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Chapter   2

Essentials of the JMX API

This chapter introduces the fundamental notion of the Java Management Extensions (JMX) API, namely managed beans, or MBeans.

An MBean is a managed Java object, similar to a JavaBeanTM, that follows the design patterns set forth in the instrumentation level of the JMX specification. An MBean can represent a device, an application, or any resource that needs to be managed. MBeans expose a management interface: a set of readable and/or writable attributes and a set of invokable operations, along with a self-description. The management interface does not change throughout the life of an MBean instance. MBeans can also emit notifications when certain defined events occur.

The JMX specification defines four types of MBean: standard MBeans, dynamic MBeans, open MBeans and model MBeans. The examples in this chapter demonstrate the simplest type of MBean, namely standard MBeans.

2.1 Standard MBeans

A standard MBean is defined by writing a Java interface called SomethingMBean and a Java class called Something that implements that interface. Every method in the interface defines either an attribute or an operation in the MBean. By default every method defines an operation. Attributes and operations are simply methods which follow certain design patterns. A standard MBean is composed of the MBean interface which lists the methods for all exposed attributes and operations, and the class which implements this interface and provides the functionality of the instrumented resource.

The following sections dissect an example standard MBean, and a simple JMX agent that manages the MBean. The code samples are taken from the directory work_dir/jmx_examples/Essential/com/example/mbeans.

2.1.1 MBean Interface

An example of a very basic MBean interface, named HelloMBean, is shown in CODE EXAMPLE 2-1.

CODE EXAMPLE 2-1 Example MBean Interface, HelloMBean
 
package com.example.mbeans; 
 
public interface HelloMBean { 
 
    public void sayHello(); 
    public int add(int x, int y); 
 
    public String getName(); 
 
    public int getCacheSize(); 
    public void setCacheSize(int size); 
} 
 

As stated previously, by convention an MBean interface takes the name of the Java class that implements it, with the suffix MBean added. So, in this case, the interface is called HelloMBean. The Hello class that implements this interface is described in Section 2.1.2 "MBean Implementation".

According to the JMX specification, an MBean interface consists of named and typed attributes that are readable and possibly writable, and named and typed operations that can be invoked by the applications that are managed by the MBean. The HelloMBean interface shown in CODE EXAMPLE 2-1, declares two operations: the Java methods add() and sayHello().

Of the two attributes that are declared by HelloMbean, Name is a read-only string, and CacheSize is an integer that can be both read and written. Getter and setter methods are declared, to allow the managed application to access and possibly change the attribute values. As defined by the JMX specification, a getter is any public method whose name begins with get and which does not return void. A getter enables a manager to read the value of the attribute, whose type is that of the returned object. A setter is any public method whose name begins with set and which takes a single parameter. A setter enables a manager to write a new value in the attribute, whose type is the same as that of the parameter.

The implementation of these operations and attributes is shown in the following section.

2.1.2 MBean Implementation

The Hello class shown in CODE EXAMPLE 2-2 implements HelloMBean.

CODE EXAMPLE 2-2 Example MBean Implementation Class, Hello
 
package com.example.mbeans; 
 
public class Hello implements HelloMBean { 
    public void sayHello() { 
        System.out.println("hello, world"); 
    } 
 
    public int add(int x, int y) { 
        return x + y; 
    } 
 
    public String getName() { 
        return this.name; 
    } 
 
 
    public int getCacheSize() { 
        return this.cacheSize; 
    } 
 
    public synchronized void setCacheSize(int size) { 
        this.cacheSize = size; 
 
        System.out.println("Cache size now " + this.cacheSize); 
    } 
 
    private final String name = "Reginald"; 
    private int cacheSize = DEFAULT_CACHE_SIZE; 
    private static final int DEFAULT_CACHE_SIZE = 200; 
} 
 

The straightforward Java class Hello provides the definitions of the operations and attributes declared by HelloMBean. As you can see, the sayHello() and add() operations are extremely simple, but real-life operations can be as simple or as sophisticated as you like.

