Why spark.sql.autoBroadcastJoinThreshold not available

[Jone Zhang]

Now i use spark1.6.0 in java
I wish the following sql to be executed in BroadcastJoin way
*select * from sample join feature*

This is my step
1.set spark.sql.autoBroadcastJoinThreshold=100M
2.HiveContext.sql("cache lazy table feature as "select * from src where
...) which result size is only 100K
3.HiveContext.sql("select * from sample join feature")
Why the join is SortMergeJoin?

Grateful for any idea!
Thanks.