JMX Connectors

This chapter introduces the concepts of standard and dynamic management beans (MBeans) and also shows how to use Java Management Extensions (JMX) technology to perform operations on MBeans, both locally and remotely.

Accessing Standard and Dynamic MBeans via the RMI Connector

As seen in Chapter 2, "Essentials of the JMX API", a standard MBean is one that statically defines its management interface through the names of the methods it contains. A dynamic MBean implements a specific Java interface and reveals its attributes and operations at run time.

The JMX technology defines a connector based on RMI. The RMI connector supports the Java Remote Method Protocol (JRMP) transport, and optionally, the Internet Inter-Object Request Broker (ORB) Protocol (IIOP) transport. This connector allows you to connect to an MBean in an MBean server from a remote location, and perform operations on it, exactly as if the operations were being performed locally.

The purpose of this example is to demonstrate the implementation of a standard MBean and a dynamic MBean. It also shows how to perform operations on them, both locally, and remotely through an RMI connection between a server and a remote client.

The RMI connector example is contained in the directory work_dir/jmx_examples/Basic.

Open the work_dir/jmx_examples/Basic directory.
Inside this directory, you will find the following files:
- Server.java
- SimpleStandardMBean.java
- SimpleStandard.java
- SimpleDynamic.java
- ClientListener.java
- Client.java
- README
Open each of the *.java files in a text editor.

Analyzing the Example Classes

The following sections analyze each of the classes used in the basic MBean example, and explain how they perform the operations described in the preceding section.

Server.java

 
public class Server { 
 
 public static void main(String[] args) { 
     try { 
          
         MBeanServer mbs = MBeanServerFactory.createMBeanServer(); 
         waitForEnterPressed(); 
 
         String domain = mbs.getDefaultDomain(); 
         waitForEnterPressed(); 
 
         String mbeanClassName = "SimpleStandard"; 
         String mbeanObjectNameStr = 
             domain + ":type=" + mbeanClassName + ",name=1"; 
         ObjectName mbeanObjectName = 
             createSimpleMBean(mbs, mbeanClassName, mbeanObjectNameStr); 
         waitForEnterPressed(); 
 
         printMBeanInfo(mbs, mbeanObjectName, mbeanClassName); 
         waitForEnterPressed(); 
 
         manageSimpleMBean(mbs, mbeanObjectName, mbeanClassName); 
         waitForEnterPressed(); 
 
         mbeanClassName = "SimpleDynamic"; 
         mbeanObjectNameStr = 
             domain + ":type=" + mbeanClassName + ",name=1"; 
         mbeanObjectName = 
             createSimpleMBean(mbs, mbeanClassName, mbeanObjectNameStr); 
         waitForEnterPressed(); 
 
         printMBeanInfo(mbs, mbeanObjectName, mbeanClassName); 
         waitForEnterPressed(); 
 
         manageSimpleMBean(mbs, mbeanObjectName, mbeanClassName); 
         waitForEnterPressed(); 
 
         [...]

Firstly, the Server.java class creates a new MBean server called mbs by calling the createMBeanServer() method of the MBeanServerFactory class.

Then, the default domain in which the MBean server is registered is obtained with a call to the getDefaultDomain() method of the MBeanServer interface. The domain is identified by the string domain.

The MBean class named SimpleStandard is also identified by a variable, in this case the string mbeanClassName. SimpleStandard is the name of the Java class for the Java object of which this MBean is an instance. The SimpleStandard.java object is examined in Section "SimpleStandard.java".

Another variable, the string mbeanObjectNameStr, is defined as the combination of the domain, plus the following key=value pairs:

The purpose of mbeanObjectNameStr is to give the MBean a human-readable identifier.

A call to createSimpleMBean() creates and registers the SimpleStandard MBean in the local MBean server, with the given object name.

The operations printMBeanInfo(), and manageSimpleMBean() are then performed on the SimpleStandard MBean. Like createSimpleMBean(), these methods are defined later in the Server.java code, and are shown in CODE EXAMPLE 3-4 and CODE EXAMPLE 3-5.

In code that is not shown here, a second MBean of the type SimpleDynamic is created and registered in the MBean server in exactly the same way as the SimpleStandard MBean. As the name suggests, this MBean is an instance of the SimpleDynamic Java object, which is examined in Section "SimpleDynamic.java".

