What causes "Cannot parse array index: " in flowable/flowable-engine?

Use a numeric index in the expression, or resolve a numeric variable as the index.

flowable/flowable-engine · error · IllegalArgumentException

Cannot parse array index:

Error message

Cannot parse array index:

What it means

ArrayELResolver.coerce throws IllegalArgumentException when an array index supplied as a String in an EL expression cannot be parsed as an integer. The original NumberFormatException is preserved as the cause.

Source

Thrown at modules/flowable-engine-common/src/main/java/org/flowable/common/engine/impl/javax/el/ArrayELResolver.java:324

			throw new PropertyNotFoundException(new ArrayIndexOutOfBoundsException(idx).getMessage());
		}
	}

	private static int coerce(Object property) {
		if (property instanceof Number) {
			return ((Number) property).intValue();
		}
		if (property instanceof Character) {
			return (Character) property;
		}
		if (property instanceof Boolean) {
			return (Boolean) property ? 1 : 0;
		}
		if (property instanceof String) {
			try {
				return Integer.parseInt((String) property);
			} catch (NumberFormatException e) {
				throw new IllegalArgumentException("Cannot parse array index: " + property, e);
			}
		}

		throw new IllegalArgumentException("Cannot coerce property to array index: " + property);
	}
}

View on GitHub (pinned to d6d39ce1c6)

Solutions

Use a numeric index in the expression, or resolve a numeric variable as the index.
If keyed access is needed, switch the base object from an array to a Map so the property is used as a key, not an index.
Check that the index variable in your expression context actually holds a number, not a String with non-digit characters.

Example fix

// before
engine.eval("${items[first]}")
// after
engine.eval("${items[0]}")  // or move 'first' into the context as an Integer

Defensive patterns

Strategy: validation

Validate before calling

if (indexProp instanceof String s && !s.matches("\\d+")) throw new IllegalArgumentException("EL array index must be numeric: " + s);

Try / catch

try { resolver.getValue(ctx, base, prop); } catch (IllegalArgumentException e) { /* log bad index literal */ }

Prevention

Only use integer literals or numeric variables as array indexes in EL
Use Map instead of array for keyed lookups
Verify index variables in the EL context hold numbers

When it happens

Trigger: Evaluating ${myArray[abc]} or ${myArray[someStringVariable]} where the property coerces to a non-numeric String like 'first' or 'id-12'.

Common situations: Copy-pasting map-style access ${data[key]} against an array instead of a Map; typos in variable names causing the index variable name to be treated as a literal string.

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

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/e543b95912df844a. Report an issue: GitHub.