RFR: 6656: Allow capturing field values with path syntax

[Marcus Hirt]

On Thu, 19 Dec 2019 11:58:07 GMT, Marcus Hirt <hirt 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!
> 
> core/org.openjdk.jmc.agent/src/main/java/org/openjdk/jmc/agent/util/AccessUtils.java line 45:
> 
>> 44: 
>> 45: public class AccessUtils {
>> 46: 	public static Field getFieldOnHierarchy(Class<?> clazz, String name) throws NoSuchFieldException {
> 
> Should probably be a final class. Class javadocs would be nice.

Private constructor throwing unsupported exception might be nice so people don't even try instantiate it.

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PR: https://git.openjdk.java.net/jmc/pull/20