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Java Dynamic Management Kit 4.0 Tutorial

Loading MBeans from a URL

In order to download an MBean, we must first have its corresponding m-let definition in an HTML file. In our example, we define the following file with three MLET tags:

Example 6-2 The M-Let File

<HTML>
<MLET
  CODE=Square.class
  ARCHIVE=Square.jar
  NAME=MLetExample:name=Square,id=1
>
<ARG TYPE=java.lang.Integer VALUE=10>
</MLET>
<MLET
  CODE=EquilateralTriangle.class
  ARCHIVE=EquilateralTriangle.jar
  NAME=MLetExample:name=EquilateralTriangle,id=1
>
<ARG TYPE=java.lang.Integer VALUE=8>
</MLET>
<MLET
  CODE=EquilateralTriangle.class
  ARCHIVE=EquilateralTriangle.jar
  NAME=MLetExample:name=EquilateralTriangle,id=2
>
<ARG TYPE=java.lang.Integer VALUE=15>
</MLET>
</HTML>

This file tells the m-let loader to create three MBeans with the given object names, using the given classes in the jar files. The ARG tag gives a parameter to be passed to the class' constructor. The number and type of ARG tags specified must match one of the public constructors for the class

The jar files must be located in the same directory as this file, regardless of whether the directory is on a local or remote host. The MLET tag may also specify a CODEBASE, which is an alternate location for the jar file. This m-let loader relies on the definition of MLET tag given by the JMX specification.

Now we are ready to call the performLoadURL method of our m-let loader. In parsing the result vector, we use our knowledge of the class names that we wrote in the m-let file.

Example 6-3 Calling the performLoadURL Method

ObjectName squareMLetClassLoader = null;
ObjectName triangleMLetClassLoader = null;

// The url string is read from the command line
Object mletParams[] = {url};
String mletSignature[] = {"java.lang.String"};
Vector mbeanList = (Vector) server.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 MBean that loaded the class Square
            squareMLetClassLoader = classLoaderObjectName;

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

The result of the call to performLoadURL is a Vector object containing as many elements as there are MLET tags in the file designated by the URL. Each element is either a vector containing the object instance of the new MBean and the object name of its class loader, or a Throwable object containing the exception or error that prevented the MBean from being loaded. The order of the elements may differ from the order of the tags in the file.

In the result, we obtain the object name of the MBeans that were created from the downloaded classes. The management architecture specified by JMX is designed so that objects are manipulated through the MBean server, not by direct reference. Therefore, downloaded classes are directly registered in the MBean server by the m-let loader, and the caller never receives a direct reference to the new object.

The object names of the class loaders are references to the internal class loader objects used by the m-let service to actually fetch the classes. We save them because they can be used if we ever need to instantiate these classes again. We will see how in the next section.