Threading stories: ThreadLocal in web applications

This week I spend reasonable time to eliminate all our ThreadLocal variables in our web applications. The reason was that they created classloader leaks and we coudn't undeploy our applications properly anymore. Classloader leaks happen when a GC root keeps referencing an application object after the application was undeployed. If an application object is still referenced after undeploy, then the whole class loader can't be garbage collected cause the considered object references your applications class file which in turn references the classloader. This will cause an OutOfMemoryError after you've undeployed and redeployed a couple of times.

Threading stories: volatile and synchronized

In my last blog about the volatile modifier I have introduced a little program that illustrates the behaviour of volatile in a Java 6 (26) Hotspot VM. Since that day I had some interesting discussions that I wanted to share in this blog. It adds another valuable insights on the volatile modifier.

Threading stories: Why volatile matters

Many years ago when I learned Java (in 2000) I was not so concerned about multithreading. In particular I wasn't concerned about the volatile modifier. I don't know why, but I never had problems without volatile, so maybe I thought it could not be so relevant. I've suddenly changed my mind when I first analyzed a wierd behaviour of an application that only existed when the application was deployed to a server JVM. Todays JVMs make a lot magic stuff to optimize runtime performance on server applications. In this blog I show you an example to get fimiliar with problems that arrize in multithreaded applications, when you don't recognize the importance of understanding how Java treats shared data in multithreaded programs.

Your web applications work - by sheer coincidence!

This blog describes a solution to a typical concurrency problem in a web application environment, and it illustrates that you - in all likelihood - cannot be sure that your application is thread-safe. It just works - by sheer coincidence.