resource file location (was: Logo file location)

[André Warnier <aw...@ice-sa.com>]

Hi.

On another thread, a discussion was started as to how to correctly 
locate and specify some data file, part of the application, and which 
needs to be read in by some code part of that same webapp.

Say that the application has a context path "/myapp".
The original idea is to locate the resource file for instance in a 
subdirectory "logos" of the web application, and for some flexibility to 
indicate this location to the webapp via an <init-param> in the webapp's 
web.xml, like

<init-param>
   <param-name>thelogo</param-name>
   <param-value>path-to-mylogo.jpg</param-value>
</init-param>

"ctx" being the ServletContext, the webapp could then obtain an input 
stream to this resource by a line like

InputStream logo_stream = 
ctx.getResourceAsStream(ctx.getInitParameter('thelogo'));

Now the question is : how does this param-value need to be set in order 
for the getResourceAsStream() to work correctly ?
According to the specification which I find here :

http://java.sun.com/products/servlet/2.5/docs/servlet-2_5-mr2/index.html
we have

java.io.InputStream 	getResourceAsStream(java.lang.String path)
       Returns the resource located at the named path as an InputStream 
object.

The trouble is, it does not really explain what is meant by a "path" in 
that context, and some of the other method descriptions on the same page 
introduce a slight doubt.

If it is a real absolute file path on the hosting system, and if on that 
system CATALINA_BASE is "/srv/tomcat/", then supposedly the parameter 
needs to be specified as

<init-param>
   <param-name>thelogo</param-name>
  <param-value>/srv/tomcat/webapps/myapp/logos/mylogo.jpg</param-value>
</init-param>

However, that is not very flexible.  If this file is an integral part of 
my webapp, then chances are that it is, relative to my webapp context, 
always at the same place. But in absolute host path terms, it is not, if 
my webapp is to be deployed on different hosts.
I would then have to change, inside of my webapp's web.xml file, the 
value of the <param-value> in function of the target host, before I 
create a war-file containing my webapp (and its web.xml), to deploy it 
on that target host.
That seems to me in flagrant contradiction of the "encapsulation" ideal 
of a webapp and even of Java.

So let's see if we can improve on that.
Maybe if we specify the parameter as follows

<init-param>
   <param-name>thelogo</param-name>
  <param-value>logos/mylogo.jpg</param-value>
</init-param>

and then used code like

InputStream logo_stream = 
ctx.getResourceAsStream(ctx.getRealPath(ctx.getInitParameter('thelogo')));

then according to

  java.lang.String 	getRealPath(java.lang.String path)
           Returns a String containing the real path for a given virtual 
path.

presumably we should get what we want (at least if "virtual path" 
mentioned above has the meaning of "relative path under this webapp's 
context path" but that is not very clear either).

However, it has many times been indicated on this list that 
getRealPath() is not guaranteed to return anything.(*)

I can see still another way to possibly obtain an input stream on this 
file, which would be to use the

java.lang.String 	getContextPath()
           Returns the context path of the web application.

and then maybe add "/logos/mylogo.jpg" to it.  But I am not quite sure 
at this stage either what "path" means here (presumably just "/myapp"), 
like when mentioned with the "path" attribute of a Context element.

So what is one to do, if one wants not to have to change the content of 
one's war file for each target host, but still allow the webapp to find 
and read the logo ?


(*) I understand that the path returned by this function could not be 
used as a path to create a file, or write to one, because it is not 
guaranteed to point to a writeable disk location.  But considering that 
this resource file is a part of my own application, which I supply 
inside of my war file which is expanded by the servlet container, and 
from which I only want to read, why can I not use this getRealPath() 
successfully then ?


