User Presses The Save Button To Save Data?

in my form i have 4 textboxes and i want all not to be empty while saving the data.I have put condition in save button,when user presses the save button to save data, to check all the textboxes not to be empty. My question is that, i want to focus to that particular textbox which is empty during saving the data.

For example: i have txt1,txt2,txt3,txt4 as textboxes in the form if txt1 is empty while saving, the focus should switch to txt1 rather user himself have to go there.