Source

  • Users can change the data

➨ Useful in limited situations

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<input type="hidden" name="userid" value="superfly"/>

COOKIES

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Cookies

  • Cookies are initially sent from the webapp to the

client to store application-specific information on the client

  • Part of an HTTP header in response to a client

Ø Every HTTP transaction includes HTTP headers Ø Not part of the HTML content

  • Client includes cookies in HTTP headers in

subsequent requests

Ø Provides way to do behavior tracking

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Process with Cookies

  • Cookies

Ø Associated with server name Ø Part of HTTP Headers

  • Example: Amazon.com

Ø Cookie stores your name, login information Ø Example: Not Sara?

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Client Web App Server Web Browser HTTP Request, Cookies Response: Cookies HTML Document

Cookies

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Cookies in Java

  • Cookies have a name and value
  • Create a Cookie object using its constructor

ØPart of javax.servlet.http.Cookie javax.servlet.http.Cookie

  • Example: store a user’s preferred language on

the client

ØApp only has to ask for this information once

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String cookie_name = "pref_language"; String cookie_value = "English"; Cookie new_cookie = new Cookie(cookie_name, cookie_value);

Sending the Cookie to the Client

  • HTTP header is sent first
  • Cookie(s) must be added to the response object

before you start writing to the client

  • Call addCookie

addCookie() on HttpServletResponse HttpServletResponse

  • bject before you call the getWriter

getWriter() method

  • Inside of doGet

doGet or doPost doPost method:

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Cookie c = new Cookie( "pref_language", "English" ); c.setMaxAge(60*60*24*365); // max age of cookie response.addCookie(c); …

  • utput = response.getWriter();
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HttpServletResponse HttpServletResponse Method

  • void

void addCookie(Cookie addCookie(Cookie)

ØAdd a Cookie to the header in the response to the client ØThe cookie will be stored on the client, depending on the max-life and if the client allows cookies

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Cookies: Maximum Ages

  • The maximum age of the cookie is how long the

cookie can live on the client, in seconds

  • When a cookie reaches its maximum age, client

deletes it

  • -1 means persists until browser exits

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c.setMaxAge(60*60*24*365); // max age of cookie

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Retrieving Cookies

  • Call getCookies

getCookies on HttpServletRequest HttpServletRequest

  • bject

ØReturns an array of Cookie objects ØRepresents all cookies that server previously sent to the client

  • For example, inside of doPost

doPost

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Cookie[] cookies = request.getCookies();

Voiding Cookies

  • May want to delete cookies when user logs out

Ø Especially for sensitive information

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// void cookie and send back to the user userid_cookie.setMaxAge(0); response.addCookie(userid_cookie);

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Why Are They “Cookies”?

  • Http Cookie, Source: Wikipedia

Ø The term "cookie" derives from "magic cookie", which is a packet of data a program receives and sends out again unchanged.

  • Magic Cookie, Source: Wikipedia

Ø The name "cookie" comes from a comparison to an unopened fortune cookie, because of the hidden information inside.

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What are challenges with using cookies?

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What are challenges with using cookies?

  • They are saved on the client machine

Ø Clients can delete or modify them

  • Increase the sizes of your network packets

Ø Send cookies on each request

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Client Web App Server Web Browser HTTP Request, Cookies Response: Cookies HTML Document

Cookies

SESSION STATE

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Session

  • One user’s visit to an application
  • Can be made up of many requests
  • Server maintains a session with a particular

client

Ø Can maintain state within that session

  • Duration of a session:

Ø If no requests from client for specified period of time (the timeout), user’s session ends Ø Timeout: typically 30 minutes

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Web Application Server

Benefits of Using Session State

  • Simpler for developer
  • Reduces network traffic

Ø Don’t need to keep passing data between client and server

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Server Web Application Server

User A’s Session State User B’s Session State User C’s Session State

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Session State in Java

  • HttpSession stores session data
  • Data is known as session attributes

Ø Have names and values

  • Store, access, and remove attributes:

