FasterXML/jackson-databind · error · IllegalArgumentException

Missing generic type information for ${type}

Error message

Missing generic type information for ${type}

What it means

Thrown by the MapEntryDeserializer constructor as a sanity check when the JavaType for a Map.Entry<K,V> does not have exactly 2 contained type parameters. Map.Entry is parameterized over key and value types; if type resolution produces a type with 0, 1, or 3+ contained types (e.g., raw Map.Entry from type erasure), the constructor rejects it. This is a type-resolution or configuration error, not a data error.

Source

Thrown at src/main/java/tools/jackson/databind/deser/jdk/MapEntryDeserializer.java:61

    /**
     * If value instances have polymorphic type information, this
     * is the type deserializer that can handle it
     */
    protected final TypeDeserializer _valueTypeDeserializer;

    /*
    /**********************************************************************
    /* Life-cycle
    /**********************************************************************
     */

    public MapEntryDeserializer(JavaType type,
            KeyDeserializer keyDeser, ValueDeserializer<Object> valueDeser,
            TypeDeserializer valueTypeDeser)
    {
        super(type);
        if (type.containedTypeCount() != 2) { // sanity check
            throw new IllegalArgumentException("Missing generic type information for "+type);
        }
        _keyDeserializer = keyDeser;
        _valueDeserializer = valueDeser;
        _valueTypeDeserializer = valueTypeDeser;
    }

    protected MapEntryDeserializer(MapEntryDeserializer src,
            KeyDeserializer keyDeser, ValueDeserializer<Object> valueDeser,
            TypeDeserializer valueTypeDeser)
    {
        super(src);
        _keyDeserializer = keyDeser;
        _valueDeserializer = valueDeser;
        _valueTypeDeserializer = valueTypeDeser;
    }

    /**
     * Factory method for constructing initial (non-contextual) instances.

View on GitHub (pinned to a50c7d2a1d)

Solutions

1. Always parameterize Map.Entry fields/parameters: Map.Entry<String,Integer> not raw Map.Entry.
2. If constructing the type programmatically, use TypeFactory.constructParametricType(Map.Entry.class, keyType, valueType) to ensure 2 contained types.
3. Check custom DeserializerFactory code for incorrect JavaType construction.
4. Use a TypeReference<Map.Entry<K,V>> when registering deserializers.

Example fix

// before: raw Map.Entry type
ObjectMapper mapper = ...;
Map.Entry result = mapper.readValue(json, Map.Entry.class);
// after: parameterized type
Map.Entry<String,Integer> result = mapper.readValue(json,
    new TypeReference<Map.Entry<String,Integer>>() {});

// Ensure Map.Entry type has exactly 2 contained types before constructing
JavaType entryType = typeFactory.constructType(new TypeReference<Map.Entry<String,Integer>>() {});
if (entryType.containedTypeCount() != 2) {
    throw new IllegalArgumentException("Map.Entry needs 2 type params: " + entryType);
}

static boolean isValidEntryType(JavaType type) {
    return type.getRawClass() == Map.Entry.class && type.containedTypeCount() == 2;
}

When it happens

Trigger: Constructing MapEntryDeserializer for a raw Map.Entry type (no generics). A custom deserializer factory that passes an incorrectly resolved JavaType. Type erasure where a field is declared as raw Map.Entry without type parameters.

Common situations: Declaring a field or parameter as raw Map.Entry (no <K,V>). Using a TypeReference or TypeFactory incorrectly to build a Map.Entry type. Framework code that constructs the deserializer manually with a wrong JavaType.

Related errors

• Type {} not Java Enum type
• Cannot update Map.Entry values
• Cannot update `Map.Entry` values
• Invalid abstract type resolution from {} to {}: latter is no
• Trying to resolve a forward reference with id [{}] that wasn

AI-assisted analysis of FasterXML/jackson-databind@a50c7d2a1d (2026-08-06). Data as JSON: /api/errors/76372ad349c625b1. Report an issue: GitHub.