my first bean (tomcat)

[ian ward]

Hello,
I'm getting more and more confused looking at old threads (and some are
very old) so I'll take the liberty of a new post for a very basic
question.

Why am I having a problem accessing my java class/bean from a jsp page
(which itself is running ok)? I'm running Tomcat 5 so I believe I need
a "myapp.xml" file in conf\catalina\localhost\ containing a context
element whose purpose is to associate an http call with the path of the
home directory where the class can be found (in \classes or in a jar in
\lib).

Then, in theory, 'jsp:usebean..etc.' will find the class I want.
Instead I get a stack trace including lines like this...

org.apache.jasper.JasperException: /jsp/listProgrammes.jsp(5,0)
org.apache.jasper.compiler.DefaultErrorHandler.jspError(DefaultErrorHandler.java:39)
org.apache.jasper.compiler.ErrorDispatcher.dispatch(ErrorDispatcher.java:409)
org.apache.jasper.compiler.ErrorDispatcher.jspError(ErrorDispatcher.java:150)
org.apache.jasper.compiler.Generator$GenerateVisitor.visit(Generator.java:1227)
org.apache.jasper.compiler.Node$UseBean.accept(Node.java:1116)
org.apache.jasper.compiler.Node$Nodes.visit(Node.java:2163)

(the top line is my jsp page which is trying to get hold of the bean)

Can anyone spell out the principles for me while I go away and check
for typos again? (Yes I have read some principles as explained in the
Tomcat docs but, well, it doesn't help for the moment.)

Many thanks
Ian

 

[John C. Bollinger]

I'm getting more and more confused looking at old threads (and some are
very old) so I'll take the liberty of a new post for a very basic
question.

Looking at very old threads along side more recent ones is bound to get
you confused because the technology changes over time.

Why am I having a problem accessing my java class/bean from a jsp page
(which itself is running ok)? I'm running Tomcat 5 so I believe I need
a "myapp.xml" file in conf\catalina\localhost\ containing a context
element whose purpose is to associate an http call with the path of the
home directory where the class can be found (in \classes or in a jar in
\lib).

JSP pages, servlets, and associated classes are aggregated together into
modular units called "Web Applications" (often referred to as
"webapps"). Before you can go anywhere, you need to assimilate the
basics of web applications and the specifics of Tomcat in that area.
The Tomcat docs would be a good place to start. Try

http://jakarta.apache.org/tomcat/tomcat-5.0-doc/appdev/deployment.html

Then, in theory, 'jsp:usebean..etc.' will find the class I want.
Instead I get a stack trace including lines like this...

org.apache.jasper.JasperException: /jsp/listProgrammes.jsp(5,0)
org.apache.jasper.compiler.DefaultErrorHandler.jspError(DefaultErrorHandler.java:39)
org.apache.jasper.compiler.ErrorDispatcher.dispatch(ErrorDispatcher.java:409)
org.apache.jasper.compiler.ErrorDispatcher.jspError(ErrorDispatcher.java:150)
org.apache.jasper.compiler.Generator$GenerateVisitor.visit(Generator.java:1227)
org.apache.jasper.compiler.Node$UseBean.accept(Node.java:1116)
org.apache.jasper.compiler.Node$Nodes.visit(Node.java:2163)

(the top line is my jsp page which is trying to get hold of the bean)

Can anyone spell out the principles for me while I go away and check
for typos again? (Yes I have read some principles as explained in the
Tomcat docs but, well, it doesn't help for the moment.)

JSP pages are translated into Java code, which is then compiled into a
servlet. Jasper is Tomcat's tool for the JSP -> Java step. Your stack
trace appears to indicate that Jasper encountered an error trying to
parse the JSP file; that probably means a syntax error in your code, and
(5,0) is probably a location close to the error. I'm not certain
offhand whether it means line 5 character 0 or character 5 line 0.

