Summary

Modify the generic signature of ClassFileBuilder, and add API to it to allow transforming elements produced by a builder handler.

Problem

In ASM there is a pattern where a user builds to a builder that immediately delegates all its output through a transform:

// ASM
ClassVisitor cv = new DelegateClassVisitor(new ClassWriter(...));
cv.visitXxx(); // write elements through the delegate

Such a pattern is not possible with current ClassFile API without advanced hacks.

A workaround for CodeBuilder for such usages exist: it is CodeBuilder::transforming(CodeTransform, Consumer). We can base the new API off this one.

We wish to introduce this to the base ClassFileBuilder class, yet the signature must represent a ClassFileTransform with a type variable to support this covariant overriding type such as CodeBuilder.

Solution

1. Expand the CodeBuilder::transforming API to the base ClassFileBuilder so all builders can enjoy this facility and cover this use pattern.

2. Declare a new type parameter in ClassFileBuilder that represents the integrated transform type, so CodeBuilder::transforming is actually overridden.

3. Provide migration helps in a release note on the original issue.

With this solution, we can emulate the ASM pattern with:

// ClassFile API
cf.build(..., clb0 -> clb0.transforming((clb, cle) -> /*process */, clb -> {
    // write elements through delegate
}));

Specification

See attachment.