RFR: 8164714: Constructor.newInstance creates instance of inner class with null outer class [v4]

[Maurizio Cimadamore]

On Wed, 19 Mar 2025 18:12:31 GMT, Chen Liang <liach at openjdk.org> wrote:

>> The Java Language Specification anticipates that inner classes always have non-null enclosing instances. It ensures the non-nullness by enforcing null checks at the use sites that provides immediately enclosing instances to inner class constructors, such as for super invocations, or an `outer.new Inner()` invocation.
>> 
>> However, the translations do not require a null check in the actual constructor, when the immediately enclosing instance is received through a mandated parameter and stored into a synthetic field or discarded.  As a result, class file constructs, such as core reflection, method handles, or arbitrary class files can pass in `null` for the immediately enclosing instance, and later execution may fail with NPE by chance if any enclosing instance is used.
>> 
>> This patch proposes to add a null check against the "outer this" in inner class constructors that call a superclass constructor, including when the "outer this" is discarded immediately thereafter (#4966) for consistency. This null check will be emitted regardless of source or target versions. This change is considered an implementation artifact like the synthetic field that captures the enclosing instance; as a result, there is no JLS change.
>> 
>> The reason for this eager NPE decision is that there is no compatibility of such NPE behaviors - any evolution of the inner classes constructed with null enclosing instances may suddenly start using an enclosing instance and fail with NPE. Therefore, there's no compatibility aspect in such out-of-spec usages of passing `null` as immediately enclosing instance, and this null check can be considered such an evolution.
>
> Chen Liang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:
> 
>  - Better docs
>  - Merge branch 'master' of https://github.com/openjdk/jdk into fix/enclosing-this-null-check
>  - Emit null check by flag/release
>  - Merge branch 'master' of https://github.com/openjdk/jdk into fix/enclosing-this-null-check
>  - Merge branch 'master' of https://github.com/openjdk/jdk into fix/enclosing-this-null-check
>  - Fix javap test
>  - 8164714: Constructor.newInstance creates instance of inner class with null outer class

The compiler changes look good - and I think more generally this looks like a good enforcement to do in the generated code (also in perspective, consider what's coming in Valhalla).

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PR Comment: https://git.openjdk.org/jdk/pull/23875#issuecomment-2743247946