Methods to get the Name attribute and to get and set the cacheSize attribute are also defined. In this example, the Name attribute value never changes, but in a real scenario it might change as the managed resource runs. For example, the attribute might represent statistics such as uptime or memory usage. Here, it is merely the name “Reginald”.

Calling the setCacheSize method allows you to alter the cacheSize attribute from its declared default value of 200. In reality, changing the cacheSize attribute could require other operations to be performed, such as discarding entries or allocating new ones. This example merely prints a message to confirm that the cache size has changed, but more sophisticated operations could be defined in the place of the simple call to println().

With the simple Hello MBean and its interface thus defined, they can now be used to manage the resource they represent, as shown in the following section.

2.1.3 Managing a Resource

As presented in the Java Management Extensions (JMX) Technology Overview, once a resource has been instrumented by MBeans, the management of that resource is performed by a JMX agent.

The core component of a JMX agent is the MBean server, a managed object server in which MBeans are registered (see the API documentation for the MBeanServer interface for details of the MBean server implementation). A JMX agent also includes a set of services to manage MBeans.

CODE EXAMPLE 2-3 presents a basic JMX agent, named Main.

CODE EXAMPLE 2-3 Creating a JMX Agent
 
package com.example.mbeans; 
 
import java.lang.management.*; 
import javax.management.*; 
 
public class Main { 
 
   public static void main(String[] args) throws Exception { 
 
      MBeanServer mbs = ManagementFactory.getPlatformMBeanServer(); 
 
      ObjectName name = new ObjectName("com.example.mbeans:type=Hello"); 
 
      Hello mbean = new Hello(); 
 
      mbs.registerMBean(mbean, name); 
 
      System.out.println("Waiting forever..."); 
      Thread.sleep(Long.MAX_VALUE); 
   } 
} 
 

The JMX agent Main begins by obtaining any MBean server that is running on the platform, by calling the getPlatformMBeanServer() method of the java.lang.management.ManagementFactory class. If there is no MBean server already running on the platform, then getPlatformMBeanServer() creates one automatically by calling the JMX method MBeanServerFactory.createMBeanServer(). The MBeanServer instance obtained by Main is named mbs.

Next, Main defines an object name for the MBean instance it will create. Every JMX MBean must have an object name. The object name is an instance of the JMX class ObjectName, and must conform to the syntax defined by the JMX specification, namely it must comprise a domain, and a list of key-properties (see the API documentation for the ObjectName class for details of this syntax). In the object name defined by Main, name, the domain is com.example.mbeans (the package in which the example MBeans are contained) and the key-property declares that this object is of the type Hello.

An instance of a Hello object is created, named mbean. This Hello object is itself an instance of the MBean Hello that was defined in Section 2.1.2 "MBean Implementation".

The Hello object named mbean is then registered as an MBean in the MBean server mbs with the object name name, by passing the object and the object name into a call to the JMX method MBeanServer.registerMBean().

With the Hello MBean registered in the MBean server, Main will simply wait for management operations to be performed on Hello. In the scope of this example, these management operations are invoking sayHello(), and add(), and getting and setting the attribute values.

2.1.4 Running the Standard MBean Example

Having examined the example classes, you can now run the example. The Java Platform, Standard Edition comes with a management and monitoring console, named JConsole, that is used to interact with the MBean in this example. JConsole is located in JavaSE_HOME/bin/jconsole, in which JavaSE_HOME is the installation directory of the Java Platform, Standard Edition (Java SE platform).

To run the example, follow the steps below, or see the README file:

  1. If you have not done so already, open work_dir/jmx_examples/Essential.

    $ cd work_dir/jmx_examples/Essential

  2. Compile the example Java classes.

    $ javac com/example/mbeans/*.java

  3. Start the Main application.

    $ java com.example.mbeans.Main

    You will see confirmation that Main is waiting for something to happen.

  4. Start JConsole in a different terminal window on the same machine.

    $ jconsole

    You will see the JConsole tool open, presenting a list of running JMX agents that you can connect to.

