Java Dynamic Management Kit 5.0 Tutorial

Creating an MBean (Method 2)

The second way to create an MBean is the single createMBean method of the MBean server. In this case, the MBean server instantiates the class and registers it all at once. As a result, the caller never has a direct reference to the new object.

Example 8–3 Creating an MBean (Method 2)

ObjectInstance httpConnectorInstance = null;
try {
    String httpConnectorClassName = "com.sun.jdmk.comm.HttpConnectorServer";
    // Let the HTTP connector server provide its default name
    httpConnectorInstance =
        myMBeanServer.createMBean(httpConnectorClassName, null);
} catch(Exception e) {
    e.printStackTrace();
    System.exit(0);
}
// We need the object name to refer to our MBean
ObjectName httpConnectorName = httpConnectorInstance.getObjectName();
echo("CLASS NAME  = " + httpConnectorInstance.getClassName());
echo("OBJECT NAME = " + httpConnectorName.toString());

// Now we demonstrate the bulk getter of the MBean server
try {
    String att1 = "Protocol";
    String att2 = "Port";
    String attNames[]= {att1, att2};
    AttributeList attList =
        myMBeanServer.getAttributes(httpConnectorName, attNames);
    Iterator attValues = attList.iterator();
    echo("\t" + att1 + "\t=" + ((Attribute) attValues.next()).getValue());
    echo("\t" + att2 + "\t=" + ((Attribute) attValues.next()).getValue());

} catch (Exception e) {
    e.printStackTrace();
    System.exit(0);
}

// Now we explicitly start the HTTP connector server
try {
    myMBeanServer.invoke(httpConnectorName, "start",
                         new Object[0], new String[0]);

    // waiting to leave starting state...
    while (new Integer(CommunicatorServer.STARTING).equals
           (myMBeanServer.getAttribute(httpConnectorName,"State"))) {
        sleep(1000);
    }
    echo("STATE = " + 
         myMBeanServer.getAttribute(httpConnectorName, "StateString"));
} catch (Exception e) {
    e.printStackTrace();
    System.exit(0);
}
[...]

One advantage of this method for creating MBeans is that the instantiation and registration are done in one call. In addition, if we have registered any class loaders in the MBean server, they will automatically be used if the class is not available locally. See Chapter 14, M-Let Class Loader for more information on class loading.

The major difference is that we no longer have a reference to our MBean instance. The object instance that was only used for display purposes in the previous example now gives us the only reference we have on the MBean: its object name.

One disadvantage of this method is that all management of the new MBean must now be done through the MBean server. For the attributes of the MBean, we need to call the generic getter and setter of the MBean server, and for the operations we need to call the invoke method. When the agent needs to access the MBean, having to go through the MBean server adds some complexity to the code. However, it does not rely on any shortcuts provided by the MBean, making the code more portable and reusable.

The createMBean method is ideal for quickly starting new MBeans that the agent application does not need to manipulate. In just one call, the new objects are instantiated and exposed for management.