Head First Servlets and JSP phần 3 docx

by more than one thread running a service method of Servlet A.

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Thread C

But you wonÕt do anything

to stop OTHER servlets!

Regardless of whether the service methods in other servlets are synchronized

or not it still means other parts of the app have access to the context attributes.

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Since we have the contex

t lock, weÕre assuming tha t once we get inside the synchronized block, the contex

t attributes are safe from other thr eads until we exi

t the block sort of Saf

e means Òsafe from any other code that ALSO synchronizes on the ServletContex

t.Ó But this is the bes

t youÕve got for making the context attributes as thr ead-safe as you can.

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it wonÕt do any good to synchronize on the Servlet, because other parts of the app will still be able to acc ess the context!

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This is a relative path (because ther eÕs

no initial forward slash (Ò/Ó)) So in this case, the Container looks f or Òresult.

jspÓ in the same logical loca tion the request is ÒinÓ (WeÕll cov er the details

of relative paths and logica l locations in the Deployment chapter.)

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(Notice that the only differenc e between these and the attribute listener s is the word ÒAttributeÓ inserted in the inter face name.)

attributeAdded() attributeRemoved() attributeReplaced()

when the session is created, and when its destroyed sessionCreated()

Lifetime of the ServletContext, which means life of the deployed app If server or app goes down, the context is destroyed (along with its attributes).

Resources you want the entire application to share, including database connections, JNDI lookup names, email addresses., etc.

Any part of the application that has direct access to the Request object That mostly means only the Servlets and JSPs to which the request is forwarded using

a RequestDispateher Also Request-related listeners.

The life of the Request, which means until the ServletÕs service() method completes In other words, for the life of the thread (stack) handling this request.

Passing model info from the controller to the view

or any other data speciÞc

to a single client request.

(NOT thread-safe!)

(Thread-safe)

+WWS6HVVLRQ Any servlet or JSP with

access to this particular session Remember, a session extends beyond a single client request to span multiple requests by the same client, which could go to different servlets.

The life of the session A session can be destroyed programmatically or can simply time-out (WeÕll

go into the details in the Session Management chapter.)

Data and resources related

to this clientÕs session, not just a single request

Something that requires an ongoing conversation with the client A shopping cart

is a typical example.

(NOT thread-safe!)

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-Option B is incorrect because ServletContext init parameters are only read at Container start-up time.

-Option E is incorrect because there is only one ServletContext object per web application

-An IllegalStateException is caused when a response has already been ÔcommittedÕ to the client (the ßush method does that), and then you attempt a forward.

(Servlet v2.4 pg 167)

-Option B is the correct method

With it you can access the container populated javax.servlet.forward.Xxx and javax.servlet.include.Xxxx attributes.

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-Options C and D refer to methods that donÕt exist.

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-Option E is incorrect because a ServletContextListener would be used for this purpose

-Option A is incorrect because these is no ServletResponseListener interface.

-Option B is incorrect because such data is typically either immediately returned or stored in request scope.

-Option C is incorrect because (since it

is not speciÞc to a particular session) it should be stored in context scope.

-Option E is incorrect because servlet context parameters should stay with the ServletContext object

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-Option E is invalid because there is no such type.

-Options A, B, and C are invalid because these types do not store attributes.

-Option D is incorrect because a servlet can invalidate a session using the HttpSession.invalidate() method

-Option C is incorrect because such parameters are forwarded in this case.

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<load-on-startup>

deployment-descriptor element determines the order of instantiation, not the number of instances.

(Servlet spec p 27)

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to give Þnal advice), and if it doesnÕt, it has to give back the next question to ask the user

We check the one incoming parameter (color) and give back the Þnal response (an array of brands that Þt that color)

This isnÕt very smart advice

Assume the NextResp onse class encapsulates the next thing to displ ay for the user, and something that indica tes whether itÕs the Þna

l advice recommendation or another ques tion.

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Then we instantia ted and used the MODEL t

o get the info we need f

or the VIEW.

Then we... data-page="14">

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ServletContextListener... chapter.)

Question for you which part of the DD does the <lis

tener> element

go into? Does it go into a <servlet>

element,