apache/maven · error · IllegalStateException

Cannot add two different pieces of metadata for: " + getKey(

Error message

Cannot add two different pieces of metadata for: " + getKey()

What it means

Thrown by ProjectArtifactMetadata.merge() when two metadata objects with the same key ('project groupId:artifactId') carry different pom files. Legacy resolution merges ArtifactMetadata instances per artifact; if the same project metadata is added twice with distinct File objects, that invariant is violated and an IllegalStateException reports the key. This is a programming/config error in the code assembling metadata, not an environment issue.

Source

Thrown at compat/maven-compat/src/main/java/org/apache/maven/project/artifact/ProjectArtifactMetadata.java:111

    public boolean storedInArtifactVersionDirectory() {
        return true;
    }

    @Override
    public String getBaseVersion() {
        return artifact.getBaseVersion();
    }

    @Override
    public Object getKey() {
        return "project " + artifact.getGroupId() + ":" + artifact.getArtifactId();
    }

    @Override
    public void merge(ArtifactMetadata metadata) {
        ProjectArtifactMetadata m = (ProjectArtifactMetadata) metadata;
        if (!m.file.equals(file)) {
            throw new IllegalStateException("Cannot add two different pieces of metadata for: " + getKey());
        }
    }

    @Override
    public void merge(org.apache.maven.repository.legacy.metadata.ArtifactMetadata metadata) {
        this.merge((ArtifactMetadata) metadata);
    }
}

View on GitHub (pinned to e4093d4e12)

Solutions

1. Find where two different pom files are attached to the same artifact (log artifact.getMetadataList() before merge) and keep only one
2. When constructing ProjectArtifactMetadata yourself, reuse the same File instance for the same artifact
3. If two artifacts genuinely need different poms, give them distinct artifactIds (or classifiers via proper attach APIs) instead of merging metadata
4. Upgrade plugins that manipulate POM metadata manually - prefer MavenProjectHelper.attachArtifact / standard install flows

Example fix

// before
artifact.addMetadata(new ProjectArtifactMetadata(artifact, pomA));
artifact.addMetadata(new ProjectArtifactMetadata(artifact, pomB)); // boom
// after
artifact.addMetadata(new ProjectArtifactMetadata(artifact, pomA));
// attach pomB to a separate artifact with its own GAV/classifier

// Before adding POM metadata, ensure no conflicting instance exists
boolean conflict = artifact.getMetadataList().stream()
    .filter(ProjectArtifactMetadata.class::isInstance)
    .map(ProjectArtifactMetadata.class::cast)
    .anyMatch(m -> !m.getFile().equals(newPomFile));
if (conflict) throw new IllegalArgumentException("pom already attached for " + artifact.getId());

When it happens

Trigger: Calling artifact.getMetadataList() flows that add two ProjectArtifactMetadata instances with different `file` values for the same groupId:artifactId - e.g. attaching metadata for both a freshly generated pom.xml and an attached alternate pom file, or constructing the metadata twice with different paths.

Common situations: Custom build extensions or plugins that add POM metadata manually; attaching two POMs to one artifact (classifier confusion); test code building fake artifacts reusing the same GAV with different temp files; rarely, mixed legacy/core metadata merging during Maven 3->4 compat paths.

Related errors

• Cannot read metadata from '{}': {}
• {} could not be retrieved from repository: {} due to an erro
• Error installing metadata: {}
• Error while deploying metadata: {}
• Unable to get dependency information: " + e.getMessage()

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/c1dee2f8cc282f51. Report an issue: GitHub.