Mapping byte code offsets in custom code attributes

[Rafael Winterhalter]

Hello,

I mentioned before that I think there is still a missing bit when it comes
to custom code attributes, and mapping them using an AttributeMapper. Many
code attributes point to a location, or area, within a method. This
typically implies specifying one or two byte code offsets within an
attribute.

To implement this, I would need a possibility to translate offsets to
labels within an attribute mapper, and vice versa. For example, if I
created byte code from a different source, where a method consists of
multiple files, I would want to mark what part of the method comes from
what file.

I do not currently know of a way to do this in the Class File API. To
resolve this, I would have hoped that the attribute reader method would be
able to issue labels that are later issued when the code instructions are
supplied. Similarly, I would have hoped that the attribute writer method
would be capable of translating labels to byte code offsets.

In the same way, I would have hoped that all attributes such as
LineNumberInfo of the LineNumberTableAttribute would expose bytecode
offsets as labels and not as integers, as the latter does not really give
any useful information by itself. This way, I could also write a custom
LineNumberTableAttribute after a method is already created by issuing
relevant labels, but by only writing the attribute when the entire method
was visited. Sometimes this is necessary when generating code, when the
information about the actual line numbers is only retrieved at a later
stage that requires knowledge of the entire method.

As I understand it, the ClassFile API is supposed to be released in Java
24. Are there still plans to integrate such a change? After that it would
require incompatible changes, therefore I had hoped this would make it into
the final version.

Thanks! Rafael
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