Parameter reflection without MethodParameters attribute

[Alex Buckley]

The new java.lang.reflect.Parameter class exposes a parameter's 1) name, 
2) finality, 3) author (via isSynthetic() and isSynthesized() - if 
neither is true, then author was human), 4) whether it's a variable 
arity parameter, 5) annotations, and 6) type.

   Sidebar: If you're wondering about "variable arity parameter",
   see JLS 8.4.1. The existence of synthetic parameters means that
   the last Parameter object from Executable.getParameters() might
   not be the variable arity parameter declared in source, so each
   Parameter object needs to expose whether it's variable arity.

The methods for (5) return fundamentally the same info as 
Method.getParameterAnnotations(), so it's clear what to return when no 
annotation attributes are present: null values and empty arrays.

But the methods for (1)-(4) are tricky when no MethodParameters 
attribute is available. Should a parameter's name be the empty string, 
with no modifiers set, and not flagged as synthetic or synthetized or 
variable arity? And then have an Executable.hasParameterMetadata() 
method to say whether these values are reliable? Maybe there should be 
Optional<T> wrappers for parameter info ... only joking, this is not the 
place to solve Java's "missing value" woes.

I think the best option is to sidestep these questions by having 
Executable.getParameters() return an empty array if no MethodParameters 
attribute is available.

SE8 and greater class files may give an empty array, while pre-SE8 class 
files must give an empty array. (The same ought to apply for 
annotations: in a pre-SE5.0 class file, an attribute called 
RuntimeVisibleAnnotations means nothing, and ought to be ignored by core 
reflection, though I have a feeling it will be exposed.)

Alex