JNIEXPORT jobject JNICALL
Java_sun_management_MemoryPoolImpl_getUsage0
  (JNIEnv *env, jobject pool)
{
    jobject usage = jmm_interface->GetMemoryPoolUsage(env, pool);
    if (usage == NULL) {
        // Throw internal error since this implementation expects the
        // pool will never become invalid.
        JNU_ThrowInternalError(env, "Memory Pool not found");
    }
    return usage;
}

This function as well as Java_sun_management_MemoryPoolImpl_getMemoryManagers0, Java_sun_management_MemoryPoolImpl_getPeakUsage0 should propagate the exception that may be returned by the jmm call rather than throw InternalError.