Re: Some questions (Servlet Init, common libraries, and controlling standard out/err)

["Craig R. McClanahan" <Cr...@eng.sun.com>]

Kirk Rasmussen wrote:

> Hi,
> I have a several questions about Tomcat 3.4.
>

Err, 3.2beta4 right?

>
> 1a) I was trying to deploy a servlet that has many dependencies on other
> classes (e.g. TOPLink, JDBC drivers)  using the WAR method of deployment.
> When it failed to load because of a missing JAR I got no error messages in
> the logs at all.  Only when I tried to access the servlet from the browser
> did I get an internal error message with no information (very generic 500
> message).  I have the debug level set to 9 for all components.  What I would
> have liked to see would be a stack trace showing me the class it was trying
> to load when it failed.  Any way to do this with Tomcat 3.2 beta 4?
>

The first thing to remember is that Java is a language that loads classes on
demand, the first time you reference them.  The fact that your servlet
references a class "Foo", and "Foo" isn't included in the WAR file anywhere, is
not going to stop you from deploying the webapp.  You only run into a problem
when your servlet actually tries to use the offending class.

This leads to the second thing to remember -- it is your *servlet*, not Tomcat,
that tries to use the offending class.  Therefore, it is your servlet's
responsibility to handle this case appropriately.  One useful mechanism is to
use the ServletContext.log() method that takes two arguments -- a message string
and an exception -- which will cause the message and the stack trace to be
written to the server's log files (in directory $TOMCAT_HOME/logs for Tomcat).

For example:

    Foo foo = null;
    try {
        foo = new Foo();
    } catch (ClassNotFoundException e) {
        getServletContext().log("Cannot find Foo", e);
    }

>
> 1b) Other than using symlinks under UNIX or changing the Java system
> classpath is there a way to share JARs across multiple webapps?  For example
> we have multiple webapps that share the same JDBC drivers. It seems like a
> waste to have to copy that JDBC driver to each lib directory.
>

The rules for this are specific to each server.  With Tomcat, the way to do this
is put the shared JAR files in the $TOMCAT_HOME/lib directory before starting
Tomcat.  You should note, however, that some Java classes will not behave
correctly when shared in this manner if they try to reference classes that are
found only in the WEB-INF/classes or WEB-INF/lib directories.

>
> 1c) Can I put subdirs under WEB-INF/lib to organize the JARs in some way?
>

No -- only the JAR files at the top level are supposed to be loaded (some
versions of Tomcat actually did scan subdirectories, but this is a bug and is
not portable to any other server).

>
> 2) Is there a way to control where standard out/err are sent other than from
> the shell?
>

Assuming you are not running under a security manager that disables this, you
can use System.setOut() and System.setErr() for this.  But servlet applications
should really be writing things to the servlet context logs (as illustrated
above).

>
> Thanks,
> Kirk
>

Craig McClanahan

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