null check in jsf expression language

问题

Please see this Expression Language

styleClass="#{obj.validationErrorMap eq null ? ' ' :  
     obj.validationErrorMap.contains('key')?'highlight_field':'highlight_row'}"

Even if the map is null, highlight_row style is getting applied.

So I changed to

styleClass="#{empty obj.validationErrorMap ? ' ' :  
     obj.validationErrorMap.contains('key')?'highlight_field':'highlight_row'}"

Even then, highlight_row is getting applied.
if the map is empty OR null I dont want any style to be applied.

Any help? and reasons for this behaviour?

回答1:

Use empty (it checks both nullness and emptiness) and group the nested ternary expression by parentheses (EL is in certain implementations/versions namely somewhat problematic with nested ternary expressions). Thus, so:

styleClass="#{empty obj.validationErrorMap ? ' ' :  
 (obj.validationErrorMap.contains('key') ? 'highlight_field' : 'highlight_row')}"

If still in vain (I would then check JBoss EL configs), use the "normal" EL approach:

styleClass="#{empty obj.validationErrorMap ? ' ' :  
 (obj.validationErrorMap['key'] ne null ? 'highlight_field' : 'highlight_row')}"

Update: as per the comments, the Map turns out to actually be a List (please work on your naming conventions). To check if a List contains an item the "normal" EL way, use JSTL fn:contains (although not explicitly documented, it works for List as well).

styleClass="#{empty obj.validationErrorMap ? ' ' :  
 (fn:contains(obj.validationErrorMap, 'key') ? 'highlight_field' : 'highlight_row')}"

来源：https://stackoverflow.com/questions/2207266/null-check-in-jsf-expression-language