[Rev 09] RFR: 6656: Allow capturing field values with path syntax
Marcus Hirt
hirt at openjdk.java.net
Thu Feb 13 14:19:42 UTC 2020
On Thu, 6 Feb 2020 16:29:10 GMT, Kangcheng Xu <kxu at openjdk.org> wrote:
>> This patch implements [JMC-6656: Allow capturing field values with path syntax](https://bugs.openjdk.java.net/browse/JMC-6656).
>>
>> In the xml configuration, a field capture looks like:
>> <field>
>> <name>a field value</name>
>> <description>a field value capture with a path syntax</description>
>> <expression>this.field.prop</expression>
>> </field>
>> See `org/openjdk/jmc/agent/test/jfrprobes_template.xml` for more examples.
>>
>> There are currently two limitations to pay attention to:
>>
>> 1. Instrumentation point cannot be in synthesized classes:
>> Instrumented classes are first loaded by obtaining the bytecode and inspected reflectively to resolve typing information. This fails if the class load is unable to provide the bytecode when `ClassLoader.getResouce()` is called. The cases where it might fail are unlikely to be for classes like proxies or auxiliaries for Java frameworks that have no visible equivalent.
>>
>> 2. Instrumentation is unable to access nestmates' private fields:
>> Before [nest-base access control](https://openjdk.java.net/jeps/181) was introduced in Java 11, accessing nestmates' private fields is, while lexically correct, forbidden by the JVM. The Java compiler works around by inserting synthetic getters/setters. However, it's not for a BCI agent since changing the class structure is not allowed during class transformation.
>>
>> Please let me know your thoughts. Thank you very much!
>
> The pull request has been updated with 1 additional commit.
Marked as reviewed by hirt (Lead).
-------------
PR: https://git.openjdk.java.net/jmc/pull/20
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