  5. Select com.example.mbeans.Main from the list in the “New Connection” window, and click on “Connect”.

    You will see a summary of your platform’s current activity.

  6. Click on the “MBeans” tab.

    This panel shows you all the MBeans currently registered in the MBean server.

  7. In the left-hand frame, expand the com.example.mbeans node in the MBean tree.

    You will see the example MBean Hello, that was created and registered by Main. If you click on Hello, you will see its associated Attributes and Operations nodes in the MBean tree.

  8. Click on the Hello MBean node in the MBean tree.

    The Hello MBean’s metadata and its associated Descriptor will be displayed.

  9. Click on the Attributes node of the Hello MBean in the MBean tree.

    This displays the MBean attributes that were defined by the Hello class.

  10. Change the value of the CacheSize attribute to 150

    In the terminal window in which you started Main, you will see confirmation of this change of attribute.

  11. Click on the Operations node of the Hello MBean in the MBean tree.

    Here you will see the two operations declared by the Hello MBean, sayHello() and add().

  12. Invoke the sayHello() operation, by clicking on the sayHello button.

    A JConsole dialogue box will inform you that the method was invoked successfully, and you will see the message “hello, world” in the terminal window in which Main is running.

  13. Provide two integers for the add() operation to add up, and click on the add button.

    You will be informed of the answer in a JConsole dialogue box.

  14. Click on “Connection” and then “Exit”, to exit JConsole.

2.2 Sending Notifications

MBeans can generate notifications, for example to signal a state change, a detected event, or a problem.

For an MBean to generate notifications, it must implement the interface NotificationBroadcaster, or its subinterface NotificationEmitter. All you need to do to send a notification is to construct an instance of the class javax.management.Notification or a subclass (such as AttributeChangedNotification), and pass it to NotificationBroadcasterSupport.sendNotification.

Every notification has a source. The source is the object name of the MBean that emitted the notification.

Every notification has a sequence number. This number can be used to order notifications coming from the same source when order matters and there is a danger of the notifications being handled in the wrong order. It is all right for the sequence number to be zero, but it is better for it to increment for each notification from a given MBean.

There is an example of a standard MBean that emits notifications in the directory work_dir/jmx_examples/Notification/com/example/mbeans. This example is essentially the same as the example seen in Section 2.1 "Standard MBeans", except that the Hello MBean implements the NotificationBroadcaster interface.

2.2.1 NotificationBroadcaster Interface

As stated above, the only difference between this example and the one presented in Section 2.1 "Standard MBeans" is the fact that the MBean implementation allows for the sending of notifications.

Notifications are activated by implementing the NotificationBroadcaster interface, as shown in CODE EXAMPLE 2-4.

CODE EXAMPLE 2-4 Implementing MBean Notifications
 
package com.example.mbeans; 
 
import javax.management.*; 
 
public class Hello 
        extends NotificationBroadcasterSupport implements HelloMBean { 
 
    public void sayHello() { 
        System.out.println("hello, world"); 
    } 
 
    public int add(int x, int y) { 
        return x + y; 
    } 
 
    public String getName() { 
        return this.name; 
    } 
 
    public int getCacheSize() { 
        return this.cacheSize; 
    } 
 
    public synchronized void setCacheSize(int size) { 
        int oldSize = this.cacheSize; 
        this.cacheSize = size; 
 
        System.out.println("Cache size now " + this.cacheSize); 
 
