flowable/flowable-engine · error · FlowableException

condition expression returns null

Error message

condition expression returns null

What it means

When evaluating a DMN rule's input condition, Flowable calls the JUEL expression via RuleExpressionCondition.evaluate and requires a Boolean answer (does the rule match or not). If the expression evaluates to null — e.g. it references an undefined variable or compares against a missing value — Flowable cannot decide the rule and throws this exception. A null condition result is never treated as 'false'; it is a hard error.

Solutions

1. Ensure every variable referenced by the decision table's input expressions is set on the execution/variable container before the DMN task runs.
2. Check the input expression in the decision table for typos or references to variables not in scope (EL is case-sensitive).
3. Make the expression null-safe, e.g. use `${var != null && var == 'X'}` so the condition always yields a Boolean.
4. Set a default value for the variable before invoking the DMN rule task.
5. Wrap the rule execution in try-catch for FlowableException and log which input expression produced the null.

Example fix

// before (DMN input expression)
${customerLevel == 'gold'}   // customerLevel is null -> error
// after
${customerLevel != null && customerLevel == 'gold'}

if (!execution.hasVariable("customerLevel")) throw new IllegalStateException("DMN input variable customerLevel missing");

boolean isBooleanResult(Object result) { return result instanceof Boolean; }

try {
    boolean hit = ruleExpression.evaluate(...);
} catch (FlowableException e) {
    if ("condition expression returns null".equals(e.getMessage())) {
        log.warn("DMN condition null, variable missing?", e);
    }
}

When it happens

Trigger: executeInputExpression calls evaluate and the JUEL expression returns null: a referenced execution variable is absent from the variable container, an EL function returns null, or the expression is a plain null-valued operand.

Common situations: Decision table input expression references a process variable that was never set; variable name typo (case-sensitive EL); a custom EL function returns Optional.empty/null; null is passed as the hit input value.

Related errors

• Could not create conclusion result
• byte array for resource
• condition expression returns non-Boolean
• Could not create execution variable
• could not create result variable

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/b63ae038e368b716. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-dmn-engine/src/main/java/org/flowable/dmn/engine/impl/el/RuleExpressionCondition.java:45

 */
public class RuleExpressionCondition {

    protected Expression expression;

    public RuleExpressionCondition(Expression expression) {
        this.expression = expression;
    }

    public boolean evaluate(Map<String, Object> variables, ELExecutionContext executionContext) {
        VariableContainerWrapper variableContainer = new VariableContainerWrapper(variables);
        variableContainer.setInstanceId(executionContext.getInstanceId());
        variableContainer.setScopeType(executionContext.getScopeType());
        variableContainer.setTenantId(executionContext.getTenantId());

        Object result = expression.getValue(variableContainer);

        if (result == null) {
            throw new FlowableException("condition expression returns null");
        }
        if (!(result instanceof Boolean)) {
            throw new FlowableException("condition expression returns non-Boolean: " + result + " (" + result.getClass().getName() + ")");
        }
        return (Boolean) result;
    }

}

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