How Can I Resolve Java 9's InaccessibleObjectException: 'Unable to make {member} accessible'?

Tackling InaccessibleObjectException in Java 9: "Unable to make {member} accessible: module {A} does not 'opens {package}' to {B}"

In Java 9, the introduction of the Platform Module System brought about stricter encapsulation. This encapsulated world sometimes throws us the dreaded InaccessibleObjectException, particularly in the context of libraries like Spring, Hibernate, and JAXB.

The culprit behind the exception: The exception implies that a specific member (method/field/class) is being restricted from access due to module system limitations. These limitations generally require the member to be public or its package to be exported. Reflection adds another layer of challenge, since it attempts to bypass these access controls.

Addressing the exception: To avoid the exception, the module system must be convinced to allow access to the restricted element. Here's how you can approach this in different scenarios:

Scenario 1: Reflective Call into JDK

The exception arises when a library/framework invokes a JDK module via reflection. Here, the JDK modules cannot be modified, so command line flags come into play.

• Solution:

  • Use '--add-opens' flag to explicitly allow reflection access to the specific package within the JDK module.

• Example:

  • To address "...Unable to make ClassLoader.defineClass accessible..."
  • Run JVM with: java --add-opens java.base/java.lang=ALL-UNNAMED

Scenario 2: Reflection over Application Code

In this case, reflection is being used within one module to access another module. As a developer, you have more control over the module being accessed here.

• Solution:

  • Modify the module descriptor of the accessed module ({A}) to:

    • Export the package to all modules: exports {package};
    • Export the package specifically to the accessing module: exports {package} to {B};
    • Open the package to all modules: opens {package};
    • Open the package specifically to the accessing module: opens {package} to {B};
    • Open the entire module: open module {A} { ... };

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