Saturday, June 18, 2005

commons-logging classloader pain

I recently learned about commons-logging's classloader behavior the hard way when moving applications that use log4j from Tomcat to WebLogic. I had naively put commons-logging.jar in my WebLogic system classpath to emulate Tomcat's behavior, and things subsequently broke. This web site provided a very detailed and erudite explanation:

Basically, it boils down to weird subtlety in commons-logging's behavior. If commons-logging.jar is in the system classpath in a J2EE environment, the "current" or "thread context" classloader for each individual webapp contains WEB-INF/lib, but commons-logging itself is loaded in the system (parent) classpath. commons-logging looks for log4j using the thread-context classloader and therefore will "see" log4j in WEB-INF/lib, but tries to actually load it in the system classpath, and breaks with a NoClassDefFoundError.

Various solutions:

  • Only put commons-logging-api.jar in the system classpath, as Tomcat does. (I didn't realize there were two different variations of the JAR at first. I know, RTFM.) This stripped-down commons-logging will pretend that log4j doesn't exist. Downside: logging output from third-party APIs like Hibernate, Digester, etc. will go who-knows-where, but not the same place as your application's log4j output.

  • Put log4j in the system classpath too. This works, but has the bad side effect that all applications will share a single log4j configuration and this makes logging unmanageable.

  • Put commons-logging.jar in your WEB-INF/lib directory. I'd heard bad things about this, especially if you want to deploy the same WAR in Tomcat. But for whatever reason it worked fine with Tomcat 4.1.31 and WebLogic 8.1. So this may be my preferred solution.

Anyhow, if this was confusing to someone who has been doing J2EE professionally for 5-6 years, I can imagine how confusing it must be to someone just trying to do this for the first time. The BileBlog puts it a lot less charitably.

Saturday, June 04, 2005

cost of supporting old browsers

I've begun to question the wisdom of standards for websites that require support for older browsers like Netscape 4. Backwards compatibility is all well and good, but I feel like often times clients--government ones in particular--forumulate these requirements without any real cost-benefit analysis.

Keeping support for Netscape 4, IE 3, etc. has a very real cost, with respect to CSS and DHTML/Javascript one-offs, or having to pass on new patterns like AJAX. But many statistics show that fewer than 1% of all browsers are Netscape 4--even going back a year or two. (See here for an example.) Most people are using IE 6.

On the other hand, a commercial client I visited had done a very careful cost-benefit analysis on operating system support--they knew exactly how many of their users were still using Windows 98 and how much it would tick them off if they were forced to upgrade. So they might be stuck supporting it, but at least they could justify the decision.

So, before making requirements to support Netscape 4, clients should do their homework, and ask themselves what the benefit is--is anyone really still using it and can't upgrade? And how much extra time and money are the developers going to spend on workarounds to support these old browsers, compared to the value delivered?