PooledUnsafeDirectByteBuf instanceof Bytebuf returns false

I’m using bytebuddy to “hook” the following method: https://github.com/netty/netty/blob/d34212439068091bcec29a8fad4df82f0a82c638/transport/src/main/java/io/netty/channel/socket/nio/NioSocketChannel.java#L347

Inside the hook, I get a reference to the method’s parameters as Object. Therefore, I’m trying to cast the first and only parameter to ByteBuf, but arguments[0] instanceof ByteBuf returns false and (ByteBuf)arguments[0] outputs: java.lang.ClassCastException: io.netty.buffer.PooledUnsafeDirectByteBuf cannot be cast to io.netty.buffer.ByteBuf After checking the code a few times, I made sure PooledUnsafeDirectByteBuf extends ByteBuf (after a long inheritance chain). Here is the relevant code:

System.out.println("Parameter: " + arguments[0]);
System.out.println("instance? " + (arguments[0] instanceof io.netty.buffer.ByteBuf));
b = (io.netty.buffer.ByteBuf) arguments[0];

Expected behavior

arguments[0] instanceof ByteBuf should return True since arguments[0] is of type PooledUnsafeDirectByteBuf (or other type that extends from ByteBuf)

Actual behavior

Parameter: PooledUnsafeDirectByteBuf(ridx: 0, widx: 0, cap: 2048)
instance? false
java.lang.ClassCastException: io.netty.buffer.PooledUnsafeDirectByteBuf cannot be cast to io.netty.buffer.ByteBuf

Steps to reproduce

The steps to reproduce are a bit complicated, If it is really necessary I will upload the smallest code I manage to make that includes a bytebuddy builder that hooks the doReadBytes method.

Netty version

I’m using this line in my build.gradle: implementation "io.netty:netty-all:4.1.73.Final"

JVM version (e.g. java -version)

------------------------------------------------------------
Gradle 7.3.2
------------------------------------------------------------

Build time:   2021-12-15 11:22:31 UTC
Revision:     26c186eb63b2b02e68d83b0dbc6ec69ab118653a

Kotlin:       1.5.31
Groovy:       3.0.9
Ant:          Apache Ant(TM) version 1.10.11 compiled on July 10 2021
JVM:          1.8.0_312 (Red Hat, Inc. 25.312-b07)
OS:           Linux 5.10.75-79.358.amzn2.x86_64 amd64

OS version (e.g. uname -a)

I’m using the new AWS linux image: [ec2-user@ip-******** netty]$ uname -a Linux ip-*********.us-east-2.compute.internal 5.10.75-79.358.amzn2.x86_64 #1 SMP Thu Nov 4 21:08:30 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Thanks for your help!

[sides-flow]

TL;DR: The problem has nothing to do with netty.

I have managed to advance some more. After a few hours of working on it and trying to understand whats going on, I tried switching the JVM to an updated version (correto-11). The newer version printed a better explanation of the problem:

java.lang.ClassCastException: class io.netty.buffer.PooledUnsafeDirectByteBuf cannot be cast to class io.netty.buffer.ByteBuf (io.netty.buffer.PooledUnsafeDirectByteBuf is in unnamed module of loader 'app'; io.netty.buffer.ByteBuf is in unnamed module of loader 'bootstrap')

This proves that you were right @raphw (as always). After some more time trying to work it out, I ended up creating another Advice class inside the AgentClass (where agentmain is located). For some reason that I still don’t fully understand, the cast worked this time. I guess that it has something to do with the class loader that loaded the Advice class, maybe… After printing the data of the argument ByteBuf I also realized that this is not the data that I was looking for 😕

I better close this issue since (now I know that) it has nothing to do with netty, but rather my lack of knowledge of how class loaders work in Java. I guess I will face this problem in the future…

[sides-flow]

Unfortunately I’m unable to use PooledUnsafeDirectByteBuf since its access modifier is package, and my code is outside the package. However, ByteBuf is public.

I will check what you said about 2 class loaders. I didn’t even think about this possibility of 2 different ByteBuf classes loaded…

If I won’t be able to figure it out myself I will upload a small snippet of the code here to make sure I didn’t do something dumb.

Thank you so much!