RFR: JDK-8315458 Implement JEP 463: Implicitly Declared Classes and Instance Main Method (Second Preview) [v26]

[Jim Laskey]

On Mon, 20 Nov 2023 13:56:13 GMT, Christian Stein <cstein at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/Class.java line 4839:
>> 
>>> 4837:     @PreviewFeature(feature=PreviewFeature.Feature.IMPLICIT_CLASSES)
>>> 4838:     @CallerSensitive
>>> 4839:     public Method getMainMethod() {
>> 
>> This is a new addition in the last few days. It's somewhat surprising as the  launch protocol has never really surfaced in the API, there's nothing to identify the main class for example. I'm in two minds on surfacing this but it does make the java launcher code much simpler.
>> 
>> Class.getMethod/getMethods return public methods, the proposed getMainMethod may return a public or non-public method and may walk the class hierarchy. So I'm not sure about the naming, it feels more like a findMainMethod to emphasize the search or just mainMethod.
>> 
>> The javadoc probably needs to be fleshed out a bit more to specify how the main method is found.  It currently says "be declared this class's hierarchy" (typo) but it has to say more and make it clear that it may return a Method with a declared class that may be a superclass.
>> 
>> "(@jls 8.4.2)", I assume that should be "{@jls 8.4.2}".
>> 
>> The return description says "the candidate main method". This begs the question if there are other candidates, how do I find them?
>> 
>> The SM permission check looks right, it has to be the same as getDeclaredMethod.
>
>> [...] it does make the java launcher code much simpler.
> 
> Yes. That's a very nice effect of surfacing the launch protocol directyl in `java.lang.Class`
> 
>> So I'm not sure about the naming, it feels more like a `findMainMethod` to emphasize the search or just `mainMethod`.
> 
> +1 for `findMainMethod`, and also an explicit `Optional<Method>` as the return type.
> 
> Or `Optional<Method> mainMethod()` similar to [ModuleDescriptor::mainClass](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/module/ModuleDescriptor.html#mainClass())

Updated.

Optional is stylizing preference. Since this method is used frequently (almost always) at startup, loading extra classes should probably be avoided. It's not hard to simply `Optional.ofNullable(cls.findMainMethod())`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16461#discussion_r1399597600