Ø Like a HashMap Ø void setAttribute(String name, Object value)

  • Values no longer need to be strings
  • Cookies and Parameters had to be strings

Ø Object getAttribute(String name) Ø void removeAttribute(String name)

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Example Session Variables

  • User gives application data
  • Application stores data in session variable

Ø session.setAttribute("username", username);

  • Application can use later in session, without user having

to give information again

Ø String username = (String) session.getAttribute("username");

  • More examples:

Ø Server computes information once, caches in session Ø Shopping carts

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name value

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Getting a Session

  • HttpServletRequest

HttpServletRequest’s getSession getSession(boolean boolean create) create) method

Ø Returns the current HttpSession HttpSession object Ø Boolean parameter specifies if a new session should be created if one does not already exist

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Other Useful Session Methods

  • setMaxInactiveInterval(),

getCreationTime(), getLastAccessedTime() Ø If want shorter than server’s timeout

  • invalidate()

Ø Invalidates session, unbinds objects bound to it

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Lab 4: Add Session Variable

  • LoginServlet will add a session variable with

name “authenticated”

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Eclipse Development Hints

  • Safe bet: restart server whenever change to a

servlet

Ø Can modify Server’s configuration

  • Under Publishing
  • Typical programming

Ø Write a few lines of code/make small changes Ø Run, test Ø Repeat

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HANDLING MULTIPLE REQUESTS

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Multiple Clients

  • Web server handles multiple requests at a time

by executing multiple threads

  • Approximately 1 thread/request

⇒Need to make sure that threads overlap in

ways that do not break the application

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Example Scenario

  • Original SurveyServlet

SurveyServlet stores the results

  • f the survey in a file on the server
  • When >1 client connects to the server at one

time, server handles both clients concurrently

Ø >1 can execute SurveyServlet SurveyServlet Ø >1 thread can read/modify file at one time Ø Can lead to inconsistent data!

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SurveyServlet Implementation

  • Operations can overlap

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SurveyServlet?animal=dog SurveyServlet?animal=cat Web Application Server file // read file // update local array // write file // print results // read file // update local array // write file // print results

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Bad Interleaving

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// read file // update local array // read file // update local array // write file // print results // write file // print results What happens in this case? Loses blue’s vote SurveyServlet?animal=dog SurveyServlet?animal=cat Web Application Server file

Critical Section

  • Sections of code that have to happen

uninterrupted or atomically

Ø Only one thread can execute at a time

  • What is the critical section in this code?

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// read file // update local array // write file // print results

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Critical Section

  • Sections of code that have to happen

uninterrupted or atomically

Ø Only one thread can execute at a time

  • What is the critical section in this code?

Ø The shared file must be read and written atomically

  • Writes cause trouble

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// read file // update local array // write file // print results

210 in 335

  • Even if only one Java statement in critical

section, synchronize it!

  • One high-level Programming Language

statement probably translates into multiple VM language statements

Ø Prevent interruption at low level

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count++; Retrieve count Add 1 to count Store count High-level: Virtual Machine level:

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Synchronization Mechanisms

  • Synchronized classes
  • Synchronized methods
  • Synchronized statements
  • Expense associated with each of these

Ø But without it, get wrong or inconsistent answers!

  • Alternative: database can handle synchronization
  • n data for you

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Project Next Steps

  • Deadline: Tuesday at midnight
  • Static Mockups

Ø Discuss use cases Ø Discuss any issues that aren’t clear/different visions

  • Revise Requirements

Ø Based on feedback

  • Set up project source code

Apr 29, 2019 Sprenkle - CS335 57

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TODO

  • Lab 4: Servlet Configuration and Session State

Ø Create a GitHub account and email to me Ø Init, destroy methods Ø Configuration parameters Ø Session state

  • Tomorrow: Static Mockups
  • Read/Summarize Quality Attributes paper by

Wed, midnight

Ø See Sakai description for details about contents

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