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Java Dynamic Management Kit 4.2 Tutorial

Asking the Agent to Load Classes

Now that the manager is connected to the client, we can "push" classes to it. We do this by first creating an m-let loader service, then having that loader create MBeans from the classes designated by our HTML file. The following code is taken from manager's runMLetExample method. The code is identical to the code of the agent example, except that we now go through the RemoteMBeanServer interface of the connector client instead of directly through the MBean server.

Example 12-6 Calling the performLoadURL Method Remotely

// Get the domain name from the Agent
String domain = connectorClient.getDefaultDomain();

// Create a new MLetSrv MBean and add it to the Agent
String mletClass = "javax.management.loading.MLetSrv";
ObjectName mletName = new ObjectName(domain + ":name=" + mletClass);
connectorClient.createMBean(mletClass, mletName);
[...]

// Create and register new Square and EquilateralTriangle MBeans
// by means of an HTML document containing MLET tags
// The url string is read from the command line
ObjectName squareMLetClassLoader = null;
ObjectName triangleMLetClassLoader = null;

Object mletParams[] = {url};
String mletSignature[] = {"java.lang.String"};
Vector mbeanList = (Vector) connectorClient.invoke(
    mletName, "performLoadURL", mletParams, mletSignature);

for (Enumeration enum = mbeanList.elements(); enum.hasMoreElements(); ) {
    Object element = enum.nextElement();
    if (element instanceof Vector) {
        // Success, we retrieve the new object name
        Vector v = (Vector) element;
        ObjectInstance objectInstance = (ObjectInstance) v.elementAt(0);
        ObjectName classLoaderObjectName = (ObjectName) v.elementAt(1);
        if (objectInstance.getClassName().equals("Square")) {
            // Retrieve the MBean that loaded the Square

            squareMLetClassLoader = classLoaderObjectName;
        } else if (objectInstance.getClassName().equals(
                       "EquilateralTriangle")) {
            // Retrieve the MBean that loaded the EquilateralTriangle
            triangleMLetClassLoader = classLoaderObjectName;
        }
        echo("\tOBJECT NAME = " + objectInstance.getObjectName());
    } else {
        // Failure, find out why
        echo("\tEXCEPTION = " + ((Throwable)element).getMessage());
    }
}

As in the agent application, we may need a shortcut for instantiating other MBeans without specifying an m-let file. Again, we can use an existing class loader from a previously loaded class to download the same classes again. We use the createMBean method of the connector client which lets us specify a class loader name. The following code is the rest of the manager's runMLetExample method, and it is also nearly identical to the agent's code.

Example 12-7 Asking the Agent to Load Classes Directly

// Create a new Square MBean from its class in the Square.jar file.
String squareClass = "Square";
ObjectName squareName = new ObjectName(
    "MLetExample:name=" + squareClass + ",id=2");
Object squareParams[] = {new Integer(12)};
String squareSignature[] = {"java.lang.Integer"};
connectorClient.createMBean(squareClass, squareName,
    squareMLetClassLoader, squareParams, squareSignature);

// Create a new EquilateralTriangle MBean from its class in the
// EquilateralTriangle.jar file.
String triangleClass = "EquilateralTriangle";
ObjectName triangleName = new ObjectName(
    "MLetExample:name=" + triangleClass + ",id=3");
Object triangleParams[] = {new Integer(20)};
String triangleSignature[] = {"java.lang.Integer"};
connectorClient.createMBean(triangleClass, triangleName,
    triangleMLetClassLoader, triangleParams, triangleSignature);

Simulating a "push" of the MBeans in this way is plausible, since the management application can specify a URL where it controls the contents of the HTML file and knows which classes are available.