        Notification n = 
            new AttributeChangeNotification(this, 
                                            sequenceNumber++, 
                                            System.currentTimeMillis(), 
                                            "CacheSize changed", 
                                            "CacheSize", 
                                            "int", 
                                            oldSize, 
                                            this.cacheSize); 
 
        sendNotification(n); 
    } 
 
    @Override 
    public MBeanNotificationInfo[] getNotificationInfo() { 
        String[] types = new String[] { 
            AttributeChangeNotification.ATTRIBUTE_CHANGE 
        }; 
        String name = AttributeChangeNotification.class.getName(); 
        String description = "An attribute of this MBean has changed"; 
        MBeanNotificationInfo info = 
            new MBeanNotificationInfo(types, name, description); 
        return new MBeanNotificationInfo[] {info}; 
    } 
 
    private final String name = "Reginald"; 
    private int cacheSize = DEFAULT_CACHE_SIZE; 
    private static final int DEFAULT_CACHE_SIZE = 200; 
 
    private long sequenceNumber = 1; 
} 
 

As you can see in CODE EXAMPLE 2-4, this Hello MBean implementation extends the NotificationBroadcasterSupport class, that itself implements the NotificationEmitter interface.

The operations and attributes are set in the same way as before, with the only exception that the cacheSize attribute’s setter method now defines a new value oldSize, which records the cacheSize attribute’s value prior to the set operation.

The notification is constructed from an instance, n, of the JMX class AttributeChangeNotification, which extends javax.management.Notification. The notification is constructed within the definition of the setCacheSize() method, from the following information, that is passed to AttributeChangeNotification as parameters:

  • The object name of the source of the notification, namely the Hello MBean, represented simply by this
  • A sequence number, which in this example is a long named sequenceNumber, that is set at 1 and that increases incrementally
  • A timestamp
  • The content of the notification message
  • The name of the attribute that has changed, in this case cacheSize
  • The type of attribute that has changed
  • The old attribute value, in this case oldSize
  • The new attribute value, in this case this.cacheSize

The notification n is then passed to the NotificationBroadcasterSupport.sendNotification() method.

Finally, the MBeanNotification is defined to describe the characteristics of the different notification instances emitted by the MBean for a given Java class of notification, which in this case is AttributeChangeNotification notifications.

The MBean interface, HelloMBean, and the JMX agent Main are identical to those used in the previous example.

2.2.2 Running the MBean Notification Example

Having examined the example classes, you can now run the example. This example again uses JConsole to interact with the Hello MBean.To run the example, follow the steps below, or see the README file:

  1. If you have not done so already, open work_dir/jmx_examples/Notification.

    $ cd work_dir/jmx_examples/Notification

  2. Compile the example Java classes.

    $ javac com/example/mbeans/*.java

  3. Start the Main application.

    $ java com.example.mbeans.Main

    You will see confirmation that Main is waiting for something to happen.

  4. Start JConsole in a different terminal window on the same machine.

    $ jconsole

    You will see the JConsole tool open, presenting a list of running JMX agents that you can connect to.

  5. Select com.example.mbeans.Main from the list in the “New Connection” window, and click on “Connect”.

    You will see a summary of your platform’s current activity.

  6. Click on the “MBeans” tab.

    This panel shows you all the MBeans currently registered in the MBean server.

  7. In the left-hand frame, expand the com.example.mbeans node in the MBean tree.

    You will see the example MBean Hello, that was created and registered by Main. If you click on Hello, you will see its associated Attributes, Operations and Notifications nodes in the MBean tree.

  8. Click on the Hello MBean node in the MBean tree.

    This displays the MBean’s metadata and its associated Descriptor.

  9. Click on the Notifications node of the Hello MBean in the MBean tree.

    You will see that the panel is blank.

  10. Click on the “Subscribe” button.

    The current number of notifications received (0), will be displayed in the Notifications node label.

  11. Click on the Attributes node of the Hello MBean in the MBean tree, and change the value of the CacheSize attribute to 150

    In the terminal window in which you started Main, you will see confirmation of this change of attribute. You will also see that the number of notifications received displayed in the Notifications node has changed to 1.

  12. Click on the Notifications node of the Hello MBean in the MBean tree again.

    You will see the details of the notification that was sent.

  13. Click on “Connection” and then “Exit”, to exit JConsole.

2.3 Introducing MXBeans

An MXBean is a new type of MBean that provides a simple way to code an MBean that only references a pre-defined set of types. In this way, you can be sure that your MBean will be usable by any client, including remote clients, without any requirement that the client have access to model-specific classes representing the types of your MBeans. MXBeans provide a convenient way to bundle related values together without requiring clients to be specially configured to handle the bundles.

In the same way as for standard MBeans, an MXBean is defined by writing a Java interface called SomethingMXBean and a Java class that implements that interface. However, unlike standard MBeans, MXBeans do not require the Java class to be called Something. Every method in the interface defines either an attribute or an operation in the MXBean. The annotation @MXBean can be also used to annotate the Java interface instead of requiring the interface’s name to be followed by the MXBean suffix.

MXBeans provide a convenient way to bundle related values together in an MBean without requiring clients to be specially configured to handle the bundles when interacting with that MBean. MXBeans already existed in the Java 2 Platform, Standard Edition (J2SE) 5.0, in the package java.lang.management. With the Java SE 6 platform, users can now define their own MXBeans, in addition to the standard set defined in java.lang.management.

The key idea behind MXBeans is that types such as java.lang.management.MemoryUsage that are referenced in the MXBean interface, java.lang.management.MemoryMXBean in this case, are mapped into a standard set of types, the so-called Open Types that are defined in the package javax.management.openmbean. The exact mapping rules appear in the MXBean specification, but to oversimplify we could say that simple types like int or String are unchanged, while complex types like MemoryUsage get mapped to the standard type CompositeDataSupport.

The operation of MXBeans is demonstrated by example programs that you will find in the directory work_dir/jmx_examples/MXBean/com/example/mxbeans after you have downloaded and unzipped the jmx_examples.zip file. The MXBean example contains the following files:

  • QueueSamplerMXBean interface.
  • QueueSampler class that implements the MXBean interface.
  • QueueSample Java type returned by the getQueueSample() method in the MXBean interface.
  • Main, the program that sets up and runs the example.

The MXBean example performs the following actions.

  • Defines a simple MXBean that manages a resource of type Queue<String>.
  • Declares a getter, getQueueSample, in the MXBean that takes a snapshot of the queue when invoked and returns a Java class QueueSample that bundles the following values together:
  • The time the snapshot was taken.
  • The queue size.
  • The head of the queue at that given time.
  • Registers the MXBean in an MBean server.

2.3.1 QueueSamplerMXBean Interface

CODE EXAMPLE 2-5 shows the code of the sample QueueSamplerMXBean interface.

CODE EXAMPLE 2-5 QueueSamplerMXBean interface
 
package com.example.mxbeans; 
 
public interface QueueSamplerMXBean { 
    public QueueSample getQueueSample(); 
    public void clearQueue(); 
} 
 

As you can see, you declare an MXBean interface in exactly the same way as you declare a standard MBean. The QueueSamplerMXBean interface declares two operations, getQueueSample and clearQueue.

2.3.2 QueueSampler Class

The QueueSampler class implements the QueueSamplerMXBean interface, as shown in CODE EXAMPLE 2-6.

CODE EXAMPLE 2-6 QueueSampler Class
package com.example.mxbeans; 
 
import java.util.Date; 
import java.util.Queue; 
 
public class QueueSampler implements QueueSamplerMXBean { 
     
    private Queue<String> queue; 
     
    public QueueSampler(Queue<String> queue) { 
       this.queue = queue; 
    } 
     
    public QueueSample getQueueSample() { 
        synchronized (queue) { 
            return new QueueSample(new Date(), queue.size(), queue.peek()); 
        } 
    } 
     
    public void clearQueue() { 
        synchronized (queue) { 
            queue.clear(); 
        } 
    } 
} 
 

The MXBean operations getQueueSample() and clearQueue() declared by the MXBean interface are defined in QueueSampler. The getQueueSample() operation simply returns an instance of the QueueSample Java type, created with the values returned by the java.util.Queue methods peek() and size() and an instance of java.util.Date.

2.3.3 QueueSample Class

The QueueSample instance returned by QueueSampler is defined in the QueueSample class shown in CODE EXAMPLE 2-7.

