[External] : more class file transformation questions

Thu Nov 28 00:01:02 UTC 2024

Hi Mark,

  1.
No, you have to do new ArrayList<>(code.elementList()) as the list returned is immutable.
  2.
To insert an instruction, just use List::add. To get an instruction, create one from the factory methods (usually named "of") of Instruction subclasses. Once you are done with the list, do
for (var e : list) codeBuilder.with(e);
  3.
Bound and unbound instructions have no difference in usage when you write them. The bound instruction can be seen as a specialized version of unbound instruction, that they are cheaper for the API to obtain and supports fast copying to another CodeBuilder when condition allows (that is, the new CodeBuilder has a constantpool that extends from the constant pool of the bound instruction). Bound instructions can only be read from Class Files; unbound instructions can always be created by factory methods on the instruction interfaces.

Regards, Chen
________________________________
From: Mark Roberts <markro at cs.washington.edu>
Sent: Wednesday, November 27, 2024 5:54 PM
To: Chen Liang <chen.l.liang at oracle.com>; classfile-api-dev at openjdk.org <classfile-api-dev at openjdk.org>
Subject: RE: [External] : more class file transformation questions

Okay, a List<CodeElement> sounds interesting.

  1.  Would I just use CodeModel::elementList() as my initial list or do I have to make a copy somehow?
  2.  How would I insert a new instruction into this list?  Builders don’t seem to return an item, perhaps I need to use the static instruction of() methods?
  3.  What is difference between Bound and Unbound instructions?

Thank you,

Mark

From: Chen Liang <chen.l.liang at oracle.com<mailto:chen.l.liang at oracle.com>>
Sent: Tuesday, November 19, 2024 3:25 PM
To: Mark Roberts <markro at cs.washington.edu<mailto:markro at cs.washington.edu>>; classfile-api-dev at openjdk.org<mailto:classfile-api-dev at openjdk.org>
Subject: Re: [External] : more class file transformation questions

Hi Mark,

ClassFileBuilder::with takes an element object, such as a FieldModel for a ClassBuilder. It does not modify the object when applied to the new builder, except implementation details like constant pool references..

ClassFileBuilder::transform takes the same type of the object as the object to build, such as a ClassModel for a ClassBuilder. In the same time, it allows you to process all element objects of that same type of object.

For example, a ClassBuilder can with a method or a field. A ClassBuilder can transform a class, and use this to merge classes, like

clb.transform(sourceClassA, ClassTransform.ACCEPT_ALL).transform(sourceClassB, ClassTransform.ACCEPT_ALL)

to merge source classes A and B into the ongoing class builder.

For your specific use case, I think what you need is a method transform that does something like:

(mb, me) -> {

    if (mb instanceof CodeModel code) {

        List<CodeElement> codeElements = ...// prepare your list of instructions

        mb.withCode(codeElements::forEach); // the list of instructions get sent to the code builderOK

    } else {

        mb.with(me);

    }

}

It is also possible to do this in CodeTransform by collecting everything in a list in accept(CodeBuilder, CodeElement) and only working on the builder in atEnd(CodeBuilder), when you can see all the elements. But that code will be a bit not straightforward compared to this pattern I am showing off, and this CodeTransform can only see all elements that is from one original model instead of those from all original models (applying the merge class A and B example).

P.S. When you reply, remember to choose "reply all" so that your reply gets sent to the classfile-api-dev list too.

Regards,

Chen Liang

________________________________

From: Mark Roberts <markro at cs.washington.edu<mailto:markro at cs.washington.edu>>
Sent: Tuesday, November 19, 2024 4:43 PM
To: Chen Liang <chen.l.liang at oracle.com<mailto:chen.l.liang at oracle.com>>
Subject: [External] : more class file transformation questions

Thank you, those suggestions really helped!

Next question, given a builder, I’m curious about the differences between the ‘with’ versus ‘transform’ methods.  I see how the transforms can simplify coding and they support composition.  The implication seems to be that with and build create new objects from old while transform modifies existing objects.  But based on the examples I’ve seen they look very similar.  Are there significant performance differences? Or other advantages?  My use case, adding instrumentation code to class files, requires inspecting every byte code and possibly modifying a lot of them.  I worry that I would have to composite many transforms where one linear walk through the byte codes would be sufficient. Am I missing something? (it wouldn’t surprise me if I was)

Thank you,

Mark

From: Chen Liang <chen.l.liang at oracle.com<mailto:chen.l.liang at oracle.com>>
Sent: Sunday, November 17, 2024 4:59 PM
To: Mark Roberts <markro at cs.washington.edu<mailto:markro at cs.washington.edu>>; classfile-api-dev at openjdk.org<mailto:classfile-api-dev at openjdk.org>
Subject: Re: [External] : RE: class file transformation questions

Hi Mark,

For performing a lot of modifications, you first can create a

private static void check(CodeBuilder builder, CodeElement element) {

    // check the elements, and if it matches a pattern, call another method

    // as long as you pass the CodeBuilder and make changes to it, your changes will be saved

}

And then refer to this as a CodeTransform in a method reference.

