Java Dynamic Management Kit 5.0 Tutorial

Loading MBeans Directly

Because the m-let loader object is a class loader, you can use it to load classes directly, without needing to define an m-let file.

Before you can load an MBean directly, you need to add the URL of its code-base to the m-let loader's internal list. Then we just use the m-let loader's object name as the class loader name when creating the MBean. Here is the code to do this in the agent example:

Example 14–5 Using the M-Let MBean as a Class Loader

// Add a new URL to the MLet class loader
// The url_1 string is read from the command line
Object mletParams_1[] = {url_1};
String mletSignature_1[] = {"java.lang.String"};
server.invoke(mletName, "addURL", mletParams_1, mletSignature_1);

// Create a Square MBean from its class in the Square.jar file.
String squareClass = "Square";
ObjectName squareName = new ObjectName(
    "MLetExample:name=" + squareClass);
Object squareParams[] = {new Integer(10)};
String squareSignature[] = {"java.lang.Integer"};
server.createMBean(squareClass, squareName, mletName,
                   squareParams, squareSignature);

You only need to add the URL to the m-let loader the first time you download a class. Once it is added, you can load it as many times as necessary by calling createMBean directly.

Because this loading mechanism does not use the MLET tag, the programmer must ensure that either the downloaded class provides its own object name or, as in Example 14–5, the agent provides one.

The fact that the m-let loader is also a class loader into which you can load multiple URLs raises the issue of name spaces. If there are two classes with the same name within the code-bases defined by the set of all URLs, the m-let loader will load one of them non-deterministically. To specify one of them precisely, do not add the URL of the second code-base to the m-let loader. Instead, create a second m-let loader MBean to which you can add the URL for the second version of the class. In this case, you will have one m-let MBean that can load one version of the class and another m-let MBean that can load the other.