flowable/flowable-engine · error · FlowableException

couldn't find a variable type that is able to serialize

Error message

couldn't find a variable type that is able to serialize 

What it means

DefaultVariableTypes.findVariableType iterates the registered VariableType list and returns the first type whose isAbleToStore(value) is true. If no registered type can handle the value's Java class, it throws FlowableException 'couldn't find a variable type that is able to serialize <value>'.

Solutions

1. Make the stored value implement a supported type (e.g. java.io.Serializable, String, Date, byte[]) or store a JSON/string representation
2. Register a custom VariableType via processEngineConfiguration.setCustomPreVariableTypes / getVariableTypes, implementing isAbleToStore and conversions
3. Check the value's class at runtime (implements Serializable?) before setting it as a variable
4. Ensure required optional modules (JPA type, JSON type) are on the classpath and registered

Example fix

// before
execution.setVariable("data", new UnserializableDto(...)); // throws
// after
public class UnserializableDto implements Serializable { ... }
execution.setVariable("data", dto);
// or register:
config.setCustomPreVariableTypes(List.of(new DtoVariableType()));

boolean isStorableVariableValue(Object v) {
    return v == null || v instanceof String || v instanceof Number || v instanceof Boolean
        || v instanceof byte[] || v instanceof java.util.Date
        || v instanceof java.io.Serializable; // plus any custom registered types
}

try {
    execution.setVariable("key", value);
} catch (org.flowable.common.engine.api.FlowableException e) {
    if (e.getMessage().startsWith("couldn't find a variable type that is able to serialize")) {
        execution.setVariable("key", jsonMapper.convertToJsonString(value));
    } else {
        throw e;
    }
}

When it happens

Trigger: Storing (setVariable/setVariableLocal) a value whose runtime class is not covered by the registered variable types — e.g. an unregistered custom bean, a POJO without a serializer, or a type removed by the JPA/Jackson type not being configured.

Common situations: Setting custom POJOs without registering a custom VariableType (e.g. SerializableType covers java.io.Serializable only if the object implements it); after upgrading Flowable where a default type (e.g. JPA entity type) is not registered; storing third-party objects that are not serializable.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

• Couldn't serialize value '' in variable ''
• Couldn't serialize value
• Failed to serialize to a RestVariable instance
• variable '
• variableName is null

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/8f9a3001218f97b5. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-variable-service/src/main/java/org/flowable/variable/service/impl/types/DefaultVariableTypes.java:68

        this.typesMap.clear();
        for (VariableType type : typesList) {
            typesMap.put(type.getTypeName(), type);
        }
    }

    @Override
    public VariableType getVariableType(String typeName) {
        return typesMap.get(typeName);
    }

    @Override
    public VariableType findVariableType(Object value) {
        for (VariableType type : typesList) {
            if (type.isAbleToStore(value)) {
                return type;
            }
        }
        throw new FlowableException("couldn't find a variable type that is able to serialize " + value);
    }

    @Override
    public int getTypeIndex(VariableType type) {
        return typesList.indexOf(type);
    }

    @Override
    public int getTypeIndex(String typeName) {
        VariableType type = typesMap.get(typeName);
        if (type != null) {
            return getTypeIndex(type);
        } else {
            return -1;
        }
    }

    @Override

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