RFR: 8193904: Uninitialized final field access and qualified this

[Remi Forax]

----- Original Message -----
> From: "Archie L. Cobbs" <duke at openjdk.org>
> To: "compiler-dev" <compiler-dev at openjdk.org>
> Sent: Wednesday, November 2, 2022 8:13:59 PM
> Subject: RFR: 8193904: Uninitialized final field access and qualified this

> If a class `MyClass` has a blank final field `foo` that has not yet been
> assigned, the compiler prevents you from accessing it via the expressions `foo`
> or `this.foo`. However, it does not prevent you from accessing it via the
> expression `MyClass.this.foo`.
> 
> Here's a simple example:
> 
> class QualifiedThis {
>    final int foo;
>    QualifiedThis() {
>        System.err.println(QualifiedThis.this.foo);   // should get an error here
>        this.foo = 42;
>    }
> }
> 
> 
> This patch fixes that omission.
> 
> I couldn't find an existing method that answers the question "Is this AST tree a
> reference to the current instance of the class I'm now compiling?" so I wrote a
> new one `TreeInfo.isThisReference()`.
> 
> The question itself is a little tricky. For example, `this` alone always is, but
> `Foo.this` is if the class being compiled is `Foo` or any supertype of `Foo`,
> *except* when `Foo` is an outer class containing the class being compiled, when
> it instead refers to that outer instance.

yes, i've always seen the qualification of this by the current class as a weird unnecessary generalization
(John in CC if there is a real reason/use case for it).

> 
> On the other hand, `Foo.super` (which, unlike `Foo.this`, can only be used to
> invoke a method), when `Foo` is a type and not a variable, refers to the
> current 'this' instance when the class being compiled is `Foo` or any supertype
> of `Foo`, period.

yes.

Rémi


> 
> -------------
> 
> Commit messages:
> - Disallow early access to final fields via qualified 'this'.
> 
> Changes: https://git.openjdk.org/jdk/pull/10956/files
> Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10956&range=00
>  Issue: https://bugs.openjdk.org/browse/JDK-8193904
>  Stats: 73 lines in 5 files changed: 70 ins; 0 del; 3 mod
>  Patch: https://git.openjdk.org/jdk/pull/10956.diff
>  Fetch: git fetch https://git.openjdk.org/jdk pull/10956/head:pull/10956
> 
> PR: https://git.openjdk.org/jdk/pull/10956