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Zoé Brossard
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Asked: 2022-05-17T13:14:30+00:00 In:

What is “warning: an illegal reflective access operation has occurred”?

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What is “warning: an illegal reflective access operation has occurred” ? I am learning about it. Can someone share with me your knowledge and experience?

an illegal reflective access operation has occurred
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  1. Best Answer
    Expert

    The cause: 

    The crux of this problem lies in the Module System#Relaxed-strong-encapsulation.

    To support the migration to Java-9, the encapsulation of modules can be loosened. This loosening of encapsulation is controlled at runtime by a new launcher option --illegal-access. And the warning is caused in the first reflective-access operation to any such package, but after that point, their is no warnings. This only warning describes the way opening the other warnings and it cannot be avoided.

    Solution:

    To solve this problem, please do the following suggestions:

    Run it with –illegal-access=deny and don’t open module packages from one to another without  declaration such as a directive(opens) or obvious using of –add-opens VM arg. You could identify static references from compiled code to JDK-internal APIs by using the jdeps tool with the –jdk-internals option.

    The warning message issued when an illegal reflective-access operation is detected has the following form:

    WARNING: Illegal reflective access by $PERPETRATOR to $VICTIM

    where: $PERPETRATORis the fully-qualified name of the type containing the code that invoked the reflective operation in question plus the code source (i.e., JAR-file path), if available, and $VICTIMis a string that describes the member being accessed, including the fully-qualified name of the enclosing type

    Remember: You have to ensure your modules don’t make those illegal reflective accesses in order not to face such warnings.    

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  2. An Oracle article that I found about Java 9 module system is available.

    By default, a type in a module is not accessible to other modules unless it’s a public type and you export its package. You expose only the packages you want to expose. With Java 9, this also applies to reflection.

    As pointed out in https://ittutoria.net/a/50251958/134894, the differences between the AccessibleObject#setAccessible for JDK8 and JDK9 are instructive. JDK9 was specifically added

    This method may be used by a caller in class C to enable access to a member of declaring class D if any of the following hold:

    • C and D are in the same module.
    • The member is public and D is public in a package that the module containing D exports to at least the module containing C.
    • The member is protected static, D is public in a package that the module containing D exports to at least the module containing C, and C is a subclass of D.
    • D is in a package that the module containing D opens to at least the module containing C. All packages in unnamed and open modules are open to all modules and so this method always succeeds when D is in an unnamed or open module.

    This highlights the importance of modules and their exports in Java 9

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