Spring Expression Language (SpEL) expression for computing the key dynamically.
Default is "", meaning all method parameters are considered as a key,
unless a custom keyGenerator() has been set.
The SpEL expression evaluates against a dedicated context that provides the
#result for a reference to the result of the method invocation, which
can only be used if beforeInvocation() is false. For supported
wrappers such as Optional, #result refers to the actual object,
not the wrapper
#root.method, #root.target, and #root.caches for
references to the method, target object, and
affected cache(s) respectively.
Shortcuts for the method name (#root.methodName) and target class
(#root.targetClass) are also available.
Method arguments can be accessed by index. For instance the second argument
can be accessed via #root.args, #p1 or #a1. Arguments
can also be accessed by name if that information is available.