Java / GraalVM Interview questions

How do you troubleshoot missing reflection metadata errors during native image build?

These errors typically surface as either a build-time warning about an unreachable class, or a runtime ClassNotFoundException/NoSuchMethodException in the finished binary.

Reproduce the failing code path on a plain JVM with the tracing agent attached: -agentlib:native-image-agent=config-merge-dir=META-INF/native-image (using merge mode so you don't overwrite existing entries).
Exercise every relevant code path, including error/edge cases, since the agent only records what actually executes.
Diff the merged config against version control to see exactly what new entries were captured.
If the missing class belongs to a well-known library, check the GraalVM Reachability Metadata Repository before hand-writing config.
Re-run native-image with -H:+PrintClassInitialization or verbose analysis output if the failure is initialization-order related rather than pure reflection.

Avoid the temptation to blanket-list "allDeclaredMethods": true across huge packages "just in case" - it balloons build time and image size while masking the specific configuration gap you actually need to find.

Take quiz

Why should config-merge-dir be used instead of config-output-dir when re-running the agent? It merges new entries with existing config instead of overwriting previously captured data

✓ Correct! Well done.

It disables reflection config entirely

✗ Try again if not.

It compiles the app directly to a native image

✗ Try again if not.

It has no functional difference

✗ Try again if not.

Why should you avoid blanket 'allDeclaredMethods: true' configs across large packages? It inflates build time and image size while hiding the specific gap that actually needs fixing

✓ Correct! Well done.

It is required for every native-image build to succeed

✗ Try again if not.

It automatically fixes all missing metadata issues with zero downside

✗ Try again if not.

It disables the Graal compiler's optimizations

✗ Try again if not.

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Java / GraalVM Interview questions

How do you troubleshoot missing reflection metadata errors during native image build?

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