Mistake in Head First Servlets & JSP

[Ericson James]

Hi,

I'm currently preparing for my SCWCD exam with a copy of OReilly - Head First Servlets And JSP - 2nd Edition (Mar 2008).

I'm hoping someone can check up what I THINK is a mistake on page 712:

The filter mappings which use <servlet-name> have urls in them. Shouldn't these be names of servlets??

I can't see it mentioned in the books online errata.

Can you also have multiple <url-patterns> or <servlet-names> as of Servlet Spec 2.5. Is this true also for the SCWCD 083 exam?

Thanks for your Help
Eurig

[Charles Lyons]

For those of us who don't own the book it would be handy if you could supply a slightly more elaborated description of the problem.

However, <servlet-name> elements in <servlet-mapping> and <filter-mapping> should always contain a contents of a corresponding <servlet-name> in a <servlet>. So from what you say alone, I believe it is indeed a mistake.

Can you also have multiple <url-patterns> or <servlet-names> as of Servlet Spec 2.5. Is this true also for the SCWCD 083 exam?

You are certainly correct that Servlet 2.5 spec allows multiple <url-pattern>s and <servlet-name>s in a single <filter-mapping>, and that only one of these may have been used in v2.4 and earlier. An educated guess would tell me that only one element is ever going to appear in a <filter-mapping> in the exam, so you won't be asked peculiarities about how many elements are allowed - so it shouldn't be a problem as far as the exam is concerned. A more typical question would be to ask you to structure a <filter-mapping> tag via drag-and-drop, and there will only be one correct answer (hence can only be either a <url-pattern> or a <servlet-name> to be used).

Hope that helps.

[Ericson James]

It helps a lot Charles. Thanks.

And as you mentioned. For those of you who don't own the book. It goes a bit like this:

<filter-mapping>
    <filter-name>Filter1</filter-name>
    <url-pattern>/Recipes/*</url-pattern>
</filter-mapping>
 
<filter-mapping>
    <filter-name>Filter2</filter-name>
    <servlet-name>/Recipes/HopsList.do</servlet-name>
</filter-mapping>
 
<filter-mapping>
    <filter-name>Filter3</filter-name>
    <url-pattern>/Recipes/Add/*</url-pattern>
</filter-mapping>
 
<filter-mapping>
    <filter-name>Filter4</filter-name>
    <servlet-name>/Recipes/Modify/ModRecipes.do</servlet-name>
</filter-mapping>
 
<filter-mapping>
    <filter-name>Filter5</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Write down the sequence in which the filters will be executed for each request path:

/Recipes/HopsReport.do
/Recipes/HopsList.do
/Recipes/Modify/ModRecipes.do
/HopsList.do
/Recipes/Add/AddRecipes.do
[ August 23, 2008: Message edited by: Eurig Jones ]

[Charles Lyons]

It could be valid - if the servlets in question have a <servlet-name> with the URL-like value; for example:

<servlet>
  <servlet-name>/Recipes/Modify/ModRecipes.do</servlet-name>
  <servlet-class>recipes.ModRecipesServlet</servlet>
</servlet>

I imagine that would be valid under the Servlet spec., but is unlikely to ever be used even so...

[Ericson James]

Good point.

Then I guess it's not an error but just a very evil question which makes you think its actualy a mistake and not just a trick question!

[Rufus Addis]

IMHO it probably is an unintentional mistake on behalf of the author of the book.

However it's *VERY* good practice for the actual exam as it encourages you to read the question slowly and carefully rather than skim it and make assumptions.

My biggest failing when I took the exam was skim a question, think I knew what knowledge it was getting me to demonstrate, and plump for an answer. I am sure I would have got a higher pass score if it'd actually slowed down, read, and re-read!

Rufus.

[Misho Chigrichenko]

to Eurig Jones:

I thought there is a mistake too, but then, just above the excercise I saw this text:

Assume Filter1 through Filter5 have been properly declared, and that the servlet names are the same as their mappings.

So remember to re-read the questions on the exam and good luck!

[Misho Chigrichenko]

In fact I forgot to share something that I see as a mistake in the book. Of course maybe I am wrong, too...

Look at the code of the

CompressionResponseWrapper 

that overrides the

getWriter()

and

getOutputStreamWriter()

methods of

HttpResponseWrapper

. Sorry, I won't paste it here, because its lengthy, so only the people who have the book could understand me...

1) As I am aware,

ServletResponse.getWriter()

should not be called after

ServletResponse.getOutputStream()

and vice-versa. If this is done, the caller should get an

IllegalStateException

...

2) The IF in method

getOutputStream()

that checks whether to throw an

IllegalStateException 

is:

if((streamUsed != null) && (streamUsed != pw)), then throw the ex

. But if you have already called

getWriter()

, you have set the

streamUsed

member var to point to exactly pw which is an instance of

PrintWriter

! So this way it is possible to call getOutputStream() after you've called

getWriter()

. Instead you get the exception if you call

getOutputStream()

twice!

3) I think it would be right to have

if((streamUsed != null) && (streamUsed [B]==[/B] pw)), then throw the ex

in

getOutputStream()

!

Please, tell me if I am wrong...

[Misho Chigrichenko]

Sorry for misusing

in such a way... I am a greenhorn, see

[Jan Sterk]

You're right.

Check the errata of The Book. It's mentioned there.
[ September 10, 2008: Message edited by: Jan Sterk ]