Java Management Extensions

Chapter 8. Dynamic Loading

In this chapter, we will discuss a facility provided by JMX that allows MBeans to be loaded into an agent dynamically. This facility, called the M-Let (short for management applet) service, is the first agent-level service we have discussed so far. There are two major sections in this chapter. The first section is an overview of the M-Let service, including the various facets of it that make it work. The second section deals with the details of the M-Let service and provides examples of code that executes in the JMX agent that uses the M-Let service.

Overview

In this section, we will look at the M-Let service, whose purpose is to provide an agent with a means to load MBeans from a Universal Resource Locator (URL). There are two ways that an agent can use the M-Let service to accomplish this. First, the agent can specify an M-Let file to the M-Let service, which uses the contents of this file to load the MBeans. The M-Let file is an XML-like text file that contains various tags that describe the MBeans to be loaded. The second method of loading MBeans is to use the M-Let service itself to load the MBeans without the use of an M-Let file. The M-Let service extends URLClassLoader from the java.net package and is thus capable of fetching bytecode from any valid URL into the JVM in which the agent is running.

The M-Let Service

In the RI, the M-Let service is implemented in a class called MLet, which implements an interface called MLetMBean (so it is instrumented ...