Java: WARNING: Unknown module: PDF4Teachers.merged.module specified to –add-exports

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Summary

The issue at hand is a Java module resolution problem. When packaging the PDF4Teachers software using jlink, a warning is raised about an unknown module PDF4Teachers.merged.module specified in --add-reads and --add-exports options. This warning leads to a subsequent error when trying to open a PDF file, resulting in an IllegalAccessError due to the module source.merged.module not being able to access classes in the org.apache.commons.logging module.

Root Cause

The root cause of this issue is the incorrect configuration of Java module dependencies. Specifically, the --add-reads and --add-exports options are used to configure module dependencies, but the module PDF4Teachers.merged.module is not properly defined or resolved. This leads to a module resolution failure, causing the IllegalAccessError when trying to access classes in other modules.

Why This Happens in Real Systems

This issue can occur in real systems when:

  • Module dependencies are not properly configured: When using jlink to package a Java application, it’s essential to correctly configure module dependencies to ensure that all required modules are included and resolved.
  • Module names are incorrect or mismatched: If module names are incorrect or mismatched, the Java runtime may not be able to resolve the dependencies, leading to module resolution failures.
  • Module versions are incompatible: If different versions of the same module are used, it can lead to compatibility issues and module resolution failures.

Real-World Impact

The real-world impact of this issue includes:

  • Application crashes or errors: The IllegalAccessError can cause the application to crash or produce errors, leading to a poor user experience.
  • Security vulnerabilities: In some cases, module resolution failures can lead to security vulnerabilities if sensitive data is exposed or if the application is compromised.
  • Maintenance and debugging challenges: Resolving module dependencies and debugging related issues can be time-consuming and challenging, especially for complex applications.

Example or Code

// Example of correct module dependency configuration
module PDF4Teachers {
    requires org.apache.commons.logging;
    exports fr.clementgre.pdf4teachers to org.controlsfx.controls;
}

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Verifying module dependencies: Carefully reviewing and verifying module dependencies to ensure they are correct and properly configured.
  • Using the correct module names and versions: Ensuring that module names and versions are correct and consistent throughout the application.
  • Configuring module resolution: Using options like --add-reads and --add-exports to configure module resolution and ensure that all required modules are included and resolved.

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience with Java modules: Limited experience with Java modules and their configuration can lead to mistakes and oversights.
  • Insufficient understanding of module dependencies: Not fully understanding how module dependencies work and how to configure them correctly can lead to module resolution failures.
  • Inadequate testing and debugging: Failing to thoroughly test and debug the application can lead to module resolution issues going undetected until later stages of development or even in production.