RFR: 8165276: Spec states to invoke the premain method in an agent class if it's public but implementation differs [v4]

[David Holmes]

On 16/12/2020 11:35 am, Serguei Spitsyn wrote:
> On Wed, 16 Dec 2020 00:50:04 GMT, David Holmes <dholmes at openjdk.org> wrote:
>> The agent class doesn't have to be public it just has to be accessible.
>>
>> The premain method should be queried for public modifier rather than just relying on a failed invocation request.
> 
> David, thank you for catching this. I'm probably missing something here.
> If the agent class is not public then the `m.canAccess(null)` check is not passed and IAE is thrown with the message:
>    `Exception in thread "main" java.lang.IllegalAccessException: method NonPublicAgent.premain must be declared public`
> 
> But the `NonPublicAgent.premain` is declared public as below:
>      public static void premain(String agentArgs, Instrumentation inst) {
>          System.out.println("premain: NonPublicAgent was loaded");
>      }
> It seems, the IAE is thrown because the agent class is not public.
> Does it mean the `m.canAccess(null)` check is not fully correct?

canAccess will check both the type accessibility and the method 
accessibility.

The premain class is not required to be public so it may not be right to 
checks its accessibility as that may fail even though the premain method 
is public and should be invoked. In that regard I think the 
setAccessible call has to remain to deal with this situation. So the 
process would be:

- load premain class via appLoader
- lookup premain method using getDeclaredMethod()
- check premain method is public and reject if not
- call setAccessible(true) in case premain class is not accessible
- do invoke

David
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> 
> PR: https://git.openjdk.java.net/jdk/pull/1694
>