Inconsistency in Constructor.getGenericParameterTypes()

[Oliver Drotbohm]

Hi Joe,

thanks for the explanation. We switched to rather iterating over ….getParameters() and take it from there. Do you think it makes sense to leave a note about this in the Javadoc?

Cheers,
Ollie

> Am 26.02.2021 um 22:38 schrieb Joe Darcy <joe.darcy at oracle.com>:
> 
> Hello Oliver,
> 
> This is long-standing if surprising and under-documented behavior.
> 
> The getGenericFoo methods, when generic type information is present, give a source-level view of the element. At a source level, the implicit outer this parameter is not present and thus omitted by constructor.getGenericParameterTypes for the constructor in question.
> 
> HTH,
> 
> -Joe
> 
> On 2/26/2021 5:41 AM, Oliver Drotbohm wrote:
>> Previously sent to the wrong list. Sorry for the double post.
>> 
>> Von: Oliver Drotbohm <odrotbohm at vmware.com>
>> Betreff: Inconsistency in Constructor.getGenericParameterTypes()
>> Datum: 25. Februar 2021 um 10:03:12 MEZ
>> An: jdk-dev at openjdk.java.net
>> 
>> Hi all,
>> 
>> we've just ran into the following issue: for a non-static, generic inner class with a constructor declaring a generic parameter, a call to constructor.getGenericParameterTypes() does not return the enclosing class parameter type. Is that by intention? If so, what's the reasoning behind that?
>> 
>> Here's a the output of a reproducer (below):
>> 
>> static class StaticGeneric<T> - names: value, string
>> static class StaticGeneric<T> - parameters: [class java.lang.Object, class java.lang.String]
>> static class StaticGeneric<T> - generic parameters: [T, class java.lang.String]
>> 
>> class NonStaticGeneric<T> - names: this$0, value, String
>> class NonStaticGeneric<T> - parameters: [class Sample, class java.lang.Object, class java.lang.String]
>> class NonStaticGeneric<T> - generic parameters: [T, class java.lang.String]
>> 
>> class NonStaticNonGeneric - names: this$0, String
>> class NonStaticNonGeneric - parameters: [class Sample, class java.lang.String]
>> class NonStaticNonGeneric - generic parameters: [class Sample, class java.lang.String]
>> 
>> Note how the constructor of the NonStaticGeneric<T> type exposes three parameter names, three parameter types but omits the enclosing class parameter in the list of generic parameter types.
>> 
>> Tested on JDK 8 to 15. Same behavior.
>> 
>> Cheers,
>> Ollie
>> 
>> 
>> class Sample {
>> 
>> 	public static void main(String[] args) {
>> 
>> 		Constructor<?> first = StaticGeneric.class.getDeclaredConstructors()[0];
>> 
>> 		System.out.println("static class StaticGeneric<T> - names: "
>> 				+ Arrays.stream(first.getParameters()).map(Parameter::getName).collect(Collectors.joining(", ")));
>> 		System.out.println("static class StaticGeneric<T> - parameters: " + Arrays.toString(first.getParameterTypes()));
>> 		System.out.println(
>> 				"static class StaticGeneric<T> - generic parameters: " + Arrays.toString(first.getGenericParameterTypes()));
>> 
>> 		System.out.println();
>> 
>> 		Constructor<?> second = NonStaticGeneric.class.getDeclaredConstructors()[0];
>> 		System.out.println("class NonStaticGeneric<T> - names: "
>> 				+ Arrays.stream(second.getParameters()).map(Parameter::getName).collect(Collectors.joining(", ")));
>> 		System.out.println("class NonStaticGeneric<T> - parameters: " + Arrays.toString(second.getParameterTypes()));
>> 		System.out
>> 				.println(
>> 						"class NonStaticGeneric<T> - generic parameters: " + Arrays.toString(second.getGenericParameterTypes()));
>> 
>> 		System.out.println();
>> 
>> 		Constructor<?> third = NonStaticNonGeneric.class.getDeclaredConstructors()[0];
>> 		System.out.println("class NonStaticNonGeneric - names: "
>> 				+ Arrays.stream(third.getParameters()).map(Parameter::getName).collect(Collectors.joining(", ")));
>> 		System.out.println("class NonStaticNonGeneric - parameters: " + Arrays.toString(third.getParameterTypes()));
>> 		System.out
>> 				.println(
>> 						"class NonStaticNonGeneric - generic parameters: " + Arrays.toString(third.getGenericParameterTypes()));
>> 	}
>> 
>> 	static class StaticGeneric<T> {
>> 		StaticGeneric(T value, String string) {}
>> 	}
>> 
>> 	class NonStaticGeneric<T> {
>> 		NonStaticGeneric(T value, String String) {}
>> 	}
>> 
>> 	class NonStaticNonGeneric {
>> 		NonStaticNonGeneric(String String) {}
>> 	}
>> }
>>