JEP draft: Prepare to Restrict The Use of JNI

[Maurizio Cimadamore]

Hi Volker,
note that the warning is generated once per module.

I think what you want is some kind of "log" mode which, while slower, 
might be more helpful to identify all the places which are performing 
JNI calls.

That said, even the "log" mode might not detect everything (as it 
basically depends on the control flow in your application). So, a 
classfile analyzer might be more complete.

If we want to enable different policies for --enable-native-access 
(similarly to what's done for -illegal-access=permit|deny|warn|debug), 
we need to split the current --enable-native-access flag in two:

* a flag to select the list of modules that are allowed to perform 
native access
* a flag to select what should happen when a native access from a 
non-allowed module is detected

Cheers
Maurizio

On 22/08/2023 14:39, Volker Simonis wrote:
> Hi,
>
> Could the warning be changed to include a stack trace (at least
> optional)? I think that can be helpful for bigger applications in
> order to identify the reason for why a JNI library was loaded.
>
> Thank you and best regards,
> Volker
>
> On Mon, Aug 21, 2023 at 5:42 AM Ron Pressler<ron.pressler at oracle.com>  wrote:
>> Hi.
>>
>> JNI has been part of the Java Platform since JDK 1.1, but it is inherently
>> unsafe, so we are proposing to restrict the use of JNI in line with the use of
>> restricted methods in the Foreign Function & Memory (FFM) API [1].
>>
>> The FFM API, expected to become permanent in JDK 22, has methods for interacting
>> with native code that are inherently unsafe. To protect the integrity of the
>> Java Platform [2], use of these methods is restricted. This means that calling
>> them causes warnings, unless the user enables unsafe native access on the
>> command line. In a future release, calling them will cause exceptions instead
>> of warnings.
>>
>> We propose to align JNI and FFM as follows. Starting in JDK 22, the Java runtime
>> will give warnings about the use of JNI, unless the user (as for FFM) enables
>> unsafe native access on the command line. In a future release, using JNI will
>> cause exceptions instead of warnings.
>>
>> In other words, our proposal is: In the future, code that uses JNI will run only
>> if unsafe native access is enabled, and to prepare for that future, JDK 22
>> should spotlight the use of JNI with warnings. We've published a draft JEP that
>> explains why using JNI is unsafe and describes the warnings [3].
>>
>> We recognize that many applications use JNI and will be impacted by this
>> proposal. It is bound to spark discussion that continues even after the
>> preparatory warnings in JDK 22. Comments are welcome.
>>
>> — Ron
>>
>> [1]:https://openjdk.org/jeps/442
>> [2]:https://openjdk.org/jeps/8305968
>> [3]:https://openjdk.org/jeps/8307341
>>
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