5.8 Filtering MBeans With the Base Agent

The base agent does very little filtering because it does very little management. Usually, filters are applied programmatically to get a list of MBeans to which some operations apply. There are no management operations in the MBean server that apply to a list of MBeans. You must loop through your list and apply the desired operation to each MBean.

Before exiting the agent application, we do a query of all MBeans so that we can unregister them properly. MBeans should be unregistered before being destroyed because they might need to perform some actions before or after being unregistered. See the Javadoc API of the MBeanRegistration interface for more information.

Example 5–4 Unregistering MBeans

public void removeMBeans() {
        
        try {
            echo("Unregistering all the registered MBeans except " +
                 "the MBean server delegate\n");
            echo("    Current MBean count = " + mbs.getMBeanCount() 
                 + "\n");
            Set allMBeans = mbs.queryNames(null, null);
            for (Iterator i = allMBeans.iterator(); i.hasNext(); ) {
                ObjectName name = (ObjectName) i.next();
                if (!name.toString().equals(ServiceName.DELEGATE)) {
                    echo("\tUnregistering " + name.toString());
                    mbs.unregisterMBean(name);
                }
            }
            echo("\n    Current MBean count = " + mbs.getMBeanCount() 
                +"\n");
            echo("done\n");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

We use the queryNames method because we only need the object names to operate on MBeans. The null object name as a filter gives us all MBeans in the MBean server. We then iterate through the resulting set and unregister each one, except for the MBean server delegate. As we shall see in 6.1.2 MBean Server Delegate, the delegate is also an MBean and so it will be returned by the query. However, if we unregister it, the MBean server can no longer function and cannot remove the rest of our MBeans.

We recognized the delegate by its standard name which is given by the static field ServiceName.DELEGATE. The ServiceName class provides standard names and other default properties for communications and service MBeans. It also provides the version strings that are exposed by the delegate MBean. Note that, because the delegate is the only MBean created directly by the MBean server, it is the only one whose name cannot be overridden during its registration. The delegate object name is always the same, so we are always sure to detect it.