RFR: 8304932: MethodTypeDescImpl can be mutated by argument passed to MethodTypeDesc.of [v2]

[Chen Liang]

On Mon, 17 Apr 2023 21:04:31 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> Have you considered that the caller of instantiating `MethodTypeDescImpl` is responsible for passing a trusted array? I think that `MethodTypeDescImpl` implementation already assumes it's a trusted array. So `MethodTypeDesc::of` to call `new MethodTypeDescImpl` with a cloned array. `MethodTypeDescImpl` should call `new MethodTypeDescImpl` instead of `MethodTypeDesc::of` as this PR did.
> 
> That way will avoid using `JavaUtilCollectionAccess` to create an immutable list without copying the input array.

My plan was that `MethodType` will expose extra entry points:

static MethodTypeDesc of(ClassDesc returnType) {
    return of(returnType, List.of());
}

static MethodTypeDesc of(ClassDesc returnType, Collection<ClassDesc> paramTypes) {
    return of(returnType, List.copyOf(paramTypes));
}


The `Collection` version can avoid copying if the user-provided list is already a null-hostile immutable list. If this API is added, I can safely swap all existing `new MethodTypeDescImpl` with `MethodTypeDesc.of` without problems.

For the backing structure for parameters, I still believe an immutable list is better than an array; unlike MethodType that needs to interact with the VM which loves arrays better, MTD is mainly user-oriented and `parameterList` is often used by programmers. By avoiding list copies in `parameterList()` alone, I think there will be immediate benefit to users.

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PR Comment: https://git.openjdk.org/jdk/pull/13186#issuecomment-1512102844