Java 9's Illegal Reflective Access: How to Identify, Trigger, and Resolve Warnings?

Understanding Illegal Reflective Access in Java 9

Illegal reflective access in Java 9 refers to certain reflective operations that violate the module encapsulation principles introduced in this version. Reflective access allows programs to access members of other classes dynamically, regardless of their visibility modifiers.

What Defines an Illegal Reflective Access?

Illegal reflective access occurs when:

• A module attempts to access a member of another module that is not explicitly declared as open to it.
• A module uses the reflection APIs to invoke a constructor, field, or method that is not accessible to the module's code.

How is Illegal Reflective Access Triggered?

The Java 9 runtime monitors reflective operations. When an illegal access is detected, a warning message is issued. This warning can be suppressed by manually specifying the --illegal-access JVM argument with one of the following modes:

• permit: Issue a warning for the first illegal access and silence subsequent ones.
• warn: Issue a warning for every illegal access.
• deny: Disable all illegal reflective operations.

Scenario that Triggers the Warning

An illegal reflective access warning is triggered when the following conditions are met:

• The --illegal-access JVM argument is set to permit or warn.
• A module attempts to access a member of another module that is not open to it.
• The reflective access occurs via static references from compiled code or through the use of the reflection APIs.

Resolving Illegal Reflective Access

To avoid illegal reflective access warnings, consider the following steps:

• Ensure that all modules declare their dependencies and open packages to each other as needed.
• Use the --add-opens VM argument to explicitly open packages between modules.
• Eliminate static references to JDK-internal APIs from compiled code.
• Set the --illegal-access JVM argument to deny to disable illegal reflective operations and enforce strong encapsulation.

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