the javac ClassWriter class provides a method writeExtraClassAttributes as follows

    /**Allows subclasses to write additional class attributes
     *
     * @return the number of attributes written
     */
    protected int writeExtraClassAttributes(ClassSymbol c) {
        return 0;
    }

The method has various problems:

1. It can be hard to provide subtypes in a modular javac
2. The field `databuf` is package-private, implying that subtypes would have to be in the same package, which again is problematic in a modular javac
3. It is unnecessarily restricted to class-level attributes. It would be more useful if it could be extended to field and method attributes as well

It would be better if a client could register to be called in a timely fashion to be able to write additional class, method, and field attributes.