public ModelMBeanOperationInterceptor() 
{
      super("ModelMBean Operation Interceptor");
      trace = log.isTraceEnabled();
}

 protected  void checkAssignable(String context,
    Class clazz,
    Object value) throws ClassNotFoundException, InvalidAttributeValueException 
{
      if (value != null && clazz.isAssignableFrom(value.getClass()) == false)
         throw new InvalidAttributeValueException(context + " has class " + value.getClass() + " loaded from " + value.getClass().getClassLoader() +
            " that is not assignable to attribute class " + clazz + " loaded from " + clazz.getClassLoader());
}

 public Object invoke(Invocation invocation) throws Throwable 
{
      // get the operation's descriptor
      Descriptor d = invocation.getDescriptor();
      Class clazz = invocation.getReturnTypeClass();
      String name = null;
      ObjectName objectName = null;
      if (trace)
      {
         if (d != null)
            name = (String) d.getFieldValue(NAME);
         objectName = invocation.getInvoker().getObjectName();
      }
      if (trace)
      {
         Object args = invocation.getArgs();
         if (args != null)
            args = Arrays.asList((Object[]) args);
         log.trace("Invoking objectName=" + objectName + " oper=" + name + " args=" + args + " desc=" + d);
      }
      long limit = CACHE_NEVER_LIMIT;
      if (d != null && clazz != null)
      {
         String timeLimit = (String) d.getFieldValue(CURRENCY_TIME_LIMIT);
         if (timeLimit != null)
            limit = Long.parseLong(timeLimit);
         // We are never stale
         if (limit == CACHE_ALWAYS_LIMIT)
         {
            String timeStamp = (String)d.getFieldValue(LAST_UPDATED_TIME_STAMP);
            if (timeStamp != null)
            {
               Object value = d.getFieldValue(CACHED_VALUE);
               if (trace)
                  log.trace("Always cache objectName=" + objectName + " oper=" + name + " value=" + value);
               checkAssignable("Cached value in descriptor ", clazz, value);
               return value;
            }
         }
         // is caching enabled
         if (limit != CACHE_NEVER_LIMIT)
         {
            String timeStamp = (String) d.getFieldValue(LAST_UPDATED_TIME_STAMP);
            long lastUpdate = (timeStamp == null) ? 0 : Long.parseLong(timeStamp);
            // if the value hasn't gone stale, return from the descriptor
            long now = System.currentTimeMillis();
            long expires = lastUpdate * 1000 + limit * 1000;
            if (now <  expires)
            {
               Object value = d.getFieldValue(CACHED_VALUE);
               if (trace)
                  log.trace("Using cache objectName=" + objectName + " oper=" + name + " value=" + value + " now=" + now + " expires=" + expires);
               checkAssignable("Cached value in descriptor ", clazz, value);
               return value;
            }
            else
            {
               if (trace)
                  log.trace("Cache expired objectName=" + objectName + " oper=" + name + " now=" + now + " expires=" + expires);
               d.removeField(CACHED_VALUE);
            }
         }
         else
         {
            // Unfortunatley we have to cope with stupid users
            if (trace)
               log.trace("Removing any cached value objectName=" + objectName + " oper=" + name + " descriptor=" + d);
            d.removeField(CACHED_VALUE);
         }
      }
      // we got here means either stale value in descriptior, or no caching
      Object value = invocation.invoke();            
      if (trace)
         log.trace("Got result objectName=" + objectName + " oper=" + name + " value=" + value);
      // update the descriptor (unless not caching)
      if (d !=null && limit != CACHE_NEVER_LIMIT)
      {
         String timestamp = Long.toString(System.currentTimeMillis()/1000);
         if (trace)
            log.trace("Cache result objectName=" + objectName + " oper=" + name + " value=" + value + " timestamp=" + timestamp);
         d.setField(CACHED_VALUE, value);
         d.setField(LAST_UPDATED_TIME_STAMP, timestamp);
      }
      return value;
}