RFR: 8335619: Add an @apiNote to j.l.i.ClassFileTransformer to warn about recursive class loading and ClassCircularityErrors
Volker Simonis
simonis at openjdk.org
Thu Jul 4 10:42:21 UTC 2024
On Wed, 3 Jul 2024 19:44:36 GMT, Chen Liang <liach 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 ...
>
> src/java.instrument/share/classes/java/lang/instrument/ClassFileTransformer.java line 169:
>
>> 167: * Transforming core JDK classes or using libraries which depend on core JDK classes
>> 168: * during transformation increases the risk for such errors. If the {@link LinkageError}
>> 169: * occurs during reference resolution (see section 5.4.3 Resolution of <cite>The
>
> Suggestion:
>
> * occurs during reference resolution (see section {@jvms 5.4.3} Resolution of <cite>The
>
> Other stylistic note: we can replace `<code>C</code>` with `{@code C}`, which is shorter.
Thanks for your suggestion @liach . I didn't knew about the `@jvms` tag which is quite handy. I've also updated the other reference to the JVMS above the new `@apiNote` which didn't used the tag either (and was wrong because the JVMS sections have changed since Java 5 :)
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PR Review Comment: https://git.openjdk.org/jdk/pull/20011#discussion_r1665513865
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