Executable.hasRealParameterData() and Parameter.isNamePresent() don't work as expected

[Daniel Fuchs]

Hi Sergey,

I believe you need to compile with `-parameters`

https://docs.oracle.com/en/java/javase/18/docs/specs/man/javac.html

best regards,

-- daniel

On 29/06/2022 07:54, Сергей Цыпанов wrote:
> This question was asked originally here: https://stackoverflow.com/questions/72787286/executable-hasrealparameterdata-and-parameter-isnamepresent-dont-work-as-ex
> but as soon as I got no answer I've decided to try it here adding some concerns regarding JavaDoc of Parameter.isNamePresent()
> 
> With ad-hoc built JDK 19 (having exposed `Executable.hasRealParameterData()`) I take the code
> 
> public class Main {
> 
>    public static void main(String[] args) throws NoSuchMethodException {
>      Method foo = Main.class.getMethod("foo", String.class, int.class);
>      System.out.println(foo.hasRealParameterData());
>    }
> 
>    public void foo(String parameter1, int parameter2) {}
> }
> 
> and compile it with
> 
> % javac Main.java
> 
> Then I run compiled Java class and it prints `false` into console. This is fine because decompiled `Main` class looks like
> 
> public class Main {
>    //...
>    public void foo(String var1, int var2) {} // parameter names are not 'real'
> }
> 
> i.e. parameter names are synthetic.
> 
> This behaviour is understandable.
> 
> Then I take the same Java sources and recompile the class with
> 
> javac -g:vars Main.java
> 
> I run the same code again and again it prints `false` to console. This puzzles me, because now the compiled code looks different:
> 
> public class Main {
>    //...
>    public void foo(String parameter1, int parameter2) {} // parameter names are 'real'
> }
> 
> Same happens if for recompilation I use plain -g flag (generates all auxiliary data).
> 
> Now let's stop calling JDK's private API and rely only on the methods available out-of-the-box,
> e.g. `Parameter.isNamePresent()` (this one calls `Executable.hasRealParameterData()` under the hood):
> 
> public static void main(String[] args) throws NoSuchMethodException {
>    Method foo = Main.class.getMethod("foo", String.class, int.class);
>    Parameter parameter1 = foo.getParameters()[0];
>    Parameter parameter2 = foo.getParameters()[1];
>    System.out.println(parameter1.isNamePresent());
>    System.out.println(parameter2.isNamePresent());
> }
> 
> public void foo(String parameter1, int parameter2) {}
> 
> And again, no matter how I compile the sources, this code prints `false false`.
> 
> So my two questions are
> 1) whether this is a bug or am I doing something wrong?
> 2) is it possible somehow to change the behaviour to have at least Parameter.isNamePresent() returning true
> 
> P.S. Parameter.isNamePresent() works unexpectedly even when I run it on conventional, not hacked JDK.
> 
> P.P.S. In compiled code I see 'real' parameter names, but if I stop at debug point in IDEA parameter name is suddenly `arg0` in `Parameter.name` field.
> 
> P.P.P.S. I think that JavaDoc of Parameter.isNamePresent() should be changed as soon as now it points out to 'MethodParameters' attribute of a class file
> which is confusing to me as to end user of Java because I cannot understand from plain Java code what are those 'MethodParameters' and what will
> be returned for any of my methods.