? 之间的区别? ? ，has_content，if_been in freemarker

?? tells if the left hand operand's value is missing (means it's Java null or you have an undefined variable there), and gives back false (missing) or true (not missing) accordingly.

?has_content is very much like ??, except it also returns false for a 0-length string or empty FTL sequence (like java.util.List, Java array, etc.) or empty FTL hash (like java.util.Map, etc.). (It doesn't return false for a 0, boolean false, etc.)

! is used to give a default value when a value is missing (again means that it's Java null or you have an undefined variable), like color!"no color". If you omit the right hand operand of !, then the default value is an empty string and empty sequence and empty hash on the same time (a multi-typed value), which is handy for writing things like <#list things! as thing>, ${foo!}.

?if_exists is the old way of writing ??. Don't use it.

While we are here, note that all these operators only cover the last step of a dotted or [] expression, like user.price!0 only handles if price is missing from user, but not if user itself is missing. To cover both possibilities, use (user.price)!0, which handles all missing variable errors thrown during the evaluation of the (), no mater where they come from.