How Can I Resolve 'Illegal Reflective Access' Warnings in Java 9?

Illegal Reflective Access in Java 9: ​​A Detailed Examination

Java 9 introduced the concept of illegal reflective access, which triggered zahlreichen on how to solve it Questions about this error message. This article will delve into the nature of illegal reflective access, the circumstances that trigger alerts, and possible solutions.

The module system in Java 9 introduces the concept of modularity#relaxing strong encapsulation, which is the key to understanding illegal reflective access. Access by code in a module to other module code and visible packages is strictly controlled. However, for облегчить переход на Java 9, the strong encapsulation of modules can be relaxed.

The runtime system can provide the following operations:

• Static access: Provided through compiled bytecode.
• Runtime calls: allows calling its runtime system with one or more packages of one or more modules as code in all unnamed modules Open.

The concept of illegal access

If the reflection API is successfully called under the following circumstances, "illegal" reflection access will be triggered:

• Normally the call will fail.
• "Illegal access" is enabled in the runtime system.

Mechanism for triggering alerts

Relaxation of encapsulation in Java 9 can be controlled at compile time via the new startup option --illegal-access. By default, this option is set to Allow. In this mode, the first reflected access operation to any such package will trigger an alert, but no further warnings will be issued. This warning contains information on how to enable further warnings.

Configuration options

The --illegal-access option can be configured in the following modes:

• debug: print for each such access Messages and stack traces.
• warn: Print a message for each such access.
• deny: Disable this type of operation.

Debugging and Repair Recommendations

• Run the application with --illegal-access=deny to find out and avoid module declarations without any ( "opens" a package from one module to another, including opens of such a directive, or explicitly using the --add-opens VM argument.
• Use the jdeps tool with the --jdk-internals option to identify static references to JDK internal APIs in compiled code.

Warning message example

A warning message that is emitted when an illegal reflective access operation is detected follows the following format:

WARNING: Illegal reflective access by $PERPETRATOR to $VICTIM

where:

• $PERPETRATOR: the qualified name of the type containing the code that triggers the reflection operation, and the available source of the code (i.e. JAR file path).
• $VICTIM: A string describing the member being accessed, including the qualified name of the enclosing type.

Precautions

To avoid such warnings and ensure future security, you need to ensure that modules do not make these illegal reflective accesses.

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