RFR: 8331671: Implement JEP 472: Prepare to Restrict the Use of JNI [v4]

[ExE Boss]

On Tue, 14 May 2024 18:10:28 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This PR implements [JEP 472](https://openjdk.org/jeps/472), by restricting the use of JNI in the following ways:
>> 
>> * `System::load` and `System::loadLibrary` are now restricted methods
>> * `Runtime::load` and `Runtime::loadLibrary` are now restricted methods
>> * binding a JNI `native` method declaration to a native implementation is now considered a restricted operation
>> 
>> This PR slightly changes the way in which the JDK deals with restricted methods, even for FFM API calls. In Java 22, the single `--enable-native-access` was used both to specify a set of modules for which native access should be allowed *and* to specify whether illegal native access (that is, native access occurring from a module not specified by `--enable-native-access`) should be treated as an error or a warning. More specifically, an error is only issued if the `--enable-native-access flag` is used at least once.
>> 
>> Here, a new flag is introduced, namely `illegal-native-access=allow/warn/deny`, which is used to specify what should happen when access to a restricted method and/or functionality is found outside the set of modules specified with `--enable-native-access`. The default policy is `warn`, but users can select `allow` to suppress the warnings, or `deny` to cause `IllegalCallerException` to be thrown. This aligns the treatment of restricted methods with other mechanisms, such as `--illegal-access` and the more recent `--sun-misc-unsafe-memory-access`.
>> 
>> Some changes were required in the package-info javadoc for `java.lang.foreign`, to reflect the changes in the command line flags described above.
>
> Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Address review comments
>    Improve warning for JNI methods, similar to what's described in JEP 472
>    Beef up tests
>  - Address review comments

src/java.base/share/classes/java/lang/Module.java line 334:

> 332:                     System.err.printf("""
> 333:                             WARNING: A native method in %s has been bound
> 334:                             WARNING: %s has been called by %s in %s

Note that this line is still not entirely correct, as for code like:

// in module a:
package a;

import b.Foo;

public class Foo {
	public static void main(String... args) {
		System.load("JNI library implementing Java_b_Bar_nativeMethod");

		Bar.nativeMethod();
	}
}


// in module b:
package b;

public class Bar {
	public static native void nativeMethod();
}


It’ll show `Bar` as the caller of `Bar::nativeMethod()`, even though the caller is `Foo` in this case, which is why I initially suggested just omitting the caller from **JNI** linkage warnings.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19213#discussion_r1601140578