I am using apex:selectlist to set values from VF page to controller. Here is the VF page screenshot - 
![enter image description here](https://i.stack.imgur.com/K6ZJt.jpg)

objectname is an apex property that stores Object name and limitset holds the maximum number of records to query.

The thing is, the code works fine when we select both the values in VF page. However, the action method - "processRequest" is not invoked if either of the objectname or limitset is not selected in the VF page. I am not able to understand why something weird is happening. I wanted to display a pageMessage in case either of the values is not selected.

Any help would be appreciated. Thanks.