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## Chapter 3

## 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.

## 3.1 Accessing Standard and Dynamic MBeans via the RMI Connector

This example demonstrates standard and dynamic MBeans only.

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 standard RMI transports, Java Remote Method Protocol (JRMP) and the Internet Inter-Object Request Broker (ORB) Protocol (IIOP). 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.

When you run this example:

1. The server:

1. Creates an MBean server
2. Registers a `SimpleStandard` and a `SimpleDynamic` MBean in the local MBean server
3. Performs local operations on the MBeans
4. Creates an RMI connector server

2. The client:

1. Creates an RMI connector
2. Registers a `SimpleStandard` and a `SimpleDynamic` MBean on the remote MBean server
3. Performs remote operations on both MBeans

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

1. 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`

2. Open each of the `*.java` files in a text editor.

### 3.1.1 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.

#### 3.1.1.1 Server.java

Due to its size, the `Server.java` class is shown in several code excerpts.

CODE EXAMPLE 3-1 MBean Example Class Server.java (Excerpt 1)

    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();

             [...]

Examining this class, you can see that the following occurs:

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 3.1.1.3 "SimpleStandard.java".

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

- The `type`, which in this case is the `mbeanClassName`.
- A `name`, to differentiate this MBean from other MBeans of the same type that might be created subsequently. In this case the name number is `1`.

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 3.1.1.4 "SimpleDynamic.java".

CODE EXAMPLE 3-2 MBean Example Class Server.java (Excerpt 2)

    [...]

    JMXServiceURL url =
      new JMXServiceURL("service:jmx:rmi:///jndi/rmi://localhost:9999/server");
    JMXConnectorServer cs =
      JMXConnectorServerFactory.newJMXConnectorServer(url, null, mbs);
    cs.start();
    waitForEnterPressed();
    cs.stop();

    [...]

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:

- The connector will use the default RMI transport, denoted by `rmi`.
- The RMI registry in which the RMI connector stub will be stored will be running on port `9999` on the local host, and the server address will be registered under the name `server`. The port `9999` specified in the example is arbitrary; you can use any available port.

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 MBean Example Class Server.java (Excerpt 3)

    [...]

    private static ObjectName createSimpleMBean(MBeanServer mbs,
                                                  String mbeanClassName,
                                                  String mbeanObjectNameStr) {
         echo("\n>>> Create the " + mbeanClassName +
              " MBean within the MBeanServer");
         echo("ObjectName = " + mbeanObjectNameStr);
         try {
              ObjectName mbeanObjectName =
                 ObjectName.getInstance(mbeanObjectNameStr);
              mbs.createMBean(mbeanClassName, mbeanObjectName);
                 return mbeanObjectName;
              } catch (Exception e) {
                echo(	"!!! Could not create the " +
                      mbeanClassName + " MBean !!!");
                e.printStackTrace();
                echo("\nEXITING...\n");
                System.exit(1);
            }
            return null;
         }

    [...]

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`.

CODE EXAMPLE 3-4 MBean Example Class Server.java (Excerpt 4)

    [...]

    private static void printMBeanInfo(MBeanServer mbs,
                                         ObjectName mbeanObjectName,
                                         String mbeanClassName) {
         MBeanInfo info = null;
         try {
             info = mbs.getMBeanInfo(mbeanObjectName);
         } catch (Exception e) {
             echo(	"!!! Could not get MBeanInfo object for " +
         	 mbeanClassName +" !!!");
             e.printStackTrace();
             return;
         }

         MBeanAttributeInfo[] attrInfo = info.getAttributes();
         if (attrInfo.length > 0) {
             for (int i = 0; i < attrInfo.length; i++) {
         	echo(" ** NAME: 	" + attrInfo[i].getName());
         	echo("    DESCR: 	" + attrInfo[i].getDescription());
         	echo("    TYPE: 	" + attrInfo[i].getType() +
                   	"READ: "+ attrInfo[i].isReadable() +
                   	"WRITE: "+ attrInfo[i].isWritable());
            }
         } else echo(" ** No attributes **");

    [...]

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:

- `MBeanConstructorInfo`, to obtain information about the MBean’s Java class.
- `MBeanOperationInfo`, to learn what operations the MBean performs, and what parameters it takes.
- `MBeanNotificationInfo`, to find out what notifications the MBean sends when its operations are performed.

CODE EXAMPLE 3-5 MBean Example Class Server.java (Excerpt 5)

    [...]

    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();
            }
       }

    [...]

CODE EXAMPLE 3-5 shows a method for managing a simple MBean.

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 3.1.1.3 "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.

#### 3.1.1.2 SimpleStandardMBean.java

The `SimpleStandardMBean.java` class is shown in CODE EXAMPLE 3-6.

CODE EXAMPLE 3-6 MBean Example Class SimpleStandardMBean.java

    public interface SimpleStandardMBean {

           public String getState();
           public void setState(String s);
           public int getNbChanges();
           public void reset();

    }

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.

#### 3.1.1.3 SimpleStandard.java

The `SimpleStandard.java` class is shown in CODE EXAMPLE 3-7.

CODE EXAMPLE 3-7 MBean Example Class 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 3.1.1.2 "SimpleStandardMBean.jav a".

The simple operations exposed by this MBean are as follows:

- To define a state.
- To update this state.
- To count the number of times the state is updated
- To reset the values of the state and the number of changes to their original value of zero
- To send a notification whenever the reset operation is invoked

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.

#### 3.1.1.4 SimpleDynamic.java

The `SimpleDynamic` class is shown in CODE EXAMPLE 3-8.

CODE EXAMPLE 3-8 MBean Example Class SimpleDynamic.java

    public class SimpleDynamic
        extends NotificationBroadcasterSupport
        implements DynamicMBean {

        public SimpleDynamic() {
            buildDynamicMBeanInfo();
        }

    [...]

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.

#### 3.1.1.5 ClientListener.java

The `ClientListener.java` class is shown in CODE EXAMPLE 3-9.

CODE EXAMPLE 3-9 MBean Example Class ClientListener.java

    public class ClientListener implements NotificationListener {
     public void handleNotification(Notification notification, Object handback)
         {
             System.out.println("\nReceived notification: " + notification);
         }
    }

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.

#### 3.1.1.6 Client.java

The `Client.java` class is shown in CODE EXAMPLE 3-10.

CODE EXAMPLE 3-10 MBean Example Class 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 3.1.1.5 "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.

### 3.1.2 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:

1. Compile the Java classes.

`$` javac *.java

2. 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 &

3. 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`.

4. 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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