You can use #set to define a variable that can be used within the template.

Our (original) template is as follows:

{ 
 "name" : $input.json('$.user.name'), 
 "time": $input.json('$.greeting.time') 
}

The template, when rewritten by using #set, is as follows:

#set($inputRoot = $input.path('$'))
{ 
 "name" : "$inputRoot.user.name", 
 "time": "$inputRoot.greeting.time"
}