Or, if you want to do a huge overhaul to a CodeModel, you can get a MethodTransform, and have a specific method like:

if (me instanceof CodeModel code) {

    methodBuilder.withCode(cob -> rebuildCode(cob, code));

}

private static void rebuildCode(CodeBuilder builder, CodeModel model) {}

This gives you a more holistic view of the code, and should be more friendly for massive transformations that almost rebuild the method code. (This one also allows you to pull instructions to a list with new ArrayList<>(model.elementList()), modify that list, and do list.forEach(builder) to send the results, which is used in some older patterns)

Regards,

Chen Liang

________________________________

From: Mark Roberts <markro at cs.washington.edu<mailto:markro at cs.washington.edu>>
Sent: Sunday, November 17, 2024 6:30 PM
To: Chen Liang <chen.l.liang at oracle.com<mailto:chen.l.liang at oracle.com>>
Subject: [External] : RE: class file transformation questions

Thank you for your help, it is much appreciated.  I have what I guess you would call a coding style question.  If you wish to perform a lot of modifications to a class file I could envision a method with hundreds of lines of code.  For a smaller example, see the chaining multiple transformations example in java/lang/classfile/ClassFileTransform.html.  I find this very (extremely?) difficult to read.  Is there any way of writing this code in a more ‘traditional’ way with several, smaller method bodies?  Or is there no way to use java.lang.classfile without chaining together large numbers of builders written as lambda functions?

Thank you,

Mark

From: Chen Liang <chen.l.liang at oracle.com<mailto:chen.l.liang at oracle.com>>
Sent: Friday, November 15, 2024 4:48 PM
To: Mark Roberts <markro at cs.washington.edu<mailto:markro at cs.washington.edu>>; classfile-api-dev at openjdk.org<mailto:classfile-api-dev at openjdk.org>
Subject: Re: class file transformation questions

Hi Mark,

  1.  For injecting code, you can override the atStart of the CodeTransform interface.  Unfortunately we don't have a utility method for you to pass a lambda 🙁 but we can always add it later.  If you wish to find an injection point, you can get the full list of code elements, and find your insertion point and add the desired instruction objects.

  1.  For adding new methods, you can also override the atStart/atEnd method of your ClassTransform, or do andThen(ClassTransform.endHandler(clb -> clb.withMethod(...))) to add methods.

  1.  To duplicate an existing method object, you can first call withMethod to create a new method, and once you have the method builder, it has a transform(otherMethod, transform) that can pipe otherMethod's contents through the transform, and finally everything goes to the method builder.

Feel free to ask more or suggest.  Unfortunately I am still trying to improve documentations for ClassFile API, so at this stage the documents may still be insufficient, and many of the useful methods are not covered by the JEP.

Regards,

Chen Liang

________________________________

From: classfile-api-dev <classfile-api-dev-retn at openjdk.org<mailto:classfile-api-dev-retn at openjdk.org>> on behalf of Mark Roberts <markro at cs.washington.edu<mailto:markro at cs.washington.edu>>
Sent: Friday, November 15, 2024 1:42 PM
To: classfile-api-dev at openjdk.org<mailto:classfile-api-dev at openjdk.org> <classfile-api-dev at openjdk.org<mailto:classfile-api-dev at openjdk.org>>
Subject: class file transformation questions

Several of our tools use the java.lang.instrument package to instrument class files when they are loaded.  We currently use BCEL to do the instrumentation, but would like to move to the new java.lang.classfile package.  I have gotten some basic transforms working in this environment and I see how to modify existing instructions.   However, we need to perform some larger modifications to the classes we instrument.

There are three basic transforms we need to perform (all on the same class file):

  1.  Injecting code into an existing method
  2.  Adding new methods
  3.  Duplicating an existing method with some changes

Any suggestions as to how to accomplish these tasks would be much appreciated.

Thank you,

Mark Roberts

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