code: public void setFilter(final String expression) throws FilterParseException {
          if (expression != null && !expression.isEmpty()) {
              this.filter = this.filterFactory.createFilter(expression);
          }
      }
docs: It is possible to specify a filter so that only packets that matches the
  filter will be passed onto the registered {@link PacketHandler}.
  
  E.g., to only accept packets of type sip with the Call-ID of "123" you
  could pass in the following filter:
  
  "sip.Call-ID == 123"
  
  @param expression
  the expression. If the expression is null or the empty string,
  it will silently be ignored.
  @throws FilterParseException
  in case the expression is not a valid filter expression.