RFR: 8294982: Implementation of Classfile API [v20]

[Adam Sotona]

On Thu, 16 Feb 2023 14:58:21 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Adam Sotona has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   added 4-byte Unicode text to Utf8EntryTest
>
> src/java.base/share/classes/jdk/internal/classfile/impl/TransformImpl.java line 51:
> 
>> 49:  * TransformImpl
>> 50:  */
>> 51: public class TransformImpl {
> 
> Understanding check: a transform implementation is made up of two runnable (start/end handlers) and a consumer. The consumer can be used e.g. to iterate and transform all the contents of a compound element (e.g. all the instruction in a code attribute).
> 
> User-defined transforms can be "resolved" that is, we can "bind" them to a specific builder, which effectively turns them into consumers (and into a concrete transform implementation that can be used).
> 
> When two transforms T1 and T2 are composed (so that T1 runs before T2), we need to implement the `resolve` method of the resulting transform so that:
> * We resolve T2 against the provided builder - this gives us a "downstream" consumer;
> * We create a new "chained" builder which wraps the provided builder, and whose `with` method calls the downstream consumer;
> * We then resolve T1 against the chained builder, and return the resulting consumer
> 
> This means that when we call `accept` on the resulting consumer, we will transform (using T1) and call `with` (passing the transformed element)  on the chained builder, which will end up calling `accept` on the downstream consumer, which will apply T2 and finally add the resulting element to the provided builder.
> 
> Correct? (Phew)

Yes, this is how it is designed and implemented.
Actually we have only a few real use cases testing this concept, one is a prototype of jdk.jfr class instrumentation at: https://github.com/openjdk/jdk/blob/212bb04e9491badb17c110787727ea19842d1528/test/jdk/jdk/classfile/AdvancedTransformationsTest.java#L293

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PR: https://git.openjdk.org/jdk/pull/10982