What causes "Cannot change fixed value" in flowable/flowable-engine?

If the field should be assignable, change the BPMN field injection from a literal to an expression, e.g. .

flowable/flowable-engine · error · ActivitiException

Cannot change fixed value

Error message

Cannot change fixed value

What it means

FixedValue wraps a literal value injected into a delegate (e.g. a field on a JavaDelegate or a DelegateExpression) that is intentionally constant. Its setValue method always throws because a fixed value cannot be assigned at runtime. The library throws this to signal that someone is trying to write to what is a read-only, statically-configured expression.

Solutions

If the field should be assignable, change the BPMN field injection from a literal to an expression, e.g. <flow:field name="x" expression="${myVar}"/>.
Remove code/assignment attempts that write to fields configured as fixed values; treat them as constants.
Use an Expression instead of FixedValue when you need read-write semantics.

Example fix

<!-- before: literal, not assignable -->
<flow:field name="amount" stringValue="100"/>
<!-- after: expression, assignable -->
<flow:field name="amount" expression="${amountVar}"/>

Defensive patterns

Strategy: validation

Validate before calling

// In a custom ELResolver or field-injection helper, reject writes to literal expressions:
if (expression instanceof FixedValue) {
  throw new IllegalArgumentException("Field 'x' is configured as a fixed literal and cannot be assigned");
}

Type guard

boolean isWritable(org.activiti.engine.impl.el.Expression e) {
  return !(e instanceof org.activiti.engine.impl.el.FixedValue);
}

Try / catch

try {
  fixedValue.setValue(newValue, container);
} catch (ActivitiException e) {
  // value is a constant; use the expression's getValue instead
}

Prevention

Treat literal field injections (stringValue=) as constants; never assign to them in delegates.
Use expression= (JUEL) instead of literals when a field must be updated.
Review delegate setters for assignments to injected fields.

When it happens

Trigger: Calling setValue(...) on a FixedValue instance — typically the UEL/EL resolver assigning a variable into a delegate field whose configured expression is a literal (value="..." in flow:field) rather than ${...} or #{...}.

Common situations: BPMN field injection with literal values being written back by the delegate, or framework code attempting to assign a value into an injected field expression that was configured as a constant.

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

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/5223bb037fbd23ee. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable5-engine/src/main/java/org/activiti/engine/impl/el/FixedValue.java:41

 * @author Frederik Heremans
 */
public class FixedValue implements Expression {

    private static final long serialVersionUID = 1L;
    private Object value;

    public FixedValue(Object value) {
        this.value = value;
    }
    
    @Override
    public Object getValue(VariableContainer variableContainer) {
        return value;
    }
    
    @Override
    public void setValue(Object value, VariableContainer variableContainer) {
        throw new ActivitiException("Cannot change fixed value");
    }

    @Override
    public String getExpressionText() {
        return value.toString();
    }

}

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