Win a copy of Head First Android this week in the Android forum!
  • Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other Pie Elite all forums
this forum made possible by our volunteer staff, including ...
  • Campbell Ritchie
  • Paul Clapham
  • Ron McLeod
  • Tim Cooke
  • Junilu Lacar
  • Rob Spoor
  • Devaka Cooray
  • Jeanne Boyarsky
Saloon Keepers:
  • Jesse Silverman
  • Stephan van Hulst
  • Tim Moores
  • Carey Brown
  • Tim Holloway
  • Jj Roberts
  • Al Hobbs
  • Piet Souris

Tomcat Classloader - load different versions of a jar

Posts: 4
Eclipse IDE Tomcat Server Java
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
I am implementing container-managed authentication for Tomcat 6. I am using a JDBCRealm, and so needed to put a mysql driver in <catalina home>\lib. This mysql-connector jar is version 5. However, one of my web apps is stuck using an older version of mysql-connector, which is in WEB-INF\lib for that web-app.

The problem I'm seeing now is that occasionally, Tomcat seems to choose one version of the mysql jar over the other, causing this web-app to break when it tries using the newer mysql jar (there are a variety of incompatibilities with this newer version that cannot be fixed at this time), OR the custom Authenticator will break since it needs the latest mysql jar.

I tried assigning the old server.loader property in Tomcat's file so that the mysql jar dependency for my JDBCRealm is isolated, Tomcat gave errors about not being able to load the Mysql classes. I am not sure, but it looks like the bootstrap loader is what loads the web-apps and their Realms (rather than the server loader), and that needs the mysql jar, which as far as I can tell, requires that everything be in <catalina home>\lib.

Is there any way to load the two different versions of the jar files and keep them separate?
Saloon Keeper
Posts: 24517
Android Eclipse IDE Tomcat Server Redhat Java Linux
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
It's very bad practice to include driver jars in webapps, and this is one reason why. What you are seeing is the kind of stuff that happens when some classes are being loaded from one location and some are loaded from another, resulting in an unholy mishmash where two or more discrete versions of a single class are in play, and which one a given function will end up with is more a game of roulette than anything else.

Unlike a certain other platform I could mention, both Java and the open-source databases take a lot of care in maintaining version inter-compatibility, although I can mention one notable offender at the binary (non-JDBC) client level. It's not MySQL, however, so that's not your problem. So in most cases, the problem would be cured by simply removing the MySQL driver from the WAR entirely.

From what you're saying, however, I understand that the newer MySQL driver does not function properly for this particular webapp. Perhaps, in part since it apparently wasn't designed according to best practices to begin with. Tomcat's class-loading rules are very explicit, so in normal cases, the WAR driver classes should take precedence within the webapp, regardelss of what other apps or the Realm are doing. However, if the webapp itself isn't well-designed, it may be making reference to resources outside of its expected boundaries.

Bottom line. Although people like to say "if it ain't broke, don't fix it", this is a naive thing to say in IT, where things get far more often broken from the outside in than vice versa. It's probably about time for that 150,000 mile overhaul on the offending webapp.
You showed up just in time for the waffles! And this tiny ad:
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop
    Bookmark Topic Watch Topic
  • New Topic