FasterXML/jackson-databind · error · IllegalStateException

Cannot update Map.Entry values

Error message

Cannot update Map.Entry values

What it means

Thrown by MapEntryDeserializer.deserialize(JsonParser, DeserializationContext, Map.Entry result) — the three-argument overload used for in-place updates (mapper.readerForUpdating(value).readValue(json)). Map.Entry instances are effectively immutable (their key/value cannot be replaced without creating a new entry), so Jackson cannot perform an update-in-place operation and throws IllegalStateException immediately.

Source

Thrown at src/main/java/tools/jackson/databind/deser/jdk/MapEntryDeserializer.java:311

            if (t == JsonToken.PROPERTY_NAME) { // most likely
                ctxt.reportInputMismatch(this,
                        "Problem binding JSON into Map.Entry: more than one entry in JSON (second field: '%s')",
                        p.currentName());
            } else {
                // how would this occur?
                ctxt.reportInputMismatch(this,
                        "Problem binding JSON into Map.Entry: unexpected content after JSON Object entry: "+t);
            }
            return null;
        }
        return new AbstractMap.SimpleEntry<Object,Object>(key, value);
    }

    @Override
    public Map.Entry<Object,Object> deserialize(JsonParser p, DeserializationContext ctxt,
            Map.Entry<Object,Object> result) throws JacksonException
    {
        throw new IllegalStateException("Cannot update Map.Entry values");
    }

    @Override
    public Object deserializeWithType(JsonParser p, DeserializationContext ctxt,
            TypeDeserializer typeDeserializer)
        throws JacksonException
    {
        // In future could check current token... for now this should be enough:
        return typeDeserializer.deserializeTypedFromObject(p, ctxt);
    }

    /*
    /**********************************************************************
    /* Alternate handlers
    /**********************************************************************
     */

    /**

View on GitHub (pinned to a50c7d2a1d)

Solutions

1. Do not use update-in-place APIs (readerForUpdating, updateValue) with Map.Entry — always deserialize fresh.
2. If you need to modify an entry, deserialize a new Map.Entry and replace the reference.
3. Use a mutable container (a small POJO or a two-element array) instead of Map.Entry if update semantics are required.
4. Restructure code to avoid the three-argument deserialize path for Map.Entry types.

Example fix

// before: attempting update-in-place
mapper.readerForUpdating(oldEntry).readValue(json);
// after: always deserialize new
Map.Entry<String,Integer> entry = mapper.readValue(json,
    new TypeReference<Map.Entry<String,Integer>>() {});

// Detect Map.Entry types and avoid update-in-place paths
if (Map.Entry.class.isAssignableFrom(targetType)) {
    throw new UnsupportedOperationException(
        "Cannot update Map.Entry — deserialize fresh instead");
}

static boolean isUpdatable(Class<?> type) {
    return !Map.Entry.class.isAssignableFrom(type);
}

try {
    mapper.readerForUpdating(oldEntry).readValue(json);
} catch (IllegalStateException e) {
    if (e.getMessage().contains("Cannot update Map.Entry")) {
        // Deserialize fresh instead
        entry = mapper.readValue(json, entryTypeRef);
    }
}

When it happens

Trigger: Calling ObjectMapper.readerForUpdating(existingEntry).readValue(json) where existingEntry is a Map.Entry. Any code path that triggers the three-argument deserialize with a non-null Map.Entry result object. Using ObjectReader.updateValue() on a Map.Entry.

Common situations: Attempting to use ObjectMapper's value-update API on a type that resolves to Map.Entry. Passing a Map.Entry to a deserialization call expecting in-place mutation. Confusing Map.Entry deserialization (create new) with update (modify existing).

Related errors

• Cannot update `Map.Entry` values
• Missing generic type information for ${type}
• Trying to resolve a forward reference with id [{}] that wasn
• Cannot call withValueDeserializer() on {}
• Cannot pass null property name

AI-assisted analysis of FasterXML/jackson-databind@a50c7d2a1d (2026-08-06). Data as JSON: /api/errors/5ee26272ea2f1ecb. Report an issue: GitHub.