Can't use static variable in @Advice.OnMethodEnter method in opentelemetry javaagent extension

Question

I'm trying to create an extension for opentelemetry javaagent. I need some static variables outside the methods to handle spans. But I get the error java.lang.ClassNotFoundException for a class where static variables are defined. The simplest code to reproduce:

public static class MethodAdvice {

        public static Integer CONST = 123;

        @Advice.OnMethodEnter(suppress = Throwable.class)
        public static void onEnter() {
            try {
                System.out.println(CONST.toString());
//                Class.forName(MethodAdvice.class.getName());
            } catch (Throwable e) {
                System.out.println("ERROR: " + e.getCause().toString());
                throw new RuntimeException(e);
            }
        }
    }

I get error when using CONST static variable for the first time. The same if I try to load the class. How can I use a static variable defined outside of the @Advice method? As I can see the opentelemetry library uses static variables outside the methods and it works, but I can't figure out how it is handled there.

Rafael Winterhalter · Accepted Answer · 2023-03-15 17:38:05Z

1

I assume that these variables are on the boot loader, or they are final and primitive such that they are inlined during compilation.

Advice code is copied to the targeted method. It is at it was written there. As a result, it will have a different scope of visibility and access.

answered Mar 15, 2023 at 17:38

Rafael Winterhalter

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trask · Accepted Answer · 2023-04-05 18:55:32Z

0

The OpenTelemetry Java agent loads Advice classes inside of its own isolated AgentClassLoader.

But the Advice method bytecode is inlined into the instrumented application classes.

And application classes do not have access to the AgentClassLoader.

You'll want to move your constant to a "helper" class, which this OTel Java agent will inject into the application class loader, and so will be accessible from your advice.

This documentation should help: https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/docs/contributing/writing-instrumentation-module.md#write-an-instrumentationmodule-step-by-step

answered Apr 5, 2023 at 18:55

trask

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1 Comment

Valeriy K. Over a year ago

Yes, you're right. I solved this issue here - stackoverflow.com/questions/75508576/…

Valeriy K. · Accepted Answer · 2023-03-17 08:35:28Z

0

I think this is how extensions are implemented in opentelemetry agent. I checked the current thread classloader, it's my application class loader. So application classloader tries to load MethodAdvice class but it is not present in the application classpath. So it is required to add the extension jar to the classpath. I've just added the extension as a dependency in my multimodule project and the issue was resolved.

UPD: But now I have another issