Attribute safety

[Adam Sotona]

I’ve added some factories below, which may simplify handling of some common custom attribute cases, for example:

        Classfile.of(Classfile.AttributeMapperOption.of(attrName -> switch (attrName.stringValue()) {

            case "MyCustomBinaryContentAttribute" -> selfContainedCustomAttribute(attrName.stringValue());

            case "MyCustomAttributeWithCPReference" -> singleConstantPoolEntryCustomAttribute(attrName.stringValue());

            default -> null;

        }));

For category #1 there might be a single factory getting attribute name and returning attribute mapper.
    static <T extends CustomAttribute<T>>
                    AttributeMapper<T> selfContainedCustomAttribute(String attributeName,
                                                                    BiFunction<AttributeMapper<T>, byte[], T> attributeFactory,
                                                                    Function<T, byte[]> contentAccessor) {
        return new AttributeMapper<T>() {
            @Override
            public String name() {
                return attributeName;
            }

            @Override
            public T readAttribute(AttributedElement enclosing, ClassReader cf, int pos) {
                return attributeFactory.apply(this, cf.readBytes(pos, cf.readInt(pos - 4)));
            }

            @Override
            public void writeAttribute(BufWriter buf, T attr) {
                buf.writeBytes(contentAccessor.apply(attr));
            }
        };
    }

    static AttributeMapper<? extends CustomAttribute> selfContainedCustomAttribute(String attributeName) {
        class SelfContainedAttribute extends CustomAttribute<SelfContainedAttribute> {
            final byte[] content;

            public SelfContainedAttribute(AttributeMapper<SelfContainedAttribute> mapper, byte[] content) {
                super(mapper);
                this.content = content;
            }
        }
        return selfContainedCustomAttribute(attributeName, SelfContainedAttribute::new, a -> a.content);
    }


For category #2 there might be more options:

  *   A factory producing mapper which throws on write when CP is not shared
This is default behavior of unknown attributes, so there is no user action needed.


  *   Or a factory producing mapper simplifying CP entries clone and re-mapping on write when CP is not shared:

    static <T extends CustomAttribute<T>>

                    AttributeMapper<T> singleConstantPoolEntryCustomAttribute(String attributeName,

                                                                              BiFunction<AttributeMapper<T>, PoolEntry, T> attributeFactory,

                                                                              Function<T, PoolEntry> entryAccessor) {

        return new AttributeMapper<T>() {

            @Override

            public String name() {

                return attributeName;

            }



            @Override

            public T readAttribute(AttributedElement enclosing, ClassReader cf, int pos) {

                return attributeFactory.apply(this, cf.readEntryOrNull(pos));

            }



            @Override

            public void writeAttribute(BufWriter buf, T attr) {

                buf.writeIndexOrZero(entryAccessor.apply(attr));

            }

        };

    }



    static AttributeMapper<? extends CustomAttribute> singleConstantPoolEntryCustomAttribute(String attributeName) {

        class SingleEntryAttribute extends CustomAttribute<SingleEntryAttribute> {

            final PoolEntry entry;



            public SingleEntryAttribute(AttributeMapper<SingleEntryAttribute> mapper, PoolEntry entry) {

                super(mapper);

                this.entry = entry;

            }

        }

        return singleConstantPoolEntryCustomAttribute(attributeName, SingleEntryAttribute::new, a -> a.entry);

    }

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