[Bug] The follow-up with skywalking-java can not coexist with other agents. #9698

[SLAMDUNK7]

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Apache SkyWalking Component

Java Agent (apache/skywalking-java)

What happened

Excuse me again. Thank you for wu-sheng's help, change the order of skywalking can resolve this problem, but thanks to our community partners for their thoughts.I find the primary cause of this problem，I hope this can help you.

I have found the reason for this problem. The problem described in #9698 will occur when the classes that skywalking is intended to enhance and they are used in agents added before skywalking.
The reason as follow :
Skywalking uses byte-buddy, when it structures byte-buddy's AgentBuilder, it select 'SinglePass (Default)' as it's 'DiscoveryStrategy'.

/**
 * A discovery strategy that considers all loaded types supplied by {@link Instrumentation#getAllLoadedClasses()}.
 */
enum SinglePass implements AgentBuilder.RedefinitionStrategy.DiscoveryStrategy {

    /**
     * The singleton instance.
     */
    INSTANCE;

    /**
     * {@inheritDoc}
     */
    public Iterable<Iterable<Class<?>>> resolve(Instrumentation instrumentation) {
        return Collections.<Iterable<Class<?>>>singleton(Arrays.<Class<?>>asList(instrumentation.getAllLoadedClasses()));
    }
}

DiscovertStrategy.SinglePass will discovery all loaded classes and then if skywaling need to enhance them in AgentBuilder.installOn, they will be retransformed.

In AgentBuilder.installOn process:

1. As first step in the picture, the classes that have been not loaded will be transform,.
2. As second step in the picture,the classes tha have been loaded will be discovered by 'DiscovertStrategy.SinglePass' and will be
   retransformed.

We quoted the 'instrumentation. RetransformClasses()' restrictions:

The retransformation must not add, remove or rename fields or methods, change the signatures of methods, or change inheritance.

It means that，when agent try to retransform classes, the above restrictions need to be observed.But i find that, when skywalking enhance a classes, it will change the class's inheritance.

If this class(can be system class) has been loaded by other agent before skywalking's premain，when skywalking enhance this class the Instrumentation.retransformClasses() will throw:

java.lang.UnsupportedOperationException: class redefinition failed: attempted to change superclass or interfaces

Because skywalking attemp to change a class's inheritace in retransform process.

What you expected to happen

I hope it can help skywalking. And i expect skywalking-java can coexist with other agents.

How to reproduce

Agent version:8.11.0
Plugin：apm-guava-eventbus-plugin
The command is like '-javaagent:xxxx-agent.jar -javaagent:skywalking-agent.jar'.

Anything else

No response

Are you willing to submit PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[wu-sheng]

Thanks for providing detailed information. I think the key point is

1. SkyWalking adds a new interface to instrumented class
2. SkyWalking add a new field to that class. That field could be read and written through interface <1>.

Those actually are why SkyWalking could provide faster performance when tracing in prod env.
We could propagate tracing context and other runtime context without many maps anf reflection.

[SLAMDUNK7]

Thanks for your answer and changing this issue to disscussion.
In our process, multiple agents are used, and it is not an ideal way to change the order of agents to be compatible with each other.
I dump classes from memory that have been enhanced by other agents. None of the other agents made changes to the class inheritance. Does skywalking provide methods or configurations to avoid modifying class inheritance? If skywalking could do this, we wouldn't care about order. Can you help me?

[wu-sheng]

Does skywalking provide methods or configurations to avoid modifying class inheritance?

No, as I mentioned, that is the key part of keeping perf good now. If there is no such field and interface, everything breaks.

[wu-sheng]

Closing for now, as no more questions to discuss.

[wranglerlmx]

After I found out about this problem, after idea 2024. x, debug mode startup will report an error. Normal after switching to idea 2023. x.