CODE EXAMPLE 2-7 QueueSample Class
package com.example.mxbeans; 
 
import java.beans.ConstructorProperties; 
import java.util.Date; 
 
public class QueueSample { 
     
    private final Date date; 
    private final int size; 
    private final String head; 
     
    @ConstructorProperties({"date", "size", "head"}) 
    public QueueSample(Date date, int size, String head) { 
        this.date = date; 
        this.size = size; 
        this.head = head; 
    } 
     
    public Date getDate() { 
        return date; 
    } 
     
    public int getSize() { 
        return size; 
    } 
     
    public String getHead() { 
        return head; 
    } 
} 
 

In QueueSample class, the MXBean framework calls all the getters in QueueSample to convert the given instance into a CompositeData and uses the @ConstructorProperties annotation to reconstruct a QueueSample instance from a CompositeData.

2.3.4 Creating and Registering the MXBean in the MBean Server

Having defined an MXBean interface and the class that implements it, as well as the Java type that is returned, the MXBean must now be created and registered in an MBean server. These actions are performed by the example class Main.

CODE EXAMPLE 2-8 MXBean example Main class
package com.example.mxbeans; 
 
import java.lang.management.ManagementFactory; 
import java.util.Queue; 
import java.util.concurrent.ArrayBlockingQueue; 
import javax.management.MBeanServer; 
import javax.management.ObjectName; 
 
public class Main { 
 
    public static void main(String[] args) throws Exception { 
        MBeanServer mbs = ManagementFactory.getPlatformMBeanServer(); 
         
        ObjectName name = 
                new ObjectName("com.example.mxbeans:type=QueueSampler"); 
         
        Queue<String> queue = new ArrayBlockingQueue<String>(10); 
        queue.add("Request-1"); 
        queue.add("Request-2"); 
        queue.add("Request-3"); 
        QueueSampler mxbean = new QueueSampler(queue); 
         
        mbs.registerMBean(mxbean, name); 
         
        System.out.println("Waiting..."); 
        Thread.sleep(Long.MAX_VALUE); 
    } 
} 
 

The Main class gets the platform MBean server, creates an object name for the MXBean QueueSampler, creates a Queue instance for the QueueSampler MXBean to process, and feeds this Queue instance to a newly created QueueSampler MXBean. The MXBean is then registered in the MBean server in exactly the same way as a standard MBean.

2.3.5 Running the MXBean Example

To run the example, follow the steps below, or see the README file:

  1. If you have not done so already, open work_dir/jmx_examples/MXBean.

    $ cd work_dir/jmx_examples/MXBean

  2. Compile the example Java classes.

    $ javac com/example/mxbeans/*.java

  3. Start the Main application.

    $ java com.example.mxbeans.Main

    You will see confirmation that Main is waiting for something to happen.

  4. Start JConsole in a different terminal window on the same machine.

    $ jconsole

    You will see the JConsole tool open, presenting a list of running JMX agents that you can connect to.

  5. Select com.example.mxbeans.Main from the list in the “New Connection” window, and click on “Connect”.

    You will see a summary of your platform’s current activity.

  6. Click on the “MBeans” tab.

    This panel shows you all the MBeans currently registered in the MBean server.

  7. In the left-hand frame, expand the com.example.mxbeans node in the MBean tree.

    You will see the example MBean QueueSampler, that was created and registered by Main. If you click on QueueSampler, you will see its associated Attributes and Operations nodes in the MBean tree.

  8. Select the Attributes node.

    You will see the QueueSample attribute appear in the right-hand pane, with its value of javax.management.openmbean.CompositeDataSupport.

  9. Double-click on the CompositeDataSupport value.

    You can see the QueueSample values date, head and size because the MXBean framework has converted the QueueSample instance into CompositeData. If you had defined QueueSampler as a Standard MBean rather than as an MXBean, JConsole would not have found the QueueSample class because it would not be in its class path. If QueueSampler had been a standard MBean, you would have received a ClassNotFoundException when retrieving the QueueSample attribute value. This demonstrates the usefulness of using MXBeans when connecting to JMX agents through generic JMX clients, like JConsole.

