MXBeans

This section explains a special type of MBean, called MXBeans.

An MXBean is a type of MBean that references only a predefined set of data 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 existed in the Java 2 Platform, Standard Edition (J2SE) 5.0 software, in the package java.lang.management. However, users can now define their own MXBeans, in addition to the standard set that is defined in java.lang.management.

The main 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. However, the general principle is for simple types such as int or String to remain unchanged, while complex types such as MemoryUsage get mapped to the standard type CompositeDataSupport.

The MXBean example consists of the following files, which are found in jmx_examples.zip:

• 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 uses these classes to perform 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

MXBean Interface

The following code shows the example QueueSamplerMXBean MXBean interface:

package com.example; 
 
public interface QueueSamplerMXBean { 
    public QueueSample getQueueSample(); 
    public void clearQueue(); 
} 

Note that you declare an MXBean interface in exactly the same way as you declare a standard MBean interface. The QueueSamplerMXBean interface declares a getter, getQueueSample and an operation, clearQueue.

Defining MXBean Operations

The MXBean operations are declared in the QueueSampler example class, as follows:

package com.example; 
 
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(); 
        } 
    } 
} 

QueueSampler defines the getQueueSample() getter and clearQueue() operation that were declared by the MXBean interface. The getQueueSample() operation returns an instance of the QueueSample Java type which was created with the values returned by the java.util.Queue methods peek() and size(), and an instance of java.util.Date.

Defining the Java Type Returned by the MXBean Interface

The QueueSample instance returned by QueueSampler is defined in the QueueSample class, as follows:

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

Creating and Registering the MXBean in the MBean Server

So far, the following have been defined: an MXBean interface and the class that implements it, as well as the Java type that is returned. Next, the MXBean must be created and registered in an MBean server. These actions are performed by the same Main example JMX agent that was used in the standard MBean example, but the relevant code was not shown in the lesson.

package com.example; 
 
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 mxbeanName = new ObjectName("com.example: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, mxbeanName);
         
        System.out.println("Waiting..."); 
        Thread.sleep(Long.MAX_VALUE); 
    } 
} 
 

The Main class performs the following actions:

• Gets the platform MBean server.
• Creates an object name for the MXBean QueueSampler.
• Creates a Queue instance for the QueueSampler MXBean to process.
• Feeds the Queue instance to a newly created QueueSampler MXBean.
• Registers the MXBean in the MBean server in exactly the same way as a standard MBean.

Running the MXBean Example

The MXBean example uses classes from the jmx_examples.zip bundle that you used in the section. This example requires version 6 of the Java SE platform. To run the MXBeans example follow these steps:

1. If you have not done so already, save jmx_examples.zip into your work_dir directory.
2. Unzip the bundle of sample classes by using the following command in a terminal window.

   unzip jmx_examples.zip
   

3. Compile the example Java classes from within the work_dir directory.

   javac com/example/*.java
   

4. Start the Main application. A confirmation that Main is waiting for something to happen is generated.

   java com.example.Main
   

5. Start JConsole in a different terminal window on the same machine. The New Connection dialog box is displayed, presenting a list of running JMX agents that you can connect to.

   jconsole
   

6. In the New Connection dialog box, select com.example.Main from the list and click Connect.

   A summary of your platform's current activity is displayed.

7. Click the MBeans tab.

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

8. In the left frame, expand the com.example node in the MBean tree.

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

9. Expand the Attributes node.

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

10. Double-click the CompositeDataSupport value.

    You 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 message when retrieving the QueueSample attribute value. The fact that JConsole finds QueueSampler demonstrates the usefulness of using MXBeans when connecting to JMX agents through generic JMX clients such as JConsole.

11. Expand the Operations node.

    A button to invoke the clearQueue operation is displayed.

12. Click the clearQueue button.

    A confirmation that the method was invoked successfully is displayed.

13. Expand the Attributes node again, and double click on the CompositeDataSupport value.

    The head and size values have been reset.

14. To close JConsole, select Connection -> Exit.