java - How to sort column with null values using display table tag -

i have list of string values in contain null values while sorting these values through display:tag,
have first display values not contain null values in ascending order , after have show null values . using following code :

<display:column title="name" property="sname" sortable="true"></display:column> 

try this:

<display:table ... defaultsort="1" defaultorder="descending"> 

where "1" index of column.