code: Filter getFilter() throws InvalidSyntaxException {
          if (filter == null && filterSpec != null) {
              filter = eventEngine.getBundleContext().createFilter(filterSpec);
          }
          return filter;
      }
docs: Get the <code>Filter</code> representing the target handler's filter
  specification.
  
  @return the compiled <code>Filter</code> instance or <code>null</code> if
  no filter specification was declared
  
  @throws InvalidSyntaxException
  if the filter specification syntax is not valid