public static  void agentmain(String agentArgs,
    Instrumentation inst) 
{
        InstrumentationFactory.setInstrumentation(inst);
}

The method that is called when a jar is added as an agent at runtime. All this method does is store the Instrumentation for later use.

 public static synchronized Instrumentation getInstrumentation() throws ClassNotFoundException, InvocationTargetException, IOException, NoSuchMethodException, IllegalAccessException 
{
        if (_inst != null || !_dynamicallyInstall)
            return _inst;
        // dynamic loading of the agent is only available in JDK 1.6+
        if (JavaVersions.VERSION <  6)
            return null;
        String agentPath = getAgentJar();
        // first obtain the PID of the currently-running process
        // ### this relies on the undocumented convention of the RuntimeMXBean's
        // ### name starting with the PID, but there appears to be no other
        // ### way to obtain the current process' id, which we need for
        // ### the attach process
        RuntimeMXBean runtime = ManagementFactory.getRuntimeMXBean();
        String pid = runtime.getName();
        if (pid.indexOf("@") != -1)
            pid = pid.substring(0, pid.indexOf("@"));
        // JDK1.6: now attach to the current VM so we can deploy a new agent
        // ### this is a Sun JVM specific feature; other JVMs may offer
        // ### this feature, but in an implementation-dependent way
        Class vmClass = Class.forName("com.sun.tools.attach.VirtualMachine");
        Object vm = vmClass.getMethod("attach", new Class[] { String.class }).
            invoke(null, new String[] { pid });
        // now deploy the actual agent, which will wind up calling agentmain()
        vm.getClass().getMethod("loadAgent", new Class[] { String.class }).
            invoke(vm, new Object[] { agentPath });
        if (_inst != null)
            return _inst;
        return null;
}

 public static synchronized  void setDynamicallyInstallAgent(boolean val) 
{
        _dynamicallyInstall = val;
}

Configures whether or not this instance should attempt to dynamically install an agent in the VM. Defaults to true.

 public static  void setInstrumentation(Instrumentation inst) 
{
         
        _inst = inst;
}