flowable/flowable-engine · error · UnsupportedOperationException

Cannot get variable value for jackson 2

Error message

Cannot get variable value for jackson 2

What it means

Jackson2JsonObjectRestVariableConverter handles Flowable JSON variables backed by Jackson's JsonNode. Its getVariableValue is intentionally unimplemented: reading a JSON variable back from a REST representation is not supported for this converter, so it always throws UnsupportedOperationException with this message.

Source

Thrown at modules/flowable-common-rest/src/main/java/org/flowable/common/rest/variable/Jackson2JsonObjectRestVariableConverter.java:48

    protected ObjectMapper objectMapper;

    public Jackson2JsonObjectRestVariableConverter(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
    }

    @Override
    public String getRestTypeName() {
        return "json";
    }

    @Override
    public Class<?> getVariableType() {
        return JsonNode.class;
    }

    @Override
    public Object getVariableValue(EngineRestVariable result) {
        throw new UnsupportedOperationException("Cannot get variable value for jackson 2");
    }

    @Override
    public void convertVariableValue(Object variableValue, EngineRestVariable result) {
        if (variableValue != null) {
            if (!(variableValue instanceof JsonNode)) {
                throw new FlowableIllegalArgumentException("Converter can only convert com.fasterxml.jackson.databind.JsonNode.");
            }
            tools.jackson.databind.JsonNode valueNode = objectMapper.readTree(variableValue.toString());
            result.setValue(valueNode);
        } else {
            result.setValue(null);
        }
    }

}

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Do not read JSON variables through this converter; store them as plain strings and parse to JsonNode client-side.
2. Upgrade to a Flowable version where JSON variable conversion via REST is supported.
3. Provide a custom RestVariableConverter for JsonNode that implements getVariableValue by parsing result.getValue() with an ObjectMapper.
4. Change the variable's REST type to 'string' so the string converter handles the read.

Example fix

// before
JsonNode node = (JsonNode) runtimeService.getVariable(executionId, "payload"); // served via jackson2 REST converter -> throws
// after
String json = (String) runtimeService.getVariable(executionId, "payload");
JsonNode node = new ObjectMapper().readTree(json);

public static boolean supportsJsonRead(RestVariableConverter c) {
    try { c.getVariableValue(null); return false; } catch (UnsupportedOperationException e) { return false; } catch (Exception e) { return true; }
}

public static boolean isJacksonJsonConverter(RestVariableConverter c) { return c instanceof Jackson2JsonObjectRestVariableConverter; }

try {
    Object value = restClient.getVariable(id, "payload");
} catch (UnsupportedOperationException e) {
    if (e.getMessage().contains("Cannot get variable value for jackson 2")) {
        // read the variable as a string and parse to JsonNode locally
    } else { throw e; }
}

When it happens

Trigger: Any REST call that resolves a variable whose REST type maps to the Jackson 2 JSON converter (getVariableType() returns JsonNode.class) and requires getVariableValue — i.e. reading a JSON-typed variable through the REST API.

Common situations: Fetching a process/task variable that was stored as a Flowable JsonNode JSON variable via the REST API; Flowable versions where JSON variable read-back was simply not wired up for Jackson 2.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Error writing form model response
• Failed to serialize to a RestVariable instance
• Failed to serialize to a RestVariable instance
• Variable name is required
• Error converting request body to RestVariable instance

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/5486b36ac82fa8ca. Report an issue: GitHub.