32.2. Projection
JBoss EL supports a limited projection syntax. A projection expression maps a sub-expression across a multi-valued (list, set, etc...) expression. For instance, the expression:
#{company.departments}
might return a list of departments. If you only need a list of department names, you must iterate over the list to retrieve the values. JBoss EL allows this with a projection expression:
#{company.departments.{d|d.name}}
The sub-expression is enclosed in braces. In this example, the expression
d.name is evaluated for each department, using d as an alias to the department object. The result of this expression will be a list of String values.
Any valid expression can be used in an expression, so —assuming you would use department names of all lengths in a company —it would also be valid to write the following:
#{company.departments.{d|d.size()}}
Projections can be nested. The following expression returns the last names of every employee in every department:
#{company.departments.{d|d.employees.{emp|emp.lastName}}}
Nested projections can be slightly tricky, however. The following expression appears to return a list of all employees in all departments:
#{company.departments.{d|d.employees}}
However, it actually returns a list containing a list of the employees for each individual department. To combine the values, it is necessary to use a slightly longer expression:
#{company.departments.{d|d.employees.{e|e}}}
This syntax cannot be parsed by either Facelets or JSP, so it cannot be used in XHTML or JSP files. Future versions of JBoss EL may accommodate the projection syntax more easily.
