flowable/flowable-engine · error · IllegalArgumentException

The field 'script' should be set on the TaskListener

Error message

The field 'script' should be set on the TaskListener

What it means

ScriptTypeTaskListener executes a script when a CMMN task/listener event fires. Before evaluating, validateParameters() (called from notify()) checks that both the 'script' and 'language' fields were configured; this error means the 'script' Expression field was never set on the listener definition.

Source

Thrown at modules/flowable-cmmn-engine/src/main/java/org/flowable/cmmn/engine/impl/listener/ScriptTypeTaskListener.java:81

        ScriptEngineRequest.Builder request = ScriptEngineRequest.builder()
                .script(script)
                .language(language)
                .scopeContainer(delegateTask)
                .traceEnhancer(trace -> trace.addTraceTag("type", "taskListener"));
        Object result = scriptingEngines.evaluate(request.build()).getResult();

        if (resultVariable != null) {
            String resultVariable = Objects.toString(this.resultVariable.getValue(delegateTask), null);
            if (resultVariable != null) {
                delegateTask.setVariable(resultVariable, result);
            }
        }
    }

    protected void validateParameters() {
        if (script == null) {
            throw new IllegalArgumentException("The field 'script' should be set on the TaskListener");
        }

        if (language == null) {
            throw new IllegalArgumentException("The field 'language' should be set on the TaskListener");
        }
    }

    public void setScript(Expression script) {
        this.script = script;
    }

    public void setLanguage(Expression language) {
        this.language = language;
    }

    public void setResultVariable(Expression resultVariable) {
        this.resultVariable = resultVariable;
    }

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Add a field named 'script' with the script text/expression to the listener definition in the CMMN XML: <flowable:field name="script"><flowable:string><![CDATA[...]]></flowable:string></flowable:field>
2. If building the listener programmatically, call setScript(expression) before deploying/executing
3. Redeploy the corrected CMMN model and re-run the case

Example fix

// before
<flowable:taskListener event="complete" class="org.flowable.cmmn.engine.impl.listener.ScriptTypeTaskListener">
  <flowable:field name="language" stringValue="javascript"/>
</flowable:taskListener>
// after
<flowable:taskListener event="complete" class="org.flowable.cmmn.engine.impl.listener.ScriptTypeTaskListener">
  <flowable:field name="script"><flowable:string><![CDATA[println('done')]]></flowable:string></flowable:field>
  <flowable:field name="language" stringValue="javascript"/>
</flowable:taskListener>

const fields = listenerDef.fields || {};
if (!fields.script || !fields.script.stringValue) throw new Error('ScriptTypeTaskListener: missing script field');

function hasScript(l) { return l != null && l.script != null && l.script.expressionText != null; }

try { deployModel(model); } catch (e) { if (String(e.message).includes("field 'script' should be set")) { fixListenerDefinition(model); } else { throw e; } }

When it happens

Trigger: A CMMN model declares a ScriptTypeTaskListener whose <flowable:field name="script"> element (or programmatic setScript) is missing, so script == null when notify() runs during plan-item/task event handling.

Common situations: Hand-edited or generated CMMN XML where the script field was omitted or misspelled (field name must be exactly 'script'); dynamically creating listener via code and forgetting setScript; model deployed from a template with placeholder script values left null.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

Related errors

• The field 'language' should be set on the TaskListener
• The field 'language' should be set on the TaskListener
• Could not find an implementation of the org.flowable.cdi.imp
• Programmatic error: task listener added to an element that i
• no SMTP host is configured for the default mail server

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/1d1f63d8288ff530. Report an issue: GitHub.