  10. Select the Operations node.

    You will see a button to invoke the clearQueue operation.

  11. Click on the clearQueue button.

    You will be informed that the method was invoked successfully.

  12. Select the Attributes node again and double click on the CompositeDataSupport value.

    The queue has been reset now.

  13. Click on “Connection” and then “Exit”, to exit JConsole.

In this example JConsole has been used as the JMX client but if you were to access your MXBean programmatically in a JMX client you write yourself, then you could do so in one of two ways:

  • Generically, using the following code.
CODE EXAMPLE 2-9 Accessing an MXBean directly
 
MBeanServer mbs = ...whatever...; 
ObjectName name = new ObjectName("com.example.mxbeans:type=QueueSampler"); 
CompositeData queueSample = (CompositeData) mbs.getAttribute(name,  
                             "QueueSample"); 
int size = (Integer) queueSample.get("size"); 
  • Via a proxy, using the following code.
CODE EXAMPLE 2-10 Accessing an MXBean via a proxy
 
MBeanServer mbs = ...whatever...; 
ObjectName name = new ObjectName("com.example.mxbeans:type=QueueSampler"); 
QueueSamplerMXBean proxy = JMX.newMXBeanProxy(mbs, name,  
                                              QueueSamplerMXBean.class); 
QueueSample queueSample = proxy.getQueueSample(); 
int size = queueSample.getSize(); 

CODE EXAMPLE 2-10 uses the newMXBeanProxy method to create the MXBean proxy. An equivalent method, newMBeanProxy, exists to create proxies for other types of MBeans. The newMBeanProxy and newMXBeanProxy methods are used in exactly the same way.

2.4 MBean Descriptors

Descriptors allow you to give additional information about MBeans to management clients. For example, a Descriptor on an MBean attribute might say what units it is measured in, or what its minimum and maximum possible values are. As of Java SE 6, Descriptors are an integrated part of the JMX API and are available in all types of MBeans.

Descriptors give you a convenient way to attach arbitrary extra metadata to your MBeans. Descriptors have always existed in the JMX API, but until Java SE 6 they were only available in conjunction with Model MBeans.

For most constructors in the classes MBean*Info (MBeanInfo, MBeanAttributeInfo, and so on), a parallel constructor exists with the same parameters plus an additional javax.management.Descriptor parameter. The same is true for OpenMBean*InfoSupport. The MBean*Info and OpenMBean*InfoSupport classes contain a getDescriptor() method.

Open MBeans return information about default and legal values from the getDefaultValue(), getLegalValues(), getMaxValue(), getMinValue() methods of OpenMBeanParameterInfo and OpenMBeanAttributeInfo. This information is now also present in the corresponding Descriptors, and other types of MBean can also return the information in their Descriptors.

MBean Descriptors are demonstrated in the example classes you will find in the directory work_dir/jmx_examples/Descriptors/com/example/mxbeans after you have downloaded and unzipped the jmx_examples.zip file. The MBean Descriptor example contains the following files.

  • Author, an annotation the supplies the name of the author of the MBean interface.
  • DisplayName, an annotation that supplies a display name for methods in the MBean interface.
  • Main, the program that sets up and runs the example.
  • QueueSamplerMXBean interface.
  • QueueSampler class that implements the MXBean interface.
  • QueueSample Java type returned by the getQueueSample() method in the MXBean interface.
  • Version, an annotation that supplies the current version of the MBean interface.

The QueueSampler MXBean in this example basically performs the same actions as the MXBean example presented in Chapter 2, "Introducing MXBeans", except with the addition of MBean Descriptors. This example shows how the DescriptorKey meta-annotation can be used to add new descriptor items to the Descriptors for a standard MBean (or an MXBean) via annotations in the standard MBean (or MXBean) interface.

