JMX: How to prevent Classloader memory leaks in a servlet container?

Question

I am wondering if or how I should deal with MBeans which are registered directly or indirectly from my application which gets deployed on a servlet container.

In most cases there are two options to retrieve a MBeanServer which you can use for registering

• create your own MBeanServer using MBeanServerFactory.createMBeanServer()

• Use ManagementFactory.getPlatformMBeanServer()

When using the first option, it's easy to deregister all MBeans: Just invoke MBeanServer.releaseMBeanServer(myMBeanServer).

But what about the second option which is used often in many 3rd party applications? (and BTW, this is also the recommended way from Sun/Oracle).

Because the platform MBeanServer is used, it won't be deregistered when the servlet context is destroyed - but even worse it still helds a reference to the web application classloader.
As a consequence all static references of the web application won't get released which results in a leak.

If you like to test this: Just deploy a simple web application which allocates a 100MB array which is references statically and which uses an oracle jdbc driver (it will register a diagnostic MBean using the platform mbean server), deployed on tomcat. Stop the application and restart it - repeat this, and you'll hit an OutOfMemoryError.

Questions:

• Do I have to deal with these issues in general or is it a problem of the servlet container and/or the 3rd party library?

• Is there a way to get all MBeans of an MBeanServer which classes are loaded by a specific ClassLoader?

• What can I do to prevent this? Do I have to keep track of all registered MBeans to the platform MBeanServer and unregister it during contextDestroyed()?

Answer 1

I'm using such an evil third-party. To ensure proper servlet context shutdown, I enumerate the beans using mbeanServer.queryMBeans(null, null) and then unregisterMBean() the beans which are in the domain of the third-party.

Set<ObjectInstance> beans = mbeanServer.queryMBeans(null, null);
for (ObjectInstance objectInstance : beans) {
    if (objectInstance.getObjectName().getDomain().equals("third-party-domain")) {
        try {
            mbeanServer.unregisterMBean(objectInstance.getObjectName());
        } catch (MBeanRegistrationException exception) {
            //error handling
        } catch (InstanceNotFoundException exception) {
            //error handling
        }
    }
}

Answer 2

What can I do to prevent this? Do I have to keep track of all registered MBeans to the platform MBeanServer and unregister it during contextDestroyed()?

This has been my standard advice. I'm not aware of a better option.