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Java Dynamic Management Kit 5.0 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 two MLET tags:

Example 14–2 The M-Let File

<HTML>
<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 two MBeans with the given object names, using the given classes in the JAR files. 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 can also specify a CODEBASE, which is an alternate location for the JAR file. The MLET tag is fully defined in the JMX specification.

To download the MBeans specified in the m-let file we call the getMBeansFromURL method and analyze the result:

Example 14–3 Calling the getMBeansFromURL Method

// the url_2 string is read from the command line
echo("\tURL = " + url_2);
Object mletParams_2[] = {url_2};
String mletSignature_2[] = {"java.lang.String"};
Set mbeanSet = (Set) server.invoke(mletName, "getMBeansFromURL",
     mletParams_2, mletSignature_2);

for (Iterator i = mbeanSet.iterator(); i.hasNext(); ) {
    Object element = i.next();
    if (element instanceof ObjectInstance) {
        // Success, we display the new MBean's name
        echo("\tOBJECT NAME = 
			" + ((ObjectInstance)element).getObjectName());
    } else {
        // Failure, we display why
        echo("\tEXCEPTION = " + ((Throwable)element).getMessage());
    }
}

The m-let loader is the class loader, and it handles just a list of code-bases that it has accessed directly. You can view this list by calling the getURLs method of the m-let loader MBean.

This behavior means that the getMBeansFromURL method does not need to return the object names of class loaders it has used. Instead it just returns either the object instance of the downloaded and registered MBean or a Throwable object in case of an error or an exception. These are returned in a Set object containing as many elements as there are MLET tags in the target m-let file.