Integrated: 8335619: Add an @apiNote to j.l.i.ClassFileTransformer to warn about recursive class loading and ClassCircularityErrors

[Volker Simonis]

On Wed, 3 Jul 2024 16:14:31 GMT, Volker Simonis <simonis at openjdk.org> wrote:

> Since Java 5 the `java.lang.instrument` package provides services that allow Java programming language agents to instrument (i.e. modify the bytecode) of programs running on the Java Virtual Machine. The `java.lang.instrument` functionality is based and implemented on top of the native Java Virtual Machine Tool Interface (JVMTI) also introduced in Java 5. But because the `java.lang.instrument` API is a pure Java API and uses Java classes to instrument Java classes it imposes some usage restrictions which are not very well documented in its API specification.
> 
> E.g. the section on ["Bytecode Instrumentation"](https://docs.oracle.com/en/java/javase/21/docs/specs/jvmti.html#bci) in the JVMTI specification explicitly warns that special "*Care must be taken to avoid perturbing dependencies, especially when instrumenting core classes*". The risk of such "perturbing dependencies" is obviously much higher in a Java API like `java.lang.instrument`, but a more detailed explanation and warning is missing from its API documentation.
> 
> The most evident class file transformation restriction is that while a class A is being loaded and transformed it is not possible to use this same class directly or transitively from the `ClassFileTransformer::transform()` method. Violating this rule will result in a `ClassCircularityError` (the exact error type is disputable as can be seen in [8164165: JVM throws incorrect exception when ClassFileTransformer.transform() triggers class loading of class already being loaded](https://bugs.openjdk.org/browse/JDK-8164165), but the result would be a `LinkageError in any case).
> 
> The risk to run into such a `ClassCircularityError` error increases with the amount of code a transforming agent is transitively using from the `transform()` method. Using popular libraries like ASM, ByteBuddy, etc. for transformation further increases the probability of running into such issues, especially if the agent aims to transform core JDK library classes.
> 
> By default, the occurrence of a `ClassCircularityError` in `ClassFileTransformer::transform()` will be handled gracefully with the only consequence that the current transformation target will be loaded unmodified (see `ClassFileTransformer` API spec: "*throwing an exception has the same effect as returning null*"). But unfortunately, it can also have a subtle but at the same time much more far-reaching consequence. If the `ClassCircularityError` occurs during the resolution of a constant pool entry in another, ...

This pull request has now been integrated.

Changeset: eec0e155
Author:    Volker Simonis <simonis at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/eec0e155f303ff4bbdab172765ca7c92c2b94cbd
Stats:     21 lines in 1 file changed: 17 ins; 0 del; 4 mod

8335619: Add an @apiNote to j.l.i.ClassFileTransformer to warn about recursive class loading and ClassCircularityErrors

Reviewed-by: alanb, stuefe, liach

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PR: https://git.openjdk.org/jdk/pull/20011