cowwoc opened a new issue, #393:
URL: https://github.com/apache/maven-build-cache-extension/issues/393
### Affected version
1.2.0
### Bug description
### Environment
- **Maven Build Cache Extension**: v1.2.0
- **Maven**: 3.9.9
- **Java**: 25
- **Project Type**: Multi-module JPMS (Java Platform Module System) project
### Problem Description
When running `./mvnw clean compile` followed by `./mvnw verify`, the build
fails with "module not found" errors despite the cache indicating a hit.
### Root Cause
The Maven Build Cache Extension creates incomplete cache entries when
builds terminate at the `compile` phase:
1. **Compile-only build** (`./mvnw clean compile`):
- Saves build metadata to cache
- **Does NOT save** `attachedOutputs` (classes directory)
- Creates cache entry with `<final>false</final>` in buildinfo.xml
- Cache directory contains only `buildinfo.xml`, no ZIP files for
classes
2. **Subsequent verify build** (`./mvnw verify`):
- Finds cached entry by checksum
- Attempts to restore from cache
- **No classes to restore** (attachedOutputs were never saved)
- Skips compilation (thinks it's cached)
- Results in "module not found" errors
### Steps to Reproduce
```bash
# Step 1: Clean and run compile-only build
./mvnw clean compile
# Step 2: Examine cache - only buildinfo.xml exists, no class ZIPs
ls ~/.m2/build-cache/v1.1/groupId/artifactId/checksum/local/
# Output: buildinfo.xml only
# Step 3: Run verify build
./mvnw clean verify
# Result: Compilation errors - "module not found: module.name"
Expected Behavior
One of the following should occur:
Option 1 (Preferred): Don't cache incomplete builds
- compile-only builds should not create cache entries
- Only package or higher phases should save to cache
Option 2: Don't restore from incomplete cache
- Mark <final>false</final> entries as non-restorable
- Force rebuild when requested phase is higher than cached phase
Option 3: Save minimal attachedOutputs for all phases
- Even compile-only builds should save the classes directory
- Ensure attachedOutputs configuration applies to all cached phases
Current Workaround
# Clear stale cache entries
rm -rf ~/.m2/build-cache/
# Always use verify or package (not compile)
./mvnw clean verify
Cache Configuration
.mvn/maven-build-cache-config.xml:
<attachedOutputs>
<dirNames>
<dirName>classes</dirName>
<dirName>test-classes</dirName>
<dirName>maven-status</dirName>
</dirNames>
</attachedOutputs>
This configuration works correctly for package/verify builds but is
ignored for compile-only builds.
Observed Behavior
- With package/verify: ✅ Works correctly
- Saves classes as artifactId-mvn-cache-ext-extra-output-N.zip
- Restores both JAR and classes directory
- Full module resolution works
- With compile only: ❌ Incomplete caching
- buildinfo.xml shows <final>false</final>
- No attachedOutputs ZIP files created
- Cache restoration fails silently
Impact
This affects JPMS projects where module-info.class must be present in
target/classes for module resolution. The incomplete cache restoration bypasses
compilation but doesn't provide the required class files.
Related Issues
This may be related to:
- MBUILDCACHE-45: "Compiled classes are not copied/restored to target"
- The documented behavior that "cache automatically kicks in on every
package or higher phase"
Suggested Fix
The extension should either:
1. Not create cache entries for builds that don't reach package phase, OR
2. Save attachedOutputs for all cached phases (including compile), OR
3. Refuse to restore from <final>false</final> entries when a higher phase
is requested
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