2.4.1 DescriptorKey Annotations

A new annotation DescriptorKey can be used to add information to the Descriptors for a standard MBean (or MXBean) via annotations in the Standard MBean (or MXBean) interface. This makes it possible for a tool that generates standard MBeans from an existing management model to include information from the model in the generated MBean interfaces, rather than in separate files.

CODE EXAMPLE 2-11 shows the definition of the annotation Author.

CODE EXAMPLE 2-11 Author Annotation
 
package com.example.mxbeans; 
 
import java.lang.annotation.Documented; 
import java.lang.annotation.ElementType; 
import java.lang.annotation.Retention; 
import java.lang.annotation.RetentionPolicy; 
import java.lang.annotation.Target; 
import javax.management.DescriptorKey; 
 
@Documented 
@Target(ElementType.TYPE) 
@Retention(RetentionPolicy.RUNTIME) 
public @interface Author { 
    @DescriptorKey("author") 
    String value(); 
} 

This annotation supplies the name of the creator of the MBean interface. A new field author will be added to the MBeanInfo Descriptor with the value defined by the @Author annotation. The files Version and DisplayName define annotations of those names in exactly the same way as for Author above. In each of Version and DisplayName, the @DescriptorKey value is “version” and “displayname” respectively.

In the case of Version, a new field version will be added to the MBeanInfo Descriptor with the value defined by the @Version annotation.

For DisplayName, new field displayName will be added to the MBeanAttributeInfo Descriptor or the MBeanOperationInfo Descriptor with the value defined by the @DisplayName annotation depending on whether the annotated method is a getter/setter or an operation, respectively.

2.4.2 Using MBean Descriptors

The QueueSamplerMXBean interface used in this example is slightly different from the one used in the MXBeans example, in as much as it implements MBean Descriptors to publish some of its information.

CODE EXAMPLE 2-12 QueueSamplerMXBean with Descriptors.
 
package com.example.mxbeans; 
 
@Author("Mr Bean") 
@Version("1.0") 
public interface QueueSamplerMXBean { 
    @DisplayName("GETTER: QueueSample") 
    public QueueSample getQueueSample(); 
    @DisplayName("OPERATION: clearQueue") 
    public void clearQueue(); 
} 
 

Here, you see that the @Author annotation is set to Mr. Bean, the @Version annotation is set to 1.0, and the @DisplayName is set to the names either of the attribute QueueSample or the operation clearQueue.

2.4.3 Running the MBean Descriptors Example

To run the example, follow the steps below, or see the README file:

  1. If you have not done so already, open work_dir/jmx_examples/Descriptors.

    $ cd work_dir/jmx_examples/Descriptors

  2. Compile the example Java classes.

    $ javac com/example/mxbeans/*.java

  3. Start the Main application.

    $ java com.example.mxbeans.Main

    You will see confirmation that Main is waiting for something to happen.

  4. Start JConsole in a different terminal window on the same machine.

    $ jconsole

    You will see the JConsole tool open, presenting a list of running JMX agents that you can connect to.

  5. Select com.example.mxbeans.Main from the list in the “New Connection” window, and click on “Connect”.

    You will see a summary of your platform’s current activity.

  6. Click on the “MBeans” tab.

    This panel shows you all the MBeans currently registered in the MBean server.

  7. In the left-hand frame, expand the com.example.mxbeans node in the MBean tree.

    You will see the example MBean QueueSampler, that was created and registered by Main. If you click on QueueSampler, you will see its associated Attributes and Operations nodes in the MBean tree.You will also see the fields author and version in the MBeanInfo Descriptor table.

  8. Expand the Attributes and Operations nodes under the QueueSampler MBean node.

    You will see the individual Attributes and Operations.

  9. Select the QueueSample node.

    You will see the field displayName in the MBeanAttributeInfo Descriptor table.

  10. Select the clearQueue node.

    You will see the field displayName in the MBeanOperationInfo Descriptor table.

  11. Click on “Connection” and then “Exit”, to exit JConsole.

 

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