MBean Server Classes

Before writing an agent application, it is important to understand the functionality of the MBean server. It is actually an interface and a factory object defined by the agent specification level of the Java Management extensions. The Java Dynamic Management Kit provides an implementation of this interface and factory. The factory object finds or creates the MBean server instance, making it possible to substitute different implementations of the MBean server.

The MBeanServer Interface

The specification of the interface defines all operations that can be applied to resources and other agent objects through the MBean server. Its methods can be divided into three main groups:

• Methods for controlling MBean instances:

  • createMBean, or instantiate and registerMBean add a new MBean to the agent

  • unregisterMBean removes an MBean from the agent

  • isRegistered and getObjectInstance associate the class name with the MBean's management name

  • addNotificationListener and removeNotificationListener control event listeners for a particular MBean

  • deserialize is used to download new MBean classes

• Methods for accessing MBean attributes and operations; these methods are identical to those presented in "The DynamicMBean Interface", except they all have an extra parameter for specifying the target MBean:

  • getMBeanInfo

  • getAttribute and getAttributes

  • setAttribute and setAttributes

  • invoke

• Methods for managing the agent as a whole:

  • getDefaultDomain (domains are a way of grouping MBeans in the agent)

  • getMBeanCount of all MBeans in an agent

  • queryMBeans and queryNames are used to find MBeans by name or by value

The MBean Server Implementation and Factory

The MBeanServerImpl class in the Java Dynamic Management Kit implements the MBeanServer interface. It is the class that will be instantiated in an agent. However, it is never instantiated directly by the agent application. Instead, you rely on the MBean server factory to return a new instance of the implementing class.

The MBeanServerFactory is a static class defined by the Java Management extensions that makes the agent application independent of the MBean server implementation. It resides in the Java virtual machine and centralizes all MBean server instantiation. It provides two static methods:

• createMBeanServer which returns a new MBean server instance

• findMBeanServer which returns a list of MBean servers in the Java virtual machine

You must use this class to create an MBean server so that other objects can obtain its reference by calling the findMBeanServer method. This method allows dynamically loaded objects to find the MBean server in an agent which has already been launched.