Minimal Agent

The minimal agent contains only the essential components of a complete agent application. These are:

• An instance of the MBean server

• Some means of communication

Example 5–1 shows the complete application from the MinimalAgent.java file:

Example 5–1 Minimal Agent

import javax.management.ObjectInstance;
import javax.management.MBeanServer;
import javax.management.MBeanServerFactory;

public class MinimalAgent {

    public static void main(String[] args) {
        
        // Instantiate the MBean server
        System.out.println("\nCreate the MBean server");
        MBeanServer server = MBeanServerFactory.createMBeanServer();
        
        // Create and start some way of communicating:
        //    - an HTML protocol adaptor
        //    - an HTTP connector server
        //    - an RMI connector server
        // Any single one of these would be sufficient
        try {

            System.out.println(
                "\nCreate and start an HTML protocol adaptor");
				ObjectInstance html = 
		server.createMBean("com.sun.jdmk.comm.HtmlAdaptorServer", 
					  null);
            server.invoke(html.getObjectName(), "start",
                          new Object[0], new String[0]);

            System.out.println(
                "\nCreate and start an HTTP connector server");
            ObjectInstance http = server.createMBean(
				"com.sun.jdmk.comm.HttpConnectorServer", 
					  null);
            server.invoke(http.getObjectName(), "start",
                          new Object[0], new String[0]);

            System.out.println(
                "\nCreate and start an RMI connector server");
            ObjectInstance rmi = server.createMBean(
                "com.sun.jdmk.comm.RmiConnectorServer", 
					  null);
            server.invoke(rmi.getObjectName(), "start",
                          new Object[0], new String[0]);

        } catch(Exception e) {
            e.printStackTrace();
            return;
        }

        System.out.println(
"\nNow, you can point your browser to http://localhost:8082/");
        System.out.println(
"or start your management application to connect to this agent.\n");

    }
}

One of the most important lines of code in this example is the instantiation of the MBean server:

MBeanServer server = MBeanServerFactory.createMBeanServer();

The MBean server is created through the static MBeanServerFactory object, and we store its object reference. Its true type is hidden by the factory object, that casts the returned object as an MBeanServer interface. The MBean server is the only functional class referenced directly in this application.

Then we just need some means of communicating (only the HTML adaptor is shown here):

ObjectInstance html = 
server.createMBean("com.sun.jdmk.comm.HtmlAdaptorServer", null);
server.invoke(html.getObjectName(), "start", new Object[0], new String[0]);

For each of the communications protocols we ask the MBean server to create the corresponding MBean. We must provide the class name of the MBean that we want the MBean server to instantiate. The MBean server instantiates the object, registers it for management, and returns its ObjectInstance reference (see ObjectInstance of an MBean).

When an MBean is created through the MBean server, you can never obtain a direct programmatic reference on an MBean. The object instance is the only handle the calling process has on the MBean. The MBean server hides the MBean object instance and only exposes its management interface.

We then ask the server to invoke the start operation of the HTML adaptor's MBean. The object instance gives us the MBean's object name which we use to identify the MBean (see Object Names). The other parameters correspond to the signature of the start operation, which takes no parameters. This operation activates the adaptor or connector so that it will respond to remote management operations on its default port. We can now connect to the HTML protocol adaptor from a web browser.