flowable/flowable-engine · warning

One of the attributes 'class', 'delegateExpression'…

Error message

One of the attributes 'class', 'delegateExpression', 'type', 'operation', or 'expression' is mandatory on serviceTask {}

What it means

A serviceTask element defines none of the required implementation attributes: activiti:class, delegateExpression, expression, a known 'type', or an operationRef for web service. The parser logs this warning and leaves the activity without a behavior; invoking the task at runtime will fail because there is nothing to execute.

Solutions

1. Add one of the required attributes, e.g. flowable:class="com.x.MyDelegate" or flowable:delegateExpression="${bean}"
2. Set a known flowable:type (mail, camel, shell, ...) or flowable:expression="${...}"
3. Provide operationRef plus WSDL imports for a web-service task

Example fix

// before
<serviceTask id="st1" name="Do work"/>
// after
<serviceTask id="st1" name="Do work" flowable:class="com.example.WorkDelegate"/>

for (ServiceTask t : model.getFlowElementsOfType(ServiceTask.class)) {
    boolean hasImpl = notEmpty(t.getImplementation()) || notEmpty(t.getType()) || notEmpty(t.getOperationRef());
    if (!hasImpl) throw new IllegalArgumentException("serviceTask " + t.getId() + " has no implementation");
}

When it happens

Trigger: executeParse of a ServiceTask reaching the final else branch: implementationType not CLASS/EXPRESSION/DELEGATEEXPRESSION/WEBSERVICE/type-branch — typically an empty <serviceTask id="x"/> or one with only unrelated attributes.

Common situations: Bare serviceTask shape dropped from a palette and never configured; import from external tooling using different attribute names; accidental removal of the class attribute during refactoring.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Invalid service task type
• One of the attributes 'type' or 'operation' is mandatory on…
• Activity needed for multi instance cannot bv found
• Could not execute shell command for + execution
• Delegate expression did neither resolve to an…

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/6dffeca599bf4654. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable5-engine/src/main/java/org/activiti/engine/impl/bpmn/parser/handler/ServiceTaskParseHandler.java:82

            activity.setActivityBehavior(bpmnParse.getActivityBehaviorFactory().createClassDelegateServiceTask(serviceTask));

            // activiti:delegateExpression
        } else if (ImplementationType.IMPLEMENTATION_TYPE_DELEGATEEXPRESSION.equalsIgnoreCase(serviceTask.getImplementationType())) {
            activity.setActivityBehavior(bpmnParse.getActivityBehaviorFactory().createServiceTaskDelegateExpressionActivityBehavior(serviceTask));

            // activiti:expression
        } else if (ImplementationType.IMPLEMENTATION_TYPE_EXPRESSION.equalsIgnoreCase(serviceTask.getImplementationType())) {
            activity.setActivityBehavior(bpmnParse.getActivityBehaviorFactory().createServiceTaskExpressionActivityBehavior(serviceTask));

            // Webservice
        } else if (ImplementationType.IMPLEMENTATION_TYPE_WEBSERVICE.equalsIgnoreCase(serviceTask.getImplementationType()) &&
                StringUtils.isNotEmpty(serviceTask.getOperationRef())) {

            WebServiceActivityBehavior webServiceActivityBehavior = bpmnParse.getActivityBehaviorFactory().createWebServiceActivityBehavior(serviceTask, bpmnParse.getBpmnModel());
            activity.setActivityBehavior(webServiceActivityBehavior);

        } else {
            LOGGER.warn("One of the attributes 'class', 'delegateExpression', 'type', 'operation', or 'expression' is mandatory on serviceTask {}", serviceTask.getId());
        }

    }
}

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