Question

I have several applications deployed on WebLogic server. Those applications expose some JMX MBeans. When I call operations on those MBeans remotely through JConsole or JRMC, they work fine.

But since JMX is not really firewall-friendly I have created another simple Servlet application that is deployed on the same server and that invokes operations on a set local MBeans. From within servlet I use ManagementFactory.getPlatformMBeanServer() to find and call MBeans of other applications deployed in the same JVM, but for some operations I get either ClassCastException or ClassNotFoundException depending on the operation that is called, but some operations work fine.

Any ideas?

Answer 1

I suspect what is happening here is that your servlet thread has a context classloader that is different from that of the MBeans that you are invoking against. Consequently, if the MBean attributes, operation parameters or return values contain types that are not core JVM classes (or classes not shared from the same root classloader), you will get ClassCast, ClassNotFound and ClassDefNotFound Exceptions.

This procedure may work for you. What you need to do is temporarily change the context classloader of the servlet thread to the same classloader as the MBean was loaded from. Once the invocation is complete, you set it back again. Since you know the ObjectName of the target MBean, the MBeanServer will supply you the correct classloader.

Here’s a basic example:

public void callMBean() throws MalformedObjectNameException, NullPointerException, InstanceNotFoundException {
    final ClassLoader currentClassLoader = Thread.currentThread().getContextClassLoader();
    try {
        ObjectName targetObjectName = new ObjectName(".....");
        MBeanServer server = ManagementFactory.getPlatformMBeanServer();
        ClassLoader tmpClassLoader = server.getClassLoaderFor(targetObjectName);
        Thread.currentThread().setContextClassLoader(tmpClassLoader);
        // ==========================================
        // Invoke operations here
        // ==========================================
    } finally {
        Thread.currentThread().setContextClassLoader(currentClassLoader);
    }
}