Java Management Extensions:
Introduction:
Imagine a system that has pluggable modules included into it,
that you could expand on as needed. Add to that the capability of
integrating seamlessly with existing applications and leveraging
on their power. All the ideas above and much more form the gist of
what JMX or Java Management Extensions wishes to bring to the Java
world.
In technical terms the Java Management Extensions provides an
architecture and API for application management and network
management using the Java programming language. It allows
Integrators to integrate independent Java agent modules into
existing systems.
Architecture
The two most important components of the JMX specifications are the
Instrument Level and the Agent Level. The instrumentation level is a
specification that defines the implementation of JMX manageable
resources. A JMX manageable resource can be an application, a
service or a device. The only condition is that the resource
needs to be written in Java or should have a Java wrapper. It also
should have been instrumented so that it can be managed by JMX
complaint applications. The instrumentation is provided by
Managed beans or MBeans. The instrumentation layer creates
resources so that they can be managed by Management agents. The
MBeans are designed to be generic, flexible and easy to
implement. They do not require the user to know the JMX agents
they would operate on. Apart from these, the instrumentation
specification provides a mechanism to notify events, this allows
MBeans to generate and notify other components of events.
The agent level defines the specification for implementing
Management agents. Management agents or MBeans control the resources
and make them available to the other applications. This level
acts as an interface to the instrumentation layer by providing a
standardized agent to manage the JMX resources. An agent
consists of an MBean server and a set of services to handle MBeans.
JMX agents provide the infrastructure and API for developers to
use MBeans or resources provided by the MBean server. They need
not know about the internal implementation of these resources or
MBeans. The instrumentation specification and the agent
specification are discussed in more detail below.
Instrumentation Layer:
The main components of the instrumentation layer are the
MBeans, the Notification model and the MBean metadata classes.
MBeans:
An MBean is a concrete Java class that includes the
implementation of its own MBean interface or an implementation
of the DynamicMBean interface. It can also optionally implement
the NotificationBroadcaster interface.
A class that implements its own MBean interface is referred to
as a standard MBean. This is the simplest type of
instrumentation available when developing new JMX manageable
resources. An MBean that implements the DynamicMBean interface
is known as a dynamic MBean, because certain elements of its
instrumentation can be controlled at runtime. The type of MBean
determines how the MBean is deployed on the MBean Server but it
has no effect on how the bean is managed. JMX agents are
designed to shield the type of MBean and provide a standard
interface. The end user is transparent to the type of bean.
A standard MBean consists of attributes and operations. The
attributes can be accessed using getter and setter methods. The
operations are static methods in the MBean interface and are
visible to JMX agents using introspection. The Dynamic MBean
allows manipulations on its methods through generic invocation
methods. The method could take the name of the attribute or the
operation as a parameter.
Notification Model:
Usually management applications need to react to state
changes or to specific conditions when they occur in an
underlying resource. The Notification Model defines a model that
allows MBeans to broadcast such management events, called
notifications. Management applications and other objects
register as listeners with the broadcaster MBean. The MBean
Notification Model of JMX enables a listener to register only
once and still receive all the different events that might occur
in the broadcaster.
The JMX Notification Model uses a generic class called
Notification to define an event type. This class may be sub-
classed to represent specific types of events. The generic event
type allows a single listener to receive numerous notifications
from different types of notification types. Objects desiring to
receive notifications sent by MBeans must implement the
NotificationListener interface. MBeans that want to broadcast
notifications have to implement the NotificationBroadcaster
interface, so that it can allow listeners to register themselves
with the MBean to receive notifications. Any type of MBean,
standard or dynamic, can be either a notification broadcaster, a
notification listener, or both at the same time.
MBean Metadata Classes:
The MBean metadata defines the classes that describe an MBean.
These classes are used both for the introspection of standard
MBeans and for the self description of all dynamic MBeans. These
classes describe the management interface of an MBean in terms
of its attributes, operations, constructors and notifications.
The JMX agent exposes all its MBeans, regardless of their type,
through the MBean metadata classes. All clients, whether
management applications or other local MBeans viewing the
management interface of an MBean, need to be able to interpret
these objects and their constructs.
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