From 79fee038fb832813db303903347f3cedd3abffc4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Arnaud=20Cogolu=C3=A8gnes?=
 <514737+acogoluegnes@users.noreply.github.com>
Date: Thu, 9 Jul 2026 09:00:55 +0200
Subject: [PATCH] Initialize frame handler's connection reference before
 sending header

_frameHandler.sendHeader() can trigger an immediate reply from the
broker, which the I/O thread processes as soon as it arrives. If that
happens before _frameHandler.initialize(this) has run, the I/O thread
dereferences a still-null AMQConnection reference (e.g. in
AmqpHandler.channelRead for the Netty transport) and the connection
fails with a NullPointerException wrapped as an IOException. This
mostly surfaces under CI-style thread scheduling on fast/local
round-trips, where the reply can outrace initialize().
---
 src/main/java/com/rabbitmq/client/impl/AMQConnection.java | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/main/java/com/rabbitmq/client/impl/AMQConnection.java b/src/main/java/com/rabbitmq/client/impl/AMQConnection.java
index dfaa8989e..6eb181c6a 100644
--- a/src/main/java/com/rabbitmq/client/impl/AMQConnection.java
+++ b/src/main/java/com/rabbitmq/client/impl/AMQConnection.java
@@ -321,6 +321,10 @@ public void start()
         // initiator) is to wait for a connection.start method to
         // arrive.
         _channel0.enqueueRpc(connStartBlocker);
+        // Must happen before the header is sent: sending it can make the broker reply
+        // immediately, and the I/O thread that reads that reply needs the connection
+        // reference to already be in place, or it hits a null reference.
+        this._frameHandler.initialize(this);
         try {
             // The following two lines are akin to AMQChannel's
             // transmit() method for this pseudo-RPC.
@@ -331,8 +335,6 @@ public void start()
             throw ioe;
         }
 
-        this._frameHandler.initialize(this);
-
         AMQP.Connection.Start connStart;
         AMQP.Connection.Tune connTune = null;
         try {