In CODE EXAMPLE 3-2, an RMI connector server is created so that operations can be performed on the MBeans remotely. A call to the class JMXServiceURL creates a new service URL called url, which serves as an address for the connector server. In this example, the service URL is given in JNDI form, rather than in encoded form (see the API documentation for the javax.management.remote.rmi package for an explanation of JNDI form). This service URL defines the following:

An RMI connector server named cs is created by calling the constructor JMXConnectorServerFactory, with the service URL url, a null environment map, and the MBean server mbs as parameters. The connector server cs is launched by calling the start() method of JMXConnectorServer, whereupon RMIConnectorServer exports the RMI object server to the RMI registry. The connection will remain open until the Enter key is pressed, as instructed by the simple method waitForEnterPressed, that is defined later in the Server code.

CODE EXAMPLE 3-3 shows the definition of the createSimpleMBean() method. In this method, the MBean instance with the object name mbeanObjectNameStr is passed to the getInstance() method of the ObjectName interface to create a new object name for registering the MBean inside the MBean server. The resulting object name instance is named mbeanObjectName. A call to the MBeanServer method createMBean() then instantiates an MBean defined by the combination of the Java object identified by mbeanClassName and the MBean instance mbeanObjectName and registers this MBean in the MBean server mbs.

In CODE EXAMPLE 3-4 we see the definition of the method printMBeanInfo(). The printMBeanInfo() method calls the MBeanServer method getMBeanInfo() to obtain details of the attributes and operations that are exposed by the MBean mbeanObjectName. MBeanAttributeInfo defines the following methods, each of which is called in turn to obtain information about the mbeanObjectName MBean’s attributes:

getName, to obtain the attribute’s name.

getDescription, to obtain the human readable description of the attribute.

getType, to obtain the class name of the attribute.

isReadable, to determine whether or not the attribute is readable.

isWritable, to determine whether or not the attribute is writable.

In code that is not shown here, calls are made to obtain information about the mbeanObjectName MBean’s constructors, operations and notifications:

[...] 
 
private static void manageSimpleMBean(MBeanServer mbs, 
                                        ObjectName mbeanObjectName, 
                                        String mbeanClassName) { 
        try { 
            printSimpleAttributes(mbs, mbeanObjectName); 
 
            Attribute stateAttribute = new Attribute("State", 
                                                     "new state"); 
            mbs.setAttribute(mbeanObjectName, stateAttribute); 
 
            printSimpleAttributes(mbs, mbeanObjectName); 
             
            echo("\n    Invoking reset operation..."); 
            mbs.invoke(mbeanObjectName, "reset", null, null); 
 
            printSimpleAttributes(mbs, mbeanObjectName); 
        } catch (Exception e) { 
            e.printStackTrace(); 
        } 
    } 
 
    private static void printSimpleAttributes( 
                                        MBeanServer mbs, 
                                        ObjectName mbeanObjectName) { 
        try { 
            String State =  
               (String) mbs.getAttribute(mbeanObjectName, "State"); 
            Integer NbChanges = 
               (Integer) mbs.getAttribute(mbeanObjectName, 
                                          "NbChanges"); 
        } catch (Exception e) { 
            echo(       "!!! Could not read attributes !!!"); 
            e.printStackTrace(); 
        } 
   } 
 
[...]

The manageSimpleMBean() method first of all calls the printSimpleAttributes() method that is also defined by Server. The printSimpleAttributes() method obtains an MBean attribute called state from the MBean mbeanObjectName, as well as another MBean attribute called NbChanges. Both of these attributes are defined in the SimpleStandard class, shown in Section "SimpleStandard.java".

The manageSimpleMBean() method then defines an attribute called stateAttribute, which is an instance of the Attribute class. The stateAttribute attribute associates a value of new state with the existing attribute state, defined by SimpleStandard. A call to the MBeanServer method setAttribute() then sets the mbeanObjectName MBean’s state to the new state defined by stateAttribute.

Finally, a call to the MBeanServer method invoke() invokes the mbeanObjectName MBean’s reset operation. The reset operation is defined in the SimpleStandard class.

