Creating an MBean (Method 1)

The methods of the MBean server enable you to create an MBean in three different ways. The base agent demonstrates all three ways, and we will discuss the advantages of each.

One way of creating an MBean consists of first instantiating its class and then registering this instance in the MBean server. Registration is the internal process of the MBean server that takes a manageable resource's MBean instance and exposes it for management.

Bold text in Example 8–2 and the following code samples highlights the important statements that vary between the three methods.

Example 8–2 Creating an MBean (Method 1)

// instantiate the HTML protocol adaptor object to use the default port
HtmlAdaptorServer htmlAdaptor = new HtmlAdaptorServer();

try {
    // We know that the HTML adaptor provides a default object name
    ObjectInstance htmlAdaptorInstance =
        myMBeanServer.registerMBean(htmlAdaptor, null);
    echo("CLASS NAME  = " + htmlAdaptorInstance.getClassName());
    echo("OBJECT NAME = " + 
    htmlAdaptorInstance.getObjectName().toString());
} catch(Exception e) {
    e.printStackTrace();
    System.exit(0);
}

// Now we need to start the html protocol adaptor explicitly
htmlAdaptor.start();
    
while (htmlAdaptor.getState() == CommunicatorServer.STARTING) {
    sleep(1000);
}
echo("STATE = " + htmlAdaptor.getStateString());
[...]

In this first case, we instantiate the HtmlAdaptorServer class and keep a reference to this object. We then pass it to the registerMBean method of the MBean server to make our instance manageable in the agent. During the registration, the instance can also obtain a reference to the MBean server, something it requires to function as a protocol adaptor.

In the minimal agent, we saw that the HTML adaptor gives itself a default name in the default domain. Its Javadoc API confirms this, so we can safely let it provide a default name. We print the object name in the object instance returned by the registration to confirm that the default was used.

Once the MBean is registered, we can perform management operations on it. Because we kept a reference to the instance, we do not need to go through the MBean server to manage this MBean. This enables us to call the start and getStateString methods directly. The fact that these methods are publicly exposed is particular to the implementation. The HTML adaptor is a dynamic MBean, so without any prior knowledge of the class, we would have to go through its DynamicMBean interface.

In a standard MBean you would call the implementation of its management interface directly. Because the HTML adaptor is a dynamic MBean, the start method is just a shortcut for the start operation. For example, we could start the adaptor and get its StateString attribute with the following calls:

htmlAdaptor.invoke("start", new Object[0], new String[0]);
echo("STATE = " + (String)htmlAdaptor.getAttribute("StateString"));

This type of shortcut is not specified by the Java Management extensions, nor is its functionality necessarily identical to that of the start operation exposed for management. In the case of the HTML adaptor, its Javadoc API confirms that it is identical, and in other cases, it is up to the MBean programmer to guarantee this functionality if it is offered.