What is Illegal Reflective Access in Java 9 and How Does It Manifest?

Illegal Reflective Access

In Java 9, illegal reflective access refers to situations where reflective APIs are used in ways that violate the enhanced encapsulation principles introduced by the module system. This can occur when code from one module attempts to access private or protected members of classes in another module without proper authorization.

Definition of Illegal Reflective Access

Illegal reflective access is defined as any reflective operation that:

• Accesses a private or protected member of a class in another module
• Calls a private method of a class in another module
• Modifies a final field of a class in another module
• Creates an instance of a class that is not publicly visible in another module

Triggering Circumstances

Illegal reflective access is triggered when:

• A class in one module uses reflection to access a private or protected member of a class in another module that is not explicitly exposed to the first module through a module declaration or VM argument.
• A class in one module uses reflection to call a private method of a class in another module that is not explicitly exposed to the first module.
• A class in one module uses reflection to modify a final field of a class in another module that is not explicitly exposed to the first module.
• A class in one module uses reflection to create an instance of a class that is not publicly visible in another module.

Module System and Relaxed Encapsulation

Java 9 introduced a relaxed-strong-encapsulation rule that allows certain limited forms of reflective access. Specifically, it allows a module's run-time system to open packages in the module to code in unnamed modules (i.e., code on the classpath). If the run-time system is invoked with this option and it succeeds in using reflection where it would have otherwise failed, then an "illegal" reflective access has occurred.

Warning Messages

When an illegal reflective access occurs, a warning message is issued in the following format:

WARNING: Illegal reflective access by $PERPETRATOR to $VICTIM

where:

• $PERPETRATOR is the fully-qualified name of the type containing the code that invoked the reflective operation, plus the code source (JAR file path) if available.
• $VICTIM describes the member being accessed, including the fully-qualified name of the enclosing type.

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