Groovy & Grails Get Value From Session

Step by Step for set and get variable
1. initial variable for user name on main.gsp :
....
g:set var="userId" value="UserName" scope="session"
...

2. get variable from var userId on Groovy :
...
println(session.userId)
...

userId is a variable on tag g:set var="userId"

3. result :
UserName