How Can I Resolve Java 9's InaccessibleObjectException?

Addressing the InaccessibleObjectException in Java 9

The InaccessibleObjectException surfaces in various scenarios with Java 9, often involving third-party libraries or frameworks that utilize reflection to interact with the Java Development Kit (JDK) or application code. Here's an explanation of the issue and how to circumvent it.

Cause and Implications

Java 9 introduced the Module System, enforcing stronger encapsulation constraints. Access to elements (e.g., classes, methods) is restricted unless specific criteria are met. These restrictions extend to reflection operations, like setAccessible(), which attempts to bypass access limitations.

Exception Message Breakdown

The exception message provides crucial information:

Unable to make {member} accessible: module {A} does not "opens {package}" to {B}

• {A}: The inaccessible module (e.g., java.base)
• {member}: The specified inaccessible element (e.g., defineClass method)
• {package}: The inaccessibility applies to elements in this package (e.g., java.lang)
• {B}: The requesting module (usually an unnamed module like @1941a8ff)

Resolution Strategies

Depending on the scenario, different strategies can resolve the issue. There are two main cases:

Case 1: Reflection Call into JDK

• Due to JDK immutability, the fix involves JVM command-line flags.
• Use --add-opens with the syntax {A}/{package}={B} to open specific packages for reflection.

Case 2: Reflection over Application Code

• The inaccessible module can be modified to open up its internals.
• Apply export or open directives in the module descriptor to configure accessibility limitations.
• Options include exporting or opening specific packages to the requesting module or all code.

Additional Considerations

• In certain scenarios, it may be necessary to apply multiple --add-opens flags.
• As a last resort, the --permit-illegal-access flag (only available in Java 9) can be utilized to enable reflective access across modules. However, this should be treated as a temporary solution due to security implications.

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