java.lang.IllegalArgumentException: Malformed \uxxxx encoding

Problem Statement

The java.lang.IllegalArgumentException: Malformed \uxxxx encoding error occurs during Maven builds when corrupted property files in your local Maven repository contain malformed Unicode escape sequences. This typically happens with the resolver-status.properties files that Maven uses to track dependency resolution status.

The error manifests as a build failure with a stack trace showing:

java.lang.IllegalArgumentException: Malformed \uxxxx encoding.
    at java.util.Properties.loadConvert(Properties.java:672)
    at java.util.Properties.load0(Properties.java:455)
    at java.util.Properties.load(Properties.java:408)

Root Cause

This error occurs due to a known Maven resolver bug (MRESOLVER-216) where multiple threads may simultaneously attempt to resolve the same artifact, corrupting the resolver-status.properties files with invalid Unicode characters like \u0000.

Solutions

Quick Fix: Delete Corrupted Files

The most common solution is to locate and delete the corrupted resolver-status.properties files:

Linux/macOS

# Find and delete all resolver-status.properties files
find ~/.m2 -name "resolver-status.properties" -delete

Windows

# Find corrupted files
FINDSTR /S /M "u0" resolver-status.properties

# Then manually delete the files listed

Targeted Cleanup

For a more targeted approach that only removes corrupted files:

# Search for files containing malformed Unicode sequences
grep -rnw ~/.m2 -e '\u0000'

# Delete only the corrupted files
find ~/.m2 -name "resolver-status.properties" -exec grep -H -e '\u0000' {} \; | sed -r 's/:.*$//g' | xargs rm

Complete Repository Reset

If the issue persists, you can delete your entire Maven repository:

rm -rf ~/.m2/repository

WARNING

This will force Maven to download all dependencies again, which may take significant time depending on your internet connection and project size.

Prevent Recurrence

To prevent future occurrences, configure Maven to use a single thread for dependency resolution:

mvn clean install -Daether.metadataResolver.threads=1

For IntelliJ IDEA users, configure this in: Settings → Build, Execution, Deployment → Maven → Runner → VM Options Add: -Daether.metadataResolver.threads=1

Advanced Debugging

If you need to identify the exact corrupted file:

1. Run Maven with debug logging:

   mvn clean install -e -X

2. Look for dependency resolution errors in the output

3. Use your IDE's debugger to set a breakpoint at java.util.Properties.loadConvert() (line 672) to inspect the corrupted file path during execution

Verification

After applying any solution, verify the fix by running:

mvn clean compile

The build should complete successfully without the encoding error.

Conclusion

The "Malformed \uxxxx encoding" error is typically caused by corrupted metadata files in your Maven repository. The most effective solutions involve:

1. Deleting corrupted resolver-status.properties files
2. Preventing concurrency issues with -Daether.metadataResolver.threads=1
3. As a last resort, clearing the entire Maven repository

This approach resolves the issue while minimizing disruption to your development workflow.