RFR: 8348240: Remove SystemDictionaryShared::lookup_super_for_unregistered_class()

[Calvin Cheung]

On Wed, 22 Jan 2025 03:28:00 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> I reimplemented `SystemDictionaryShared::lookup_super_for_unregistered_class()` with Java code. This removes hacks in `SystemDictionary::resolve_with_circularity_detection()` to facilitate future clean-ups there that are planned by @coleenp. Please see the [JBS issue](https://bugs.openjdk.org/browse/JDK-8348240) for a discussion of why the hack was there.
> 
> The new Java code is in the `jdk/internal/misc/CDS$UnregisteredClassLoader` class. I also simplified `UnregisteredClasses::create_classloader()` by moving some unwieldy C++ `JavaCalls` code into Java.
> 
> ### How this works:
> 
> For example, let's assume you have the following classfiles in foo.jar:
> 
> 
> interface MyInterface1 {}
> interface MyInterface2 {}
> class MyClass extends Object implements MyInterface1, MyInterface2 {}
> 
> 
> The CDS classlist file can look like this:
> 
> 
> java/lang/Object id: 1
> MyInterface1: id: 2 super: 1 source: foo.jar
> MyInterface2: id: 3 super: 1 source: foo.jar
> MyClass: id: 4 super: 1 interfaces: 2 3 source: foo.jar
> 
> 
> When CDS handles the `MyClass: id: 4` line, it calls `CDS$UnregisteredClassLoader::load()` with the following parameters:
> 
> - `name` = "MyClass"
> - `superClass` = java.lang.Object
> - `interfaces` = {MyInterface1, MyInterface2} 
> 
> `CDS$UnregisteredClassLoader::load()` will start parsing MyClass.class from foo.jar. The ClassFileParser will see symbolic references to the supertypes of `MyClass`. These supertypes will be resolved by `CDS$UnregisteredClassLoader::findClass()`, using the classes provided by `superClass` and `interfaces`.

Looks like a good cleanup. I have couple of comments.

src/hotspot/share/cds/unregisteredClasses.cpp line 72:

> 70: 
> 71:   JavaValue result(T_OBJECT);
> 72:   JavaCallArguments args(2);

Should the 2 be increased to 3?

src/hotspot/share/classfile/vmSymbols.hpp line 742:

> 740:   template(toFileURL_name,                                  "toFileURL")                                          \
> 741:   template(toFileURL_signature,                             "(Ljava/lang/String;)Ljava/net/URL;")                 \
> 742:   template(url_array_classloader_void_signature,            "([Ljava/net/URL;Ljava/lang/ClassLoader;)V")          \

I think you can also remove the following since `URLClassLoader` is not used directly anymore:
`do_klass(URLClassLoader_klass,                        java_net_URLClassLoader                               ) `

-------------

PR Review: https://git.openjdk.org/jdk/pull/23226#pullrequestreview-2568629771
PR Review Comment: https://git.openjdk.org/jdk/pull/23226#discussion_r1926215557
PR Review Comment: https://git.openjdk.org/jdk/pull/23226#discussion_r1926205535