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Re: resource file location (was: Logo file location)

[André Warnier <aw...@ice-sa.com>]

Caldarale, Charles R wrote:
>> From: André Warnier [mailto:aw@ice-sa.com]
>> Subject: resource file location (was: Logo file location)
>>
>> The trouble is, it does not really explain what is meant by a "path" in
>> that context, and some of the other method descriptions on the same
>> page introduce a slight doubt.
> 
> Sure it does:
> 
> "The path must begin with a "/" and is interpreted as relative to the current context root.
> 
> "This method allows the servlet container to make a resource available to servlets from any source. Resources can be located on a local or remote file system, in a database, or in a .war file.
> 
> "The servlet container must implement the URL handlers and URLConnection objects that are necessary to access the resource."
> 
> The path parameter is clearly a URL for any resource in the domain of the context.  What the URL actually maps to depends on the configuration of the webapp and container.
> 
>> If it is a real absolute file path on the hosting system
> 
> There is no requirement and certainly no guarantee that the host even has a file system.
> 
>> if on that system CATALINA_BASE is "/srv/tomcat/", then supposedly the
>> parameter needs to be specified as
>> <init-param>
>>    <param-name>thelogo</param-name>
>>   <param-value>/srv/tomcat/webapps/myapp/logos/mylogo.jpg</param-value>
>> </init-param>
> 
> Nope.  Read the API spec again: "interpreted as relative to the current context root."  The proper setting is just <param-value>/logos/mylogo.jpg</param-value>.
> 
>> That seems to me in flagrant contradiction of the "encapsulation" ideal
>> of a webapp and even of Java.
> 
> It would be, which is why the spec doesn't do it that way.
> 
>> and then used code like
>>
>> InputStream logo_stream =
>> ctx.getResourceAsStream(ctx.getRealPath(ctx.getInitParameter('thelogo')
>> ));
> 
> Again, getRealPath() should NEVER (well, almost never) be used, since there's no requirement for it to return anything useful.
> 
>> So what is one to do, if one wants not to have to change the content of
>> one's war file for each target host, but still allow the webapp to find
>> and read the logo ?
> 
> You don't - you just have to read the spec correctly, and not ignore the definition of the parameter.
> 
Chuck, Chris,

Ok, I get it now.  I just read the Method Summary part of that page, 
never scrolled down to the Method Detail descriptions, so I missed the 
very clear definition of "path".
Mea culpa.
Thanks for showing me the errors of my ways.
And I swear that I'll never use getRealPath().





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RE: resource file location (was: Logo file location)

["Caldarale, Charles R" <Ch...@unisys.com>]

> From: André Warnier [mailto:aw@ice-sa.com]
> Subject: resource file location (was: Logo file location)
> 
> The trouble is, it does not really explain what is meant by a "path" in
> that context, and some of the other method descriptions on the same
> page introduce a slight doubt.

Sure it does:

"The path must begin with a "/" and is interpreted as relative to the current context root.

"This method allows the servlet container to make a resource available to servlets from any source. Resources can be located on a local or remote file system, in a database, or in a .war file.

"The servlet container must implement the URL handlers and URLConnection objects that are necessary to access the resource."

The path parameter is clearly a URL for any resource in the domain of the context.  What the URL actually maps to depends on the configuration of the webapp and container.

> If it is a real absolute file path on the hosting system

There is no requirement and certainly no guarantee that the host even has a file system.

> if on that system CATALINA_BASE is "/srv/tomcat/", then supposedly the
> parameter needs to be specified as
> <init-param>
>    <param-name>thelogo</param-name>
>   <param-value>/srv/tomcat/webapps/myapp/logos/mylogo.jpg</param-value>
> </init-param>

Nope.  Read the API spec again: "interpreted as relative to the current context root."  The proper setting is just <param-value>/logos/mylogo.jpg</param-value>.

> That seems to me in flagrant contradiction of the "encapsulation" ideal
> of a webapp and even of Java.

It would be, which is why the spec doesn't do it that way.

> and then used code like
> 
> InputStream logo_stream =
> ctx.getResourceAsStream(ctx.getRealPath(ctx.getInitParameter('thelogo')
> ));

Again, getRealPath() should NEVER (well, almost never) be used, since there's no requirement for it to return anything useful.

> So what is one to do, if one wants not to have to change the content of
> one's war file for each target host, but still allow the webapp to find
> and read the logo ?

You don't - you just have to read the spec correctly, and not ignore the definition of the parameter.

 - Chuck


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