apache/maven · warning

Unknown reference type for {}: {}, using default

Error message

Unknown reference type for {}: {}, using default

What it means

Maven 4 interns model objects (Dependency, Plugin, ...) in DefaultModelObjectPool to cut memory use. The reference strength of a pooled class can be tuned per simple class name via the system property 'maven.model.processor.referenceType.<ClassName>'. The value must match a Cache.ReferenceType constant (NONE, SOFT, WEAK, HARD, case-insensitive); valueOf() throws IllegalArgumentException for anything else and this warn is emitted, falling back to the default reference type.

Source

Thrown at impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultModelObjectPool.java:132

        return Cache.newCache(referenceType);
    }

    /**
     * Gets the reference type to use for a specific object type.
     * Checks for per-type configuration first, then falls back to default.
     */
    private Cache.ReferenceType getReferenceTypeForClass(Class<?> objectType) {
        String className = objectType.getSimpleName();

        // Check for per-type configuration first
        String perTypeProperty = Constants.MAVEN_MODEL_PROCESSOR_REFERENCE_TYPE_PREFIX + className;
        String perTypeValue = getProperty(perTypeProperty, null);

        if (perTypeValue != null) {
            try {
                return Cache.ReferenceType.valueOf(perTypeValue.toUpperCase());
            } catch (IllegalArgumentException e) {
                LOGGER.warn("Unknown reference type for " + className + ": " + perTypeValue + ", using default");
            }
        }

        // Fall back to default reference type
        return getDefaultReferenceType();
    }

    /**
     * Gets the default reference type from system properties.
     */
    private Cache.ReferenceType getDefaultReferenceType() {
        try {
            String referenceTypeProperty =
                    getProperty(Constants.MAVEN_MODEL_PROCESSOR_REFERENCE_TYPE, Cache.ReferenceType.HARD.name());
            return Cache.ReferenceType.valueOf(referenceTypeProperty.toUpperCase());
        } catch (IllegalArgumentException e) {
            LOGGER.warn("Unknown default reference type, using HARD");
            return Cache.ReferenceType.HARD;

View on GitHub (pinned to e4093d4e12)

Solutions

1. Set the property to one of the exact enum names NONE, SOFT, WEAK or HARD (any case): -Dmaven.model.processor.referenceType.Dependency=SOFT
2. Remove surrounding whitespace/quotes from the value in MAVEN_OPTS, .mvn/jvm.config or the CI variable
3. Delete the per-type property entirely and rely on the global 'maven.model.processor.referenceType' if per-class tuning was not intended

Example fix

# before
export MAVEN_OPTS="$MAVEN_OPTS -Dmaven.model.processor.referenceType.Dependency=softish"

# after
export MAVEN_OPTS="$MAVEN_OPTS -Dmaven.model.processor.referenceType.Dependency=SOFT"

// before launching Maven / setting the system property
static final Set<String> VALID_REF_TYPES = Set.of("NONE", "SOFT", "WEAK", "HARD");

static boolean isValidRefType(String value) {
    return value != null && VALID_REF_TYPES.contains(value.strip().toUpperCase(Locale.ROOT));
}

// guard
String v = System.getProperty("maven.model.processor.referenceType.Dependency");
if (v != null && !isValidRefType(v)) {
    throw new IllegalArgumentException("Fix maven.model.processor.referenceType.Dependency: " + v);
}

When it happens

Trigger: Starting Maven (or an embedded Maven instance) with -Dmaven.model.processor.referenceType.Dependency=<bad>, where <bad> is not NONE/SOFT/WEAK/HARD (e.g. 'softish', 'STRONG', or a value with leading/trailing whitespace, since it is never trimmed). getReferenceTypeForClass() is consulted on every internObject() call for that class, so the warn repeats.

Common situations: Typos in MAVEN_OPTS or CI environment variables; copy-pasting tuning advice written for a different library; values quoted with stray spaces (' SOFT '); expecting PascalCase names that do not exist (e.g. 'WEAK_REFERENCE').

Related errors

• Unknown default reference type, using HARD
• Ranges overlap: {}
• Only fully-qualified sets allowed in multiple set scenario:
• Single version must be surrounded by []: {}
• Range defies version ordering: {}

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/f8c4c6821fa29c8a. Report an issue: GitHub.