RFR: 8238197: JFR: Rework setting and getting EventHandler

[Jaroslav Bachorik]

On Wed, 9 Jun 2021 21:29:51 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> Greetings,
> 
> please help review this changeset which moves the setting and getting of event handlers to native to avoid allocations.
> 
> Testing: jdk_jfr, stress
> 
> Thanks
> Markus

src/hotspot/share/jfr/jni/jfrJavaSupport.cpp line 777:

> 775:                                                          descriptor);
> 776:   return typed_field_holder != NULL ? typed_field_holder : ik->find_field(vmSymbols::eventHandler_name(),
> 777:                                                                           vmSymbols::object_signature(), // untyped

When would be the event handler field untyped?

src/hotspot/share/jfr/jni/jfrJavaSupport.cpp line 811:

> 809:   assert(java_lang_Class::as_Klass(h_mirror()) == field_holder, "invariant");
> 810:   const oop handler_oop = JNIHandles::resolve(handler);
> 811:   assert(handler_oop != NULL, "invariant");

Should this be rather 

if (handler_oop == NULL) {
  return false;
}

to have it consistent with the code in `get_handler_field_descriptor()`?

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PR: https://git.openjdk.java.net/jdk/pull/4445