Cannot expose Java Instrumentation for tomcat jvm

I am using a tomcat7 instance to run a Java application. My application needs the Java instrumentation exposed. This is done with a javaagent and I pass the agent at startup to the JVM in the setenv.bat script.

set JAVA_OPTS=%JAVA_OPTS% -javaagent:"C:\path\to\agent.jar"

In the manifest file I have the required section:

Premain-Class: package.name.agent.ExposeInstrumentation

In the premain method of the agent class assign the instrumentation provided by the JVM to a static variable accessible through a static method

public final class ExposeInstrumentation {
private static Instrumentation s_instrumentation;
public static void premain(String arguments, Instrumentation instrumentation) {
    s_instrumentation = instrumentation;
}

public static Instrumentation getInstrumentation() {
    return s_instrumentation;
}
}

But in my code when I do this:

Instrumentation instrumentation = ExposeInstrumentation.getInstrumentation();

getInstrumentation() returns null;

What is the problem?

UPDATE

I did some further debugging and premain gets executed and s_instrumentation receives the instrumentation, but when I call getInstrumentation later on in my code s_instumentation is set to null. This is strange I tought the value remains valid thought the life of the program.