You should also be aware that like XML tags but unlike HTML tags, JSP
tags are case-sensitive. The correct spelling, therefore, is either
<jsp:useBean .../> or <jsp:useBean ...>...</jsp:useBean>. To get more
specific help, you'll need to show us the code that Jasper fails on.

 

[ian ward]

John,

thanks for your reply. I may have come over a bit more
helpless/clueless than I am - I hope! I have looked at the Tomcat page
you indicate and I do have some idea about the architecture of a webapp
and where things should go but I am a bit concerned about some of the
other files which need setting. I am finding the doc a bit wordy to
wade thru and I'd like to see a 'recipe' for getting a Bean (capital
'b' in useBean, I checked) going, the main steps, 'cos I seem to be
missing/hashing one of them. By the way, are you sure I've got a syntax
kind of error in my jsp? the offending line is simple enough.....

<jsp:useBean id="dialogueSystem" scope="request"
class="controllers.impl.C_SystemImpl" />

.....my impression is that it can't interprete it 'cos it can't find the
thing and it can't find the thing 'cos it's relying on information in
another file which isn't correct. Example, here is the contents of my
myapp.xml file in conf\Catalina\localhost\

<Context path="/myapp" docBase="C:/jakarta-tomcat-5.0.28/webapps/myapp"
debug="0" reloadable="true">

</Context>

....oh yeah, I get a bit confused with which way the obliques should go
as well (yes, I'm on Windows)!
Last thought, I've noticed that the jsp-examples don't seem to need any
of this stuff - maybe I'm chasing a red herring, even though it is
emphasised on one those Tomcat doc pages!

Ian

 

[John C. Bollinger]

thanks for your reply. I may have come over a bit more
helpless/clueless than I am - I hope! I have looked at the Tomcat page
you indicate and I do have some idea about the architecture of a webapp
and where things should go but I am a bit concerned about some of the
other files which need setting. I am finding the doc a bit wordy to
wade thru and I'd like to see a 'recipe' for getting a Bean (capital
'b' in useBean, I checked) going, the main steps, 'cos I seem to be
missing/hashing one of them.

Recipe:

1) Write Java source code for your bean. It should have a no-arg
constructor, and, technically, it should be Serializable. Make sure to
assign the bean to a named package.

2) Compile the bean's Java code to get a class file for it.

3) Put the bean's class file in the webapp's WEB-INF/classes directory,
or jar it up and put the jar in the webapp's WEB-INF/lib directory. In
either case, make sure the path (within WEB-INF/classes or within the
jar) maps correctly onto the bean's package name.

4) Use a <jsp:useBean .../> action to declare your JSP's use of a bean
instance; depending on the bean's scope and the application flow the
instance may or may not be newly created for any particular page view.

By the way, are you sure I've got a syntax
kind of error in my jsp? the offending line is simple enough.....

<jsp:useBean id="dialogueSystem" scope="request"
class="controllers.impl.C_SystemImpl" />

No, I'm not *sure* that you have a syntax error, but I think it highly
likely based on the stack trace. It is possible, however, that the
error is not in your useBean action itself but rather in something else
nearby in the JSP source.

....my impression is that it can't interprete it 'cos it can't find the
thing and it can't find the thing 'cos it's relying on information in
another file which isn't correct.

The page translator -- which is where the error appears to occur -- does
not need to find the class file for your bean. It simply uses the
useBean attributes to produce some Java source code. If the class file
is out of place then you should get a different error from a different
component (details and timing depend on the servlet container).

Example, here is the contents of my
myapp.xml file in conf\Catalina\localhost\

<Context path="/myapp" docBase="C:/jakarta-tomcat-5.0.28/webapps/myapp"
debug="0" reloadable="true">

</Context>

As far as I can tell, this whole myapp.xml business is unnecessary with
Tomcat 5. I don't know whether Tomcat does anything with that file at
all if it is present. For the time being, I'd remove it altogether and
see what happens; unless Tomcat starts having trouble finding your
webapp, you can probably keep it gone.

...oh yeah, I get a bit confused with which way the obliques should go
as well (yes, I'm on Windows)!