SimpleStandardMBean.java

The SimpleStandardMBean.java class is a straightforward JMX specification management interface for the MBean SimpleStandard. This interface exposes the four operations defined by SimpleStandard for management through a JMX agent.

SimpleStandard.java

 
public class SimpleStandard 
    extends NotificationBroadcasterSupport 
    implements SimpleStandardMBean { 
    public String getState() { 
       return state; 
    } 
    public void setState(String s) { 
       state = s; 
       nbChanges++; 
    } 
     
    public int getNbChanges() { 
        return nbChanges; 
    } 
     
    public void reset() { 
       AttributeChangeNotification acn =  
           new AttributeChangeNotification(this, 
                                          0, 
                                          0, 
                                          "NbChanges reset", 
                                          "NbChanges", 
                                          "Integer", 
                                          new Integer(nbChanges), 
                                          new Integer(0)); 
       state = "initial state"; 
       nbChanges = 0; 
       nbResets++; 
       sendNotification(acn); 
    } 
     
    public int getNbResets() { 
       return nbResets; 
    } 
 
    public MBeanNotificationInfo[] getNotificationInfo() { 
        return new MBeanNotificationInfo[] { 
          new MBeanNotificationInfo( 
          new String[] { 
            AttributeChangeNotification.ATTRIBUTE_CHANGE }, 
            AttributeChangeNotification.class.getName(), 
            "This notification is emitted when the reset()  
             method is called.") 
        }; 
    } 
     
    private String state = "initial state"; 
    private int nbChanges = 0; 
    private int nbResets = 0; 
 
}

The SimpleStandard class defines a straightforward JMX specification standard MBean.

The SimpleStandard MBean exposes operations and attributes for management by implementing the corresponding SimpleStandardMBean interface, shown in Section "SimpleStandardMBean.java".

The notification emitted by the reset operation is an instance of the class AttributeChangeNotification, which collects information about the number of changes carried out on the State attribute before calling reset. The content of the notification sent is defined by the MBeanNotificationInfo instance.

SimpleDynamic.java

The SimpleDynamic dynamic MBean shows how to expose attributes and operations for management at runtime, by implementing the DynamicMBean interface. It starts by defining a method, buildDynamicMBeanInfo(), for obtaining information for the MBean dynamically. The buildDynamicMBeanInfo() method builds the MBeanInfo for the dynamic MBean.

The rest of the code of SimpleDynamic corresponds to the implementation of the DynamicMBean interface. The attributes, operations and notifications exposed are identical to those exposed by the SimpleStandard MBean.

ClientListener.java

The ClientListener class implements a straightforward JMX specification notification listener.

The handleNotification() method of the NotificationListener interface is called upon reception of a notification, and prints out a message to confirm that a notification has been received.

Client.java

 
public class Client { 
 
  public static void main(String[] args) { 
    try { 
      // Create an RMI connector client 
      // 
      JMXServiceURL url = new JMXServiceURL( 
         "service:jmx:rmi:///jndi/rmi://localhost:9999/server"); 
      JMXConnector jmxc = JMXConnectorFactory.connect(url, null); 
      ClientListener listener = new ClientListener(); 
      MBeanServerConnection mbsc = jmxc.getMBeanServerConnection(); 
      waitForEnterPressed();       
       
      // Get domains from MBeanServer 
      // 
      String domains[] = mbsc.getDomains(); 
      for (int i = 0; i < domains.length; i++) { 
          System.out.println("Domain[" + i + "] = " + domains[i]); 
      } 
      waitForEnterPressed();       
 
      String domain = mbsc.getDefaultDomain();       
   
      // Create SimpleStandard MBean  
      ObjectName mbeanName =  
             new ObjectName(domain +":type=SimpleStandard,name=2"); 
      mbsc.createMBean("SimpleStandard", stdMBeanName, null, null); 
      waitForEnterPressed();       
             
      // Create SimpleDynamic MBean 
      ObjectName dynMBeanName = 
          new ObjectName(domain +":type=SimpleDynamic,name=2"); 
      echo("\nCreate SimpleDynamic MBean..."); 
      mbsc.createMBean("SimpleDynamic", dynMBeanName, null, null); 
      waitForEnterPressed(); 
       
      // Get MBean count 
      echo("\nMBean count = " + mbsc.getMBeanCount()); 
 
