Package io.netty.buffer

Class JfrEventRegistration

java.lang.Object

io.netty.buffer.JfrEventRegistration

All Implemented Interfaces:

FlightRecorderListener

final class JfrEventRegistration
extends java.lang.Object
implements FlightRecorderListener

Registers the allocator events with JFR once a Flight Recorder has been initialized.

The events are @Registered(false), because registering the first event class sets up JFR's metadata repository, which is expensive and pointless in processes that never record. Until an event class is registered, JFR does not instrument it, so Event.isEnabled() is a constant false and the JIT removes the allocation path guards entirely. Registering it later retransforms the class, which deoptimizes the guards, exactly like starting a recording in a running process.

JFR notifies listeners while it holds the PlatformRecorder class lock. It registers its own JDK events under the same lock, and the event classes are loaded by this class's initializer and have trivial initializers, so registering from the callback does not wait for anything that may in turn wait for that lock.

A Flight Recorder is initialized at most once per JVM, so the listener removes itself after registering the events. Otherwise JFR's static listener list would keep this class, and with it the class loader that loaded Netty, reachable for the lifetime of the JVM. JFR iterates over a copy of the list, so this is safe from the callback.

If no Flight Recorder is ever initialized, the listener stays in that list, so the class loader that loaded Netty can't be unloaded, for example on a webapp redeploy. Holding the listener weakly doesn't help: before JDK 25, addListener stores the caller's AccessControlContext with it, which references Netty's protection domain and so its class loader. Set -Dio.netty.jfr.enabled=false to avoid this.

This must not be initialized from an event class initializer: registration initializes the event classes.

Field Summary

Fields

Modifier and Type	Field	Description
private static java.lang.Class<?>[]	EVENT_CLASSES
private static InternalLogger	logger

Constructor Summary

Constructors

Modifier	Constructor	Description
private	JfrEventRegistration()

Method Summary

Modifier and Type	Method	Description
(package private) static void	init()	Make sure the allocator events are registered once a Flight Recorder exists.
void	recorderInitialized(FlightRecorder recorder)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

logger

private static final InternalLogger logger

EVENT_CLASSES

private static final java.lang.Class<?>[] EVENT_CLASSES

Constructor Detail

JfrEventRegistration

private JfrEventRegistration()

Method Detail

init

static void init()

Make sure the allocator events are registered once a Flight Recorder exists. Allocators call this, guarded by PlatformDependent.isJfrEnabled(), before they can emit events. The class initializer does the work.

recorderInitialized

public void recorderInitialized(FlightRecorder recorder)

Specified by:

recorderInitialized in interface FlightRecorderListener