The way you have put them should work.

Last thought, I've noticed that the jsp-examples don't seem to need any
of this stuff - maybe I'm chasing a red herring, even though it is
emphasised on one those Tomcat doc pages!

For which version of Tomcat? I don't see anything about it on the page
I pointed you to, and it certainly ought to be there if the file is
necessary.

 

[ian ward]

John,
Very many thanks for the recipe. It looks reassuringly/worryingly
familiar!
Good point about misleading line numbers ie the error could be
somewhere else...but as I have had the useBean line in different places
with no change I'll not get my hopes up. It is true that if I'm in an
unfamiliar area of work then I tend to opt for the most complex
diagnosis and it's often a typo or something similar.

Maybe your point 3 needs looking at more carefully, I was pretty sure I
had that covered...however, off the top of my head I'm not sure how I
verify it...I'll try to work on that, it may be the most promising.

Finally - I found all the stuff about context here......
C:\jakarta-tomcat-5.0.28\webapps\tomcat-docs\config\context.html.....but
I don't remember anymore why I started looking for it....oh, yeah, I've
got a book which mentions it (although it's an oldish book and, as this
page also mentions in passing, it talks about putting the context
reference in the server.xml file which is old hat apparently).
But, as you say, it maybe IS a red herring and that's why I hope your
recipe works and I don't have to go wading through more
unnecessary/misleading/confusing doc!!

Thanks again
Ian

 

[ian ward]

Hello John (or anyone else who happens, like me, to be working on
Sunday),

Looking at point 3 from your mail, as I said I would in my previous
mail, I believe I have a problem because the package name of my Bean
has two levels. I have found this by including into my webapp a Bean I
know works (numguess from the tomcat jsp-examples folder) in its own
package. Then I put a version of the Bean in one of my packages, still
ok and then I put a version in a subpackage at which point I get a
problem.

Is this normal, explainable, avoidable?

Thanks
Ian

 

[John C. Bollinger]

Looking at point 3 from your mail, as I said I would in my previous
mail, I believe I have a problem because the package name of my Bean
has two levels. I have found this by including into my webapp a Bean I
know works (numguess from the tomcat jsp-examples folder) in its own
package. Then I put a version of the Bean in one of my packages, still
ok and then I put a version in a subpackage at which point I get a
problem.

Is this normal, explainable, avoidable?

No, maybe, probably.

Point #3, reprise:

3) Put the bean's class file in the webapp's WEB-INF/classes directory,
or jar it up and put the jar in the webapp's WEB-INF/lib directory. In
either case, make sure the path (within WEB-INF/classes or within the
jar) maps correctly onto the bean's package name.


So to be as specific as possible, given a bean (or any other class used
by your webapp) with fully-qualified class name org.foo.Bar, you can do
either one of these:

a) put Bar.class in directory WEB-INF/classes/org/foo/

or

b) create a jar <somename>.jar containing the class file as an entry
named org/foo/Bar.class, then put the jar in WEB-INF/lib/.

You may of course put many class files into the same jar file. You may
put multiple jar files into WEB-INF/lib/, and multiple class files into
WEB-INF/classes/. It is best to avoid putting the same class in both
places, though technically it isn't an error.

I assure you that this works for many, many people, including me, with
package names containing various numbers of components, on a wide
variety of application servers including Tomcat 5. If it is not working
for you then the only possibilities I see are an erroneous JSP page
(more likely), a bad or misplaced class file (doesn't seem to fit the
facts), or a broken Tomcat installation (not inconceivable).

 

[ian ward]

John,
well, it seems to work for me now, I'm happy to say, but I'm not sure
why!
I fiddled a bit, and the last fiddle I did (although I'm not sure I
didn't already fiddle this but maybe something else was out of place at
the time) was to change the scope of the useBean action from session to
request and from then on it behaved itself, even if I changed it back
to session.
Thanks for your clear and explicit replies, I'll know where to come for
the next prob
Ian