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Java Dynamic Management Kit 5.1 Getting Started Guide

2.1 Instrumenting Resources as MBeans

The instrumentation level of the JMX specification defines standards for making resources manageable in the Java programming language. The instrumentation of a manageable resource is provided by one or more management beans, or MBeans. An MBean is a Java object that exposes attributes and operations for management. These attributes and operations enable any Java dynamic management agent to recognize and manage the MBean.

The design patterns for MBeans give the developer explicit control over how a resource, device, or application is managed. For example, attribute patterns enable you to distinguish between a read-only and a read-write property in an MBean. The set of all attributes and operations exposed to management through the design patterns is called the management interface of an MBean.

Any resource that you want to make accessible through an agent can be represented as an MBean. Both the agent application and remote managers can access MBeans in an agent. MBeans can generate notification events that are sent to all local or remote listeners. For more information about managing MBeans remotely, see 2.3 Communication Components.

You can make resources accessible through Java DMK agents even if they are not instrumented as MBeans, by using MBean Interceptors. See 2.6.1.4 Server authentication for details.

You can also download MBeans from a web server and plug them into an agent at any time, in response to a demand from the management application. This is called dynamic class loading and means that future services and applications can be loaded on the fly and without any downtime. For example, dynamic class loading can be used to provide rapid, low-cost delivery of end-user applications across very large bases of Java technology enabled devices, such as desktop PCs or Web phones.

There are four types of MBean:

2.1.1 Standard MBeans

Standard MBeans are Java objects that conform to certain design patterns that are derived from the JavaBeans component model. Standard MBeans allow you to define your management interface straightforwardly in a Java interface. The method names of this interface determine getters and setters for attributes and the names of operations. The class implementation of this interface contains the equivalent methods for reading and writing the MBean's attributes and for invoking its operations, respectively.

The management interface of a standard MBean is static, and this interface is exposed statically. Standard MBeans are static because the management interface is defined by the source code of the Java interface. Attribute and operation names are determined at compilation time and cannot be altered at runtime. Changes to the interface must be recompiled.

Standard MBeans are the quickest and easiest type of MBeans to implement. They are suited to creating MBeans for new manageable resources and for data structures that are defined in advance and that are unlikely to change often.

2.1.2 Dynamic MBeans

Dynamic MBeans do not have getter and setter methods for each attribute and operation. Instead, dynamic MBeans have generic methods for getting or setting an attribute by name, and for invoking operations by name. These methods are common to all dynamic MBeans and are defined by the DynamicMBean interface.

The management interface is determined by the set of attribute and operation names to which these methods respond. The getMBeanInfo method of the DynamicMBean interface must also return a data structure that describes the management interface. This metadata contains the attribute and operation names, their types, and the notifications that can be sent if the MBean is a broadcaster.

Dynamic MBeans provide a simple way to wrap existing Java objects that do not follow the design patterns for standard MBeans. You can also implement dynamic MBeans to access resources that are not based on Java technology by using the Java Native Interface (JNI).

The management interface of a dynamic MBean is static, but this interface is exposed dynamically when the MBean server calls its getMBeanInfo method. The implementation of a dynamic MBean can be complex, for example, if the MBean determines its own management interface based on existing conditions when it is instantiated.

2.1.3 Model MBeans

A model MBean is a generic, configurable, dynamic MBean that you can use to instrument a resource at runtime. A model MBean is an MBean template. The caller tells the model MBean what management interface to expose. The caller also determines how attributes and operations are implemented, by designating a target object on which attribute access and operation invocation are actually performed.

The model MBean implementation class is mandated by the JMX specification and therefore is always available for instantiation in an agent. Management applications can use model MBeans to instrument resources on the fly.

To instrument a resource and expose it dynamically, you need to:

The management interface of a model MBean is dynamic, and it is also exposed dynamically. The application that configures a model MBean can modify its management interface at any time. The application can also change its implementation by designating a new target object.

Management applications access all types of MBeans in the same manner, and most applications are not aware of the various MBean types. However, if a manager understands model MBeans, it can obtain additional management information about the managed resource. This information includes behavioral and runtime metadata that is specific to model MBeans.

2.1.4 Open MBeans

Open MBeans allow management applications and the users to understand and use new managed objects as the objects are discovered at runtime. These MBeans are called open because they rely on a small, predefined set of universal Java types and they advertise their functionality.

Management applications and open MBeans are thus able to share and use management data and operations at runtime without requiring the recompiling, reassembly, or expensive dynamic linking of management applications. In the same way, human operators can use the newly discovered managed object intelligently without having to consult additional documentation.

To provide its own description to management applications, an open MBean must be a dynamic MBean. Beyond the DynamicMBean interface, no corresponding open interface exists that must be implemented. Instead, an MBean earns its openness by providing a descriptively rich metadata and by using only certain predefined data types in its management interface.

An open MBean has attributes, operations, constructors, and possibly notifications like any other MBeans. An open MBean is a dynamic MBean with the same behavior and all of the same functionality. An open MBean also has the responsibility of providing its own description. However, all of the object types that the MBean manipulates, its attribute types, its operation parameters and return types, and its constructor parameters, must belong to a defined set of basic data types. It is the developer's responsibility to implement the open MBean fully by using these data types only.

An MBean indicates whether it is open or not through the MBeanInfo object it returns. Open MBeans return an OpenMBeanInfo interface. This interface is implemented by OpenMBeanInfoSupport, which inherits from MBeanInfo. If an MBean is marked as open in this manner, it is a guarantee that a management application compliant with the JMX specification can immediately make use of all attributes and operations without requiring additional classes.

Since open MBeans are also dynamic MBeans and they provide their own description, the MBean server does not check the accuracy of the OpenMBeanInfo object. The developer of an open MBean must guarantee that the management interface relies on the basic data types and provides a rich, human-readable description. As a rule, the description provided by the various parts of an open MBean must be suitable for displaying to a user through a graphical user interface (GUI).