java-native-access/jna · error · java.lang.IllegalArgumentException

Native size for type "cls.getName()" is unknown

Error message

Native size for type "cls.getName()" is unknown

What it means

The no-value Native.getNativeSize(Class) enumerates every category JNA can size (primitives, wrappers, Structure, arrays, Pointer, Buffer, Callback, String, WString, etc.). If the class matches none, JNA cannot know how many bytes it occupies natively and throws this IllegalArgumentException. It signals an unsupported mapping, mirroring error 364 but for the type-only size lookup.

Source

Thrown at src/com/sun/jna/Native.java:1515

        if (cls == char.class || cls == Character.class) return WCHAR_SIZE;
        if (cls == int.class || cls == Integer.class) return 4;
        if (cls == long.class || cls == Long.class) return 8;
        if (cls == float.class || cls == Float.class) return 4;
        if (cls == double.class || cls == Double.class) return 8;
        if (Structure.class.isAssignableFrom(cls)) {
            if (Structure.ByValue.class.isAssignableFrom(cls)) {
                return Structure.size((Class<? extends Structure>) cls);
            }
            return POINTER_SIZE;
        }
        if (Pointer.class.isAssignableFrom(cls)
            || (Platform.HAS_BUFFERS && Buffers.isBuffer(cls))
            || Callback.class.isAssignableFrom(cls)
            || String.class == cls
            || WString.class == cls) {
            return POINTER_SIZE;
        }
        throw new IllegalArgumentException("Native size for type \"" + cls.getName()
                                           + "\" is unknown");
    }

    /**
     * @param cls The Java class
     * @return {@code true} whether the given class is supported as a native argument type.
     */
    public static boolean isSupportedNativeType(Class<?> cls) {
        if (Structure.class.isAssignableFrom(cls)) {
            return true;
        }
        try {
            return getNativeSize(cls) != 0;
        }
        catch(IllegalArgumentException e) {
            return false;
        }
    }

View on GitHub (pinned to d036ad9781)

Solutions

  1. Only call Native.getNativeSize for JNA-supported classes; for structs use Structure.size(class) instead.
  2. Convert unsupported types to supported mappings (Structure, IntegerType, PointerType, arrays of primitives, String/WString).
  3. If you need the pointer-sized opaque value, use Pointer.class which resolves to POINTER_SIZE.
  4. Check Native.isSupportedNativeType(cls) (the guard right below this throw) before querying sizes.

Example fix

// before
int sz = Native.getNativeSize(MyPojo.class); // unknown native size

// after
if (Native.isSupportedNativeType(MyPojo.class)) {
    int sz = Native.getNativeSize(MyPojo.class);
} else {
    int sz = Structure.size(MyStruct.class); // map it as a Structure
}
Defensive patterns

Strategy: validation

Validate before calling

if (!Native.isSupportedNativeType(cls)) {
    throw new IllegalArgumentException("Cannot query native size for unsupported type: " + cls);
}
int size = Native.getNativeSize(cls);

Type guard

static boolean hasKnownNativeSize(Class<?> cls) {
    return Native.isSupportedNativeType(cls);
}

Try / catch

try {
    return Native.getNativeSize(cls);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("is unknown")) {
        return Structure.size((Class<? extends Structure>) cls); // if it is a struct
    } else { throw e; }
}

Prevention

When it happens

Trigger: Calling Native.getNativeSize(SomeClass.class) with an unsupported class, or mapping structures/callbacks whose field/parameter types are not JNA-mappable, causing internal size queries on unknown types.

Common situations: Size-querying a custom class to plan memory allocations, using boxed generic types (List, Optional) as mapped fields, or typos where an unsupported wrapper type replaced a primitive in a Structure definition.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of java-native-access/jna@d036ad9781 (2026-09-12). Data as JSON: /api/errors/e65c5f554476696f. Report an issue: GitHub.