flowable/flowable-engine · error · FlowableIllegalArgumentException

Priority expression does not resolve to a number: + activeTa

Error message

Priority expression does not resolve to a number: + activeTaskPriority

What it means

The user task's flowable:priority expression resolved to an object that is neither a String nor a Number, so the engine cannot derive an integer priority. This is the else-branch of handlePriority and throws FlowableIllegalArgumentException. (The message references the local field activeTaskPriority in the source, but the offending value is the resolved expression result.)

Source

Thrown at modules/flowable-engine/src/main/java/org/flowable/engine/impl/bpmn/behavior/UserTaskActivityBehavior.java:294

            }
        }
    }

    protected void handlePriority(CreateUserTaskBeforeContext beforeContext, ExpressionManager expressionManager, TaskEntity task, DelegateExecution execution,
            String activeTaskPriority) {
        if (StringUtils.isNotEmpty(beforeContext.getPriority())) {
            final Object priority = expressionManager.createExpression(beforeContext.getPriority()).getValue(execution);
            if (priority != null) {
                if (priority instanceof String) {
                    try {
                        task.setPriority(Integer.valueOf((String) priority));
                    } catch (NumberFormatException e) {
                        throw new FlowableIllegalArgumentException("Priority does not resolve to a number: " + priority, e);
                    }
                } else if (priority instanceof Number) {
                    task.setPriority(((Number) priority).intValue());
                } else {
                    throw new FlowableIllegalArgumentException("Priority expression does not resolve to a number: " + activeTaskPriority);
                }
            }
        }
    }

    protected void handleCategory(CreateUserTaskBeforeContext beforeContext, ExpressionManager expressionManager, TaskEntity task,
            DelegateExecution execution) {
        if (StringUtils.isNotEmpty(beforeContext.getCategory())) {
            String category = null;
            try {
                Object categoryValue = expressionManager.createExpression(beforeContext.getCategory()).getValue(execution);
                if (categoryValue != null) {
                    category = categoryValue.toString();
                }
            } catch (FlowableException e) {
                category = beforeContext.getCategory();
                LOGGER.warn("property not found in task category expression {}", e.getMessage());
            }

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Make the expression return a Number (Integer) directly, e.g. ${bean.priorityValue} returning int
2. Unwrap the enum/DTO: return the numeric field instead of the object
3. Coerce in a delegate/execution listener: setVariable to an Integer before the user task
4. Fix the type of the underlying process variable

Example fix

// before
public Priority getPriority() { return Priority.HIGH; }
// after
public int getPriority() { return 100; }

Object p = priorityExpr.getValue(execution);
if (p != null && !(p instanceof Number) && !(p instanceof String)) {
    throw new IllegalArgumentException("Priority must be Number or numeric String, got " + p.getClass());
}

boolean isNumericPriority(Object v) {
    return v instanceof Number || (v instanceof String && ((String) v).matches("\\d+"));
}

try {
    taskService.complete(taskId, vars);
} catch (FlowableIllegalArgumentException e) {
    if (e.getMessage().contains("Priority expression")) {
        // unwrap the object to its numeric value and retry
    }
}

When it happens

Trigger: flowable:priority expression evaluating to Boolean, Map, custom object, or null-adjacent wrapper that is not Number/String.

Common situations: Spring bean method returning a Priority enum or custom DTO; expression returning a Boolean flag mistakenly used as priority; variable set to JSON-serialized object by a preceding service task.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Priority expression does not resolve to a number: %s
• Priority expression does not resolve to a number:
• Due date expression does not resolve to a Date, Instant, Loc
• Priority does not resolve to a number: + priority
• Category expression does not resolve to a string: %s

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/b53bbf6c067eac2f. Report an issue: GitHub.