Java EE Training

8/9/2019 Java EE Training

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Agenda

Implementing session tracking from scratch

Using basic session tracking

Understanding the session-tracking API eren a ng e ween server an rowser

sessions

Storing immutable objects vs. storing

Tracking user access counts

Accumulatin user urchases Implementing a shopping cart

Building an online store5

2010 Marty Hall

Overview

Customized Java EE Training: http://courses.coreservlets.com/Servlets, JSP, JSF 2.0, Struts, Ajax, GWT 2.0, Spring, Hibernate, SOAP & RESTful Web Services, Java 6.

Developed and taught by well-known author and developer. At public venues or onsite at yourlocation.6


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Rolling Your Own Session

IdeaClient appends some extra data on the end of each URL

that identifies the session

about that session

E.g., http://host/path/file.html;jsessionid=1234

AdvantageWorks even if cookies are disabled or unsupported

sa van ages

Must encode all URLs that refer to your own site

Fails for bookmarks and links from other sites

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Rolling Your Own Session

Idea:

Advantage

Works even if cookies are disabled or unsupported

DisadvantagesLots of tedious processing

All pages must be the result of form submissions

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Tracking API



Session Tracking Basics

Access the session objectCall request.getSession to get HttpSession object

This is a hashtable associated with the user

session.

Call etAttribute on the Htt Session ob ect cast thereturn value to the appropriate type, and check whetherthe result is null.

.Use setAttribute with a key and a value.

.Call removeAttribute discards a specific value.

Call invalidate to discard an entire session.12


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Session Tracking Basics:

HttpSession session = request.getSession();

SomeClass value =

(SomeClass)session.getAttribute("someID");==

value = new SomeClass(...);session.setAttribute("someID", value);

doSomethingWith(value);

}

Do not need to call setAttribute again (after modifying value) if the modified. , ,

new object reference, and you must call setAttribute again. However, callsetAttribute every time if you want to support distributed sessions (where asingle app is distributed across multiple nodes in a cluster).

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To Synchronize or Not to

The J2EE blueprints say not to botherThere are no race conditions when multiple different

users access the page simultaneously

,same user to access the session concurrently

The rise of Ajax makes synchronizationimportantWith Ajax calls, it is actually quite likely that two

Performance tip

Use the session or perhaps the value from the session asthe label of the synchronized block

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What Changes if Server Uses

Session tracking code:No change

Code that generates hypertext links back to

Pass URL through response.encodeURL. If server is usin cookies this returns URL unchan ed

If server is using URL rewriting, this appends the sessioninfo to the URL

. .String url = "order-page.html";url = response.encodeURL(url);

Pass URL through response.encodeRedirectURL

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HttpSession Methods

getAttributeExtracts a previously stored value from a session object.

Returns null if no value is associated with given name.

Associates a value with a name. Monitor changes: values

implement HttpSessionBindingListener. removeAttribute

Removes values associated with name.

getAttributeNamesReturns names of all attributes in the session.

geReturns the unique identifier.

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HttpSession Methods

isNewDetermines if session is new to client(not to page)

getCreationTime eturns t me at w c sess on was rst create

getLastAccessedTime

getMaxInactiveInterval, setMaxInactiveIntervalGets or sets the amount of time session should o without

access before being invalidated

invalidate Invalidates current session

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2010 Marty Hall

Storing Simple Values




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A Servlet that Shows Per-Client

public class ShowSession extends HttpServlet {public void doGet(HttpServletRequest request,

HttpServletResponse response)

throws ServletException, IOException {response.setContentType("text/html");= .

synchronized(sesssion) {String heading;Integer accessCount =(Integer)session.getAttribute("accessCount");

if (accessCount == null) {accessCount = new Integer(0);

" ",} else {heading = "Welcome Back";accessCount =

new Integer(accessCount.intValue() + 1);}session.setAttribute("accessCount", accessCount);

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PrintWriter out = response.getWriter();out.println(docType +"\n" +"" + title + "\n" +"\n" +"\n" +" " " ""Information on Your Session:\n" +"\n" +"\n" +" Info TypeValue\n" +" Number of Previous Accesses\n" +" " + accessCount + " n" +"\n" +"");

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Storing Lists of Values



Aside: Compilation Warnings re

HttpSession does not use genericsSince it was written pre-Java5. So, following is illegal:

HttpSession session =request.getSession();

Typecasting to a generic type results in acompilation warning

p ess on sess on = reques .ge ess on ;

List listOfBooks =

(List)session.getAttribute("book-list");

Still compiles and runs, but warning is annoying

You can su ress warnin sPut the following before line of code that does typecast:

@SuppressWarnings("unchecked")24


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Accumulating a List

public class ShowItems extends HttpServlet {

ublic void doPost Htt ServletRe uest re uest

HttpServletResponse response)

throws ServletException, IOException {HttpSession session = request.getSession();

synchronized(session) {

@SuppressWarnings("unchecked")

List previousItems =

" ".

if (previousItems == null) {

previousItems = new ArrayList();

session.setAttribute("previousItems", previousItems);

}

String newItem = request.getParameter("newItem");

if ((newItem != null) &&newI em. r m .equa s ""

previousItems.add(newItem);

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Accumulating a List

response.setContentType("text/html");PrintWriter out = response.getWriter();String title = "Items Purchased";String docType ="\n";

ou .pr n n oc ype"\n" +"" + title + "\n" +"\n" +" " " "

if (previousItems.size() == 0) {out.println("No items");

} else {out. rintln ""for(String item: previousItems) {out.println(" " + item);

}out.println("");

}out.println("");

}}}26


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Accumulating a List

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Accumulating a List

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Advanced Features



Distributed and Persistent

Some servers support distributed Web apps oa a anc ng use o sen eren reques s o eren

machines. Sessions should still work even if different hosts are hit. On some servers, you must call setAttribute to trigger replication

,you better load balancing

Some servers suport persistent sessions

(as long as browser stays open). Very important for web4!

Tomcat 5 and 6 support this

,the java.io.Serializable interface There are no methods in this interface; it is just a flag:

pu c c ass y ess on a a mp emen s er a za e...

}

Builtin classes like String and ArrayList are already Serializable30


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Letting Sessions Live Across

IssueBy default, Java sessions are based on cookies that live in

the browsers memory, but go away when the browser isclosed. This is often but not alwa s what ou want.

SolutionExplicitly send out the JSESSIONID cookie.

Do this at the beginning of the users actions

Call setMaxAge first

Using a cookie with a large maxAge makes no senseunless the session timeout (inactiveInterval) is also large

An overly large session timeout can waste server memory

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An On-Line Bookstore

Session tracking code stays the same as insimple examples

Shopping cart class is relatively complex ent es tems y a un que cata og

Does not repeat items in the cart

Instead each entr has a count associated with it If count reaches zero, item is deleted from cart

Pages built automatically from objects thatave escr p ons o oo s

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Wrap-up



Summary

Sessions do not travel across networkOnly unique identifier does

Get the session request.get ess on

Extract data from session .

Do typecast and check for null

If you cast to a generic type, use @SuppressWarnings

Put data in session session.setAttribute

us om c asses n sess onsShould implement Serializable

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Questions?



Documents

Java EE Training