flowable/flowable-engine · error · IllegalArgumentException

Cannot parse array index:

Error message

Cannot parse array index: 

What it means

JsonNodeELResolver.toIndex throws IllegalArgumentException('Cannot parse array index: ' + property) when the property used to address a JSON array element is a String that is not a valid integer. Array access requires a numeric index; a non-numeric string cannot be coerced.

Solutions

1. Use a numeric index (literal or integer variable) when the base is a JSON array
2. Validate with a regex or Integer.parseInt before using a variable as an index
3. If key-based access is intended, make the base a JSON object, not an array

Example fix

// before
${jsonArray["first"]}  // IllegalArgumentException
// after
${jsonArray[0]}

boolean isValidArrayIndex = s != null && s.matches("\\d+");

boolean isValidArrayIndex(Object p) { return p instanceof Number || (p instanceof String s && s.matches("\\d+")); }

try { expr.getValue(ctx); } catch (FlowableException e) { if (e.getCause() instanceof IllegalArgumentException iae && iae.getMessage().startsWith("Cannot parse array index")) { throw new IllegalArgumentException("Use a numeric index for JSON arrays", iae); } throw e; }

When it happens

Trigger: Evaluating an expression like ${jsonVar["abc"]} or ${jsonVar[someStringVar]} where the target base is a JSON array and the property string does not parse via Integer.valueOf.

Common situations: Typos in expressions using quoted keys on arrays instead of objects; passing a user-supplied or variable string as an index without validating it is numeric; mixing up object property and array index syntax.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• Cannot coerce property to array index:
• error.property.method.access
• error.property.method.invocation
• error.property.method.notfound
• error.property.method.returntype

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/56af907377b3f87a. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-engine-common/src/main/java/org/flowable/common/engine/impl/el/JsonNodeELResolver.java:367

        } else if (value != null) {
            node.set(index, value.toString());

        } else {
            node.setNull(index);
        }

        context.setPropertyResolved(true);
    }

    protected int toIndex(Object property) {
        int index;
        if (property instanceof Number) {
            index = ((Number) property).intValue();
        } else if (property instanceof String) {
            try {
                index = Integer.valueOf((String) property);
            } catch (NumberFormatException e) {
                throw new IllegalArgumentException("Cannot parse array index: " + property, e);
            }
        } else {
            throw new IllegalArgumentException("Cannot coerce property to array index: " + property);
        }

        return index;
    }

    /**
     * Test whether the given base should be resolved by this ELResolver.
     * 
     * @param base
     *            The bean to analyze.
     * @return base != null
     */
    private final boolean isResolvable(Object base) {
        return JsonUtil.isJsonNode(base);
    }

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