      // Query MBean names 
      echo("\nQuery MBeanServer MBeans:"); 
      Set names = mbsc.queryNames(null, null); 
      for (Iterator i = names.iterator(); i.hasNext(); ) { 
      echo(     "ObjectName = " + (ObjectName) i.next()); 
      } 
      waitForEnterPressed(); 
       
      mbsc.setAttribute(stdMBeanName, 
                        new Attribute("State", "changed state")); 
 
      SimpleStandardMBean proxy = JMX.newMBeanProxy( 
          mbsc, stdMBeanName, SimpleStandardMBean.class, true); 
      echo("\nState = " + proxy.getState()); 
 
      ClientListener listener = new ClientListener(); 
      mbsc.addNotificationListener(stdMBeanName, listener, null, null); 
 
      mbsc.invoke(stdMBeanName, "reset", null, null); 
 
      mbsc.removeNotificationListener(stdMBeanName, listener); 
      mbsc.unregisterMBean(stdMBeanName); 
       
      [...] 
       
      jmxc.close(); 
    } catch (Exception e) { 
      e.printStackTrace(); 
    } 
  } 
} 
[...]

The Client.java class creates an RMI connector client that is configured to connect to the RMI connector server created by Server.java.

As you can see, Client.java defines the same service URL url as that defined by Server.java. This allows the connector client to retrieve the RMI connector server stub named server from the RMI registry running on port 9999 of the local host, and to connect to the RMI connector server.

With the RMI registry thus identified, the connector client can be created. The connector client, jmxc, is an instance of the interface JMXConnector, created by the connect() method of JMXConnectorFactory. The connect() method is passed the parameters url and a null environment map when it is called.

The Client also creates an instance of ClientListener, to listen for notifications, as shown in Section "ClientListener.java".

An instance of a JMX specification MBeanServerConnection, named mbsc, is then created by calling the getMBeanServerConnection() method of the JMXConnector instance jmxc.

The connector client is now connected to the MBean server created by Server.java, and can register MBeans and perform operations on them with the connection remaining completely transparent to both ends.

The client creates and registers the SimpleStandard MBean and the SimpleDynamic MBean in the MBean server with a call to the createMBean() method of MBeanServerConnection, and performs the operations defined by SimpleStandard and SimpleDynamic as if they were local JMX specification MBean operations.

MBean proxies allow you to access an MBean through a Java interface, allowing you to make calls on the proxy rather than having to write lengthy code to access a remote MBean. An MBean proxy for SimpleStandardMBean is created here by calling the method newMBeanProxy() in the javax.management.JMX class, passing it the MBean’s MBeanServerConnection, object name, the class name of the MBean interface and true, to signify that the proxy must behave as a NotificationBroadcaster. You can make proxies for MXBeans in exactly the same way as for standard MBeans, by simply calling newMXBeanProxy() instead of newMBeanProxy().

The code for the different operations performed on SimpleDynamic is not shown here, because the operations are the same as those performed on SimpleStandard.

Finally, the client unregisters the SimpleStandard MBean and closes the connection. The final removeNotificationListener is optional, as listeners registered by a remote client are removed when that client is closed.

Running the MBean Example

Having examined the example classes, you can now run the example. To run the example, follow the steps below, or see the README file:

Compile the Java classes.
$ javac *.java
Start an RMI registry on port 9999 of the local host.
The RMI registry will be used by the Server to register the RMI connector stub.

$ rmiregistry 9999 &
Start the Server class.
$ java -classpath . Server

You will see confirmation of the creation of the MBean server and the creation of the SimpleStandard MBean in the MBean server. You will then be prompted to press the Enter key to obtain information about, and then to perform operations on, the SimpleStandard MBean.

Once the operations on the SimpleStandard have completed, the process will be repeated for the SimpleDynamic MBean.

Once both the MBeans have been created and their operations performed, you see the creation of an RMI connector server, to allow operations to be performed on the MBeans from the remote Client.
Start the Client class in another terminal window.
$ java -classpath . Client

You will see confirmation of the creation of the RMI connector client and of the connection with the connector server. You will also be informed of the domain name, and the creation and registration of SimpleStandard and SimpleDynamic MBeans. The client will then perform operations on SimpleStandard and SimpleDynamic MBeans, before unregistering them.

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Chapter 3