FasterXML/jackson-databind · error · IllegalArgumentException

Cannot pass null modifier

Error message

Cannot pass null modifier

What it means

DeserializerFactoryConfig.withDeserializerModifier(ValueDeserializerModifier) registers a modifier that can post-process or replace deserializers built by the factory. It rejects null because the factory pipeline assumes a non-null modifier when iterating; a null would NPE during bean deserializer construction. This is the path SimpleModule.setDeserializerModifier and direct factory-config wiring use.

Source

Thrown at src/main/java/tools/jackson/databind/cfg/DeserializerFactoryConfig.java:125

    {
        if (additional == null) {
            throw new IllegalArgumentException("Cannot pass null KeyDeserializers");
        }
        KeyDeserializers[] all = ArrayBuilders.insertInListNoDup(_additionalKeyDeserializers, additional);
        return new DeserializerFactoryConfig(_additionalDeserializers, all, _modifiers,
                _valueInstantiators);
    }

    /**
     * Fluent/factory method used to construct a configuration object that
     * has same configuration as this instance plus one additional
     * deserialiazer modifier. Added modifier has the highest priority (that is, it
     * gets called before any already registered modifier).
     */
    public DeserializerFactoryConfig withDeserializerModifier(ValueDeserializerModifier modifier)
    {
        if (modifier == null) {
            throw new IllegalArgumentException("Cannot pass null modifier");
        }
        ValueDeserializerModifier[] all = ArrayBuilders.insertInListNoDup(_modifiers, modifier);
        return new DeserializerFactoryConfig(_additionalDeserializers, _additionalKeyDeserializers,
                all, _valueInstantiators);
    }

    /**
     * Fluent/factory method used to construct a configuration object that
     * has same configuration as this instance plus specified additional
     * value instantiator provider object.
     * Added instantiator provider has the highest priority (that is, it
     * gets called before any already registered resolver).
     *
     * @param instantiators Object that can provide {@link tools.jackson.databind.deser.ValueInstantiator}s for
     *    constructing POJO values during deserialization
     */
    public DeserializerFactoryConfig withValueInstantiators(ValueInstantiators instantiators)
    {

View on GitHub (pinned to a50c7d2a1d)

Solutions

1. Only register when non-null: if (mod != null) cfg.withDeserializerModifier(mod).
2. Inject/construct the modifier eagerly; fail bean wiring if the dependency is missing.
3. To 'clear' modifiers, construct a fresh DeserializerFactoryConfig rather than passing null.
4. Add a unit test for your module asserting it never registers a null modifier.

Example fix

// before
ValueDeserializerModifier mod = enableTweaks ? new MyModifier() : null;
cfg.withDeserializerModifier(mod); // throws
// after
ValueDeserializerModifier mod = enableTweaks ? new MyModifier() : null;
if (mod != null) cfg.withDeserializerModifier(mod);

ValueDeserializerModifier mod = ...;
if (mod == null) throw new IllegalArgumentException("modifier must not be null");
cfg.withDeserializerModifier(mod);

// non-null reference check

try {
    cfg.withDeserializerModifier(mod);
} catch (IllegalArgumentException e) {
    if (e.getMessage().equals("Cannot pass null modifier")) {
        // skip modifier registration
    }
    throw e;
}

When it happens

Trigger: Calling config.withDeserializerModifier(null); a module registering a modifier conditionally and forwarding null when disabled; a field that was supposed to hold a modifier but was never assigned due to a DI failure.

Common situations: Feature-flagged modifier registration; Spring/Hazelcast bean for the modifier missing; copy-paste of a module where the modifier argument was dropped; test setup that clears modifiers by passing null instead of building a fresh config.

Related errors

• Cannot pass null Deserializers
• Cannot pass null KeyDeserializers
• Cannot pass null resolver
• Module ({}) without defined name
• Module ({}) without defined version

AI-assisted analysis of FasterXML/jackson-databind@a50c7d2a1d (2026-08-06). Data as JSON: /api/errors/b3ed26598ebb3893. Report an issue: GitHub.