RFR: JDK-8323706 Move SimpleSelector and CompoundSelector to internal packages

[John Hendrikx]

On Fri, 19 Jan 2024 00:00:32 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> Moves `SimpleSelector` and `CompoundSelector` to internal packages.
>> 
>> This can be done with only a minor API break, as `SimpleSelector` and `CompoundSelector` were public before.  However, these classes could not be constructed by 3rd parties.  The only way to access them was by doing a cast (generally they're accessed via `Selector` not by their sub types).  The reason they were public at all was because the CSS engine needs to be able to access them from internal packages.
>> 
>> This change fixes a mistake (or possibly something that couldn't be modelled at the time) when the CSS API was first made public. The intention was always to have a `Selector` interface/abstract class, with private implementations (`SimpleSelector` and `CompoundSelector`).
>> 
>> This PR as said has a small API break.  The other changes are (AFAICS) source and binary compatible:
>> 
>> - Made `Selector` `sealed` only permitting `SimpleSelector` and `CompoundSelector` -- as `Selector` had a package private constructor, there are no concerns with pre-existing subclasses
>> - `Selector` has a few more methods that are now `protected` -- given that the class is now sealed, these modified methods are not accessible (they may still require rudimentary documentation I suppose)
>> - `Selector` now has a `public` default constructor -- as the class is sealed, it is inaccessible
>> - `SimpleSelector` and `CompoundSelector` have a few more `public` methods, but they're internal now, so it is irrelevant
>> - `createMatch` was implemented directly in `Selector` to avoid having to expose package private fields in `Match` for use by `CompoundSelector`
>> - No need anymore for the `SimpleSelectorShim`
>
> modules/javafx.graphics/src/main/java/javafx/css/Selector.java line 179:
> 
>> 177:      * @throws IOException if reading from {@code DataInputStream} fails
>> 178:      */
>> 179:     protected static Selector readBinary(int bssVersion, DataInputStream is, String[] strings)
> 
> This is an API addition, so will need an `@since 23`. Since you are adding new API here, should it be an instance method rather than a static method? Or is is called without having an instance of Selector in some cases?

I'll add the `@since 23`, however it can't be called by anyone except FX itself.

Some background: the readBinary/writeBinary methods are ultimately called via the publicly accessible methods `loadBinary` and `saveBinary` in `javafx.css.Stylesheet`.  

The reason it needs to be `protected` now is because `CompoundSelector` is using this (but IMHO, it shouldn't have).  Why I say it shouldn't?  That's because it already knows what it will be reading will all be `SimpleSelector`s, as it stores a counter (a `short`) and then loads that many `SimpleSelector`s.  However, by not taking the direct route of using `SimpleSelector#readBinary` (and the corresponding `SimpleSelector#writeBinary`) there is an additional `byte` prefix indicating the type of selector -- this isn't needed and wastes some space in the binary format.

Changing that now however would make the binary format incompatible with earlier versions, so I think making this `protected` is a reasonable solution.  It also mirrors the `writeBinary` method then, which was already `protected`.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1333#discussion_r1458688819