Java 9 and beyond, I suggest VarHandle API instead:

// A lookup object that can see the private members of this class.
    MethodHandles.Lookup lookup = MethodHandles.lookup();
    VarHandle fieldHandle = null;
    Class<?> currentClass = config.getClass();

    // Loop to find the field in the class hierarchy
    while (currentClass != null && fieldHandle == null) {
        try {
            // Get a special lookup that can access private members of the target class.
            // This requires `--add-opens` if the target is in another module.
            MethodHandles.Lookup privateLookup = MethodHandles.privateLookupIn(currentClass, lookup);
            // Find a handle for the field by name and type.
            fieldHandle = privateLookup.findVarHandle(currentClass, "perThreadHardLimitMB", int.class);
        } catch (IllegalAccessException | NoSuchFieldException e) {
            // Field not in this class, try the superclass.
            currentClass = currentClass.getSuperclass();
        }
    }

    try {
        if (fieldHandle != null) {
            // Set the value using the handle.
            fieldHandle.set(config, limitMB);
            log.info("Set perThreadHardLimitMB to {} MB via VarHandle", limitMB);
        } else {
            log.error("Could not find VarHandle for perThreadHardLimitMB field");
        }
    } catch (Throwable t) { // VarHandle.set can throw Throwable
        log.error("Failed to set per-thread RAM limit via VarHandle", t);
    }

Thanks, I have updated the pull request to use VarHandle instead of reflection.