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

[Alan Bateman]

On Tue, 8 Dec 2020 11:41:33 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> This change have been already reviewed by Mandy, Alan and David.
> Now, the PR approval is needed.
> The push was postponed because the CSR was not approved at that time (it is now):
>    https://bugs.openjdk.java.net/browse/JDK-8248189
> Investigation of existing popular java agents was requested by Joe.
> ----------
> 
> The java.lang.instrument spec:
>   https://docs.oracle.com/en/java/javase/15/docs/api/java.instrument/java/lang/instrument/package-summary.html
> 
> Summary:
>   The java.lang.instrument spec clearly states:
>     "The agent class must implement a public static premain method
>      similar in principle to the main application entry point."
>   Current implementation of sun/instrument/InstrumentationImpl.java
>   allows the premain method be non-public which violates the spec.
>   This fix aligns the implementation with the spec.
> 
> Testing:
>   A mach5 run of jdk_instrument tests is in progress.

src/java.instrument/share/classes/sun/instrument/InstrumentationImpl.java line 501:

> 499:             String msg = "method " + classname + "." +  methodname + " must be declared public";
> 500:             throw new IllegalAccessException(msg);
> 501:         }

If canAccess fails then it means that javaAgentClass is not public or the premain method is not public. The invoke will fail but I agree eat explicit canAccess check means we get finer control on the exception message.

(I can't help feeling that we should do a bit more cleanup here and not use getDeclaredMethod or be concerned with inheritance. This is because the Premain-Class is meant to specify the agent class. Also can't have a public static premain in super class and a non-public static premain in a sub-class).

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