Raible Designs

Saturday August 10, 2002

Explanation of Tomcat's Class Loader from the Chief Developer of Tomcat and Struts:

> On Fri, 9 Aug 2002, Matt Raible wrote:
>
> Struts Dev Team - please verify my hypothesis below.
>
> I believe the Struts JARs and the DBCP JARs from common should be the 
> same.  After all, Struts does use the DBCP in it's distribution.  You 
> might want to upgrade to the latest build of Struts (which includes 
> DBCP and Logging, among others).  I know they're planning on releasing 
> 1.1 Beta 2 this weekend.
>

The story is somewhat convoluted (I *hate* class loaders :-), but it goes 
like this:

If you are going to use a JNDI datasource, then both your webapp AND 
Tomcat need to see the same copy of the JDBC driver and the dbcp classes
 (for 4.1.x) -- and the way to make that happen is to put the driver into 
common/lib.

Because commons-dbcp.jar is there, it's dependent jars (such as 
commons-collections.jar, jdbc2_0-stdext.jar, and (recently added) 
commons-lang.jar) need to be in common/lib as well.  Plus, of course, 
jndi.jar if you're on a 1.2 system.

Having any of these JARs duplicated inside the webapp causes a problem
 in 4.x based systems, because they try to use the local copy first -- and a
 copy if foo.bar.MyClass loaded from the webapp is *not* the same class 
as a copy of foo.bar.MyClass loaded from common/lib.  This is a place where 
Tomcat's modified class loader model causes grief (but only because you're 
trying to use the same classes that Tomcat internally is trying to use -- if you 
were using something like GenericDataSource with your own copy of the 
connection pool, it would not be a problem).

One workaround for this would be to try turning off the modified loader 
delegation model, by putting something like this in your server.xml file (or in
 a context configuration file on Tomcat 4.1.x.):

  <Context path="/myapp" ...>
    <Loader delegate="false"/>
  </Context>

This *should* let you have a copy of all the JAR files inside your webapp, 
as well as in common/lib, but not cause any conflicts.

Please let me know, one way or the other, if this works -- it's clearly a 
desireable goal that you can ship a WAR file with all the included commons 
JARs but not have problems like this.  I want to review how Tomcat's default 
configuration can make this easier.  And, from a Struts application perspective 
(not just for Roller), this is a pretty serious usability issue.

> HTH,
>
> Matt
>

Craig McClanahan

PS:  In Tomcat 4.1.x, the "deploy" command of the manager app lets you include 
a context configuration file in the WAR, at location "META-INF/context.xml".  So it 
might still be possible to configure a single WAR that works correctly in Tomcat 
4.1.x and still works in other servers (that would obviously ignore such a file).

Thanks for the clear and concise definition of how this all works Craig. So basically, in Tomcat 4.x, don't keep duplicates of your JARs in $CATALINA_HOME/common/lib AND in yourApp/WEB-INF/lib, OR turn off the modified loader delegation as specified above. Tomcat 4.1.x looks very cool, I'm just curious to know why they're on a Beta Release of 4.1.8, but still no official release? Posted in Java at Aug 10 2002, 12:49:00 AM MDT Add a Comment