oracle/graal · warning · ArrayIndexOutOfBoundsException

Index should be an int

Error message

Index should be an int

What it means

ArrayIndexOutOfBoundsException ('Index should be an int') thrown by the ProxyArray view of a host object's member keys in EspressoExternalHostProxies.getMemberKeys: the polyglot long index passed to get(long) overflows int range, so Math.toIntExact(index) throws ArithmeticException, which is converted to this AIOOBE. Polyglot array indices are longs, but the backing Java key array is indexed by int, so any index outside [0, Integer.MAX_VALUE] (including negatives or values > 2^31-1) cannot be served.

Source

Thrown at espresso-compiler-stub/src/com.oracle.truffle.espresso.vmaccess/src/com/oracle/truffle/espresso/vmaccess/EspressoExternalHostProxies.java:363

    private Value computeGuestMethodMap(Class<?> hostClass, EspressoExternalResolvedInstanceType guestType) {
        Map<String, ProxyExecutable> map = computeMethodMap(hostClass, guestType);
        ProxyObject invocables = new ProxyObject() {
            @Override
            public Object getMember(String key) {
                return map.get(key);
            }

            @Override
            public Object getMemberKeys() {
                Object[] keys = map.keySet().toArray();
                return new ProxyArray() {
                    @Override
                    public Object get(long index) {
                        try {
                            return keys[Math.toIntExact(index)];
                        } catch (ArithmeticException e) {
                            throw new ArrayIndexOutOfBoundsException("Index should be an int");
                        }
                    }

                    @Override
                    public void set(long index, Value value) {
                        throw new UnsupportedOperationException("set() not supported.");
                    }

                    @Override
                    public long getSize() {
                        return keys.length;
                    }
                };
            }

            @Override
            public boolean hasMember(String key) {
                return map.containsKey(key);

View on GitHub (pinned to a66e9ccd1d)

Solutions

  1. Guest-side: clamp/validate indices to 0 <= index < getSize() before accessing member keys.
  2. Host-side: treat this exception as 'index out of the representable range' and return undefined/holes for huge indices instead of failing the iteration.
  3. If a guest engine probes with sentinel indices, ensure the engine's interop layer is current — newer Truffle versions probe less aggressively.
  4. Report a bug only if the index is within [0, size) and still fails; that would indicate a genuine provider defect.

Example fix

// before (guest interop caller)
for (let i = 0; i < 2 ** 33; i++) readKeyAt(i); // long probe hits get(Long.MAX)

// after
const n = memberKeys.getSize(); // long
for (let i = 0; i < n && i < 2147483647; i++) readKeyAt(i);
Defensive patterns

Strategy: validation

Validate before calling

// guest/interop side: bounds-check against size before read
long n = memberKeys.getSize();
if (index < 0 || index >= n || index > Integer.MAX_VALUE) {
    return undefinedOrError(index); // out of representable range
}
return memberKeys.get(index);

Try / catch

try { return keysProxy.get(idx); } catch (ArrayIndexOutOfBoundsException e) { /* index not representable as int: treat as hole/undefined */ }

Prevention

When it happens

Trigger: Guest language code iterating the array-shaped mirror of a host object's member keys with an index larger than Integer.MAX_VALUE or negative; a guest runtime probing ProxyArray.getSize/get bounds incorrectly and calling get() with a huge sentinel index.

Common situations: JS or other guest engines performing sparse-array probing (reading index 2^32 or beyond to detect array holes); guest code passing an unvalidated user-supplied index straight into the polyglot array access; interop with languages whose arrays are long-indexed (e.g. some native engines).

Related errors

AI-assisted analysis of oracle/graal@a66e9ccd1d (2026-08-14). Data as JSON: /api/errors/026b1f29cfebb1c6. Report an issue: GitHub.