oracle/graal · error · IllegalArgumentException

Could not unwrap an array constant: {}

Error message

Could not unwrap an array constant: {}

What it means

copyMemory requires the source constant to be unwrappable to a live host array via snippetReflection.asObject(Object.class, src). The type check passed (it is a primitive array type), but asObject returned null, meaning the constant is not a materialized host object — e.g. null, a placeholder, or a constant created by a different reflection provider.

Source

Thrown at compiler/src/jdk.graal.compiler.hostvmaccess/src/jdk/graal/compiler/hostvmaccess/HostVMAccess.java:490

    private ResolvedJavaType lookupType(String name, ClassLoader loader) {
        try {
            Class<?> cls = Class.forName(name, false, loader);
            return providers.getMetaAccess().lookupJavaType(cls);
        } catch (ClassNotFoundException e) {
            return null;
        }
    }

    @Override
    public void copyMemory(JavaConstant src, int srcFrom, int srcTo, byte[] dst, int dstFrom) {
        ResolvedJavaType arrayType = getProviders().getMetaAccess().lookupJavaType(src);
        if (arrayType == null || !arrayType.isArray() || !arrayType.getComponentType().isPrimitive()) {
            throw new IllegalArgumentException("Expected a primitive array constant, got " + src);
        }
        var array = providers.getSnippetReflection().asObject(Object.class, src);
        if (array == null) {
            throw new IllegalArgumentException("Could not unwrap an array constant: " + src);
        }
        int sourceArrayEnd = Array.getLength(array) * arrayType.getComponentType().getJavaKind().getByteCount();
        if (srcFrom < 0 || srcTo > sourceArrayEnd || srcTo < srcFrom) {
            throw new IllegalArgumentException(
                            "Invalid input range: " + srcFrom + ".." + srcTo + " for array of length " + Array.getLength(array) + " with kind " + arrayType.getComponentType().getJavaKind());
        }
        int bytesToCopy = srcTo - srcFrom;
        if (dstFrom < 0 || dstFrom > dst.length - bytesToCopy) {
            throw new IllegalArgumentException("Invalid output range: " + dstFrom + ".." + (dstFrom + bytesToCopy) + " for array of length " + dst.length);
        }
        var unsafe = Unsafe.getUnsafe();
        unsafe.copyMemory(array, unsafe.arrayBaseOffset(array.getClass()) + srcFrom, dst, Unsafe.ARRAY_BYTE_BASE_OFFSET + dstFrom, bytesToCopy);
    }

    /**
     * Host mode performs the unaligned read with {@link Unsafe} against the unwrapped hosted array
     * object.
     */

View on GitHub (pinned to a66e9ccd1d)

Solutions

1. Pre-check with providers.getSnippetReflection().asObject(Object.class, srcConst) and require non-null before copying
2. Rebuild the constant from the live array via snippetReflection.forObject on the same runtime
3. Reject non-materializable constants upstream instead of relying on copyMemory's guard

Example fix

// before
vmAccess.copyMemory(srcConst, 0, len, dst, 0);

// after
Object liveArray = providers.getSnippetReflection().asObject(Object.class, srcConst);
if (liveArray == null) {
    throw new IllegalStateException("src constant has no live host object");
}
vmAccess.copyMemory(srcConst, 0, len, dst, 0);

if (providers.getSnippetReflection().asObject(Object.class, srcConst) == null) {
    throw new IllegalStateException("copyMemory source not materializable: " + srcConst);
}

When it happens

Trigger: Calling copyMemory with a constant that types as int[]/long[] etc. but cannot be materialized: NULL_POINTER, constants from another Graal runtime or guest VM, or synthesized constants with no backing object.

Common situations: AOT/image-build constants leaking into host-mode APIs; cross-VMAccess constant reuse; tests with hand-built constants.

Related errors

• Could not unwrap array: {}
• No original class for type {}
• Invalid input range: {}..{} for array of length {} with kind
• Invalid output range: {}..{} for array of length {}
• Expected an espresso constant got a {}

AI-assisted analysis of oracle/graal@a66e9ccd1d (2026-08-14). Data as JSON: /api/errors/8c615c6d1bc70fb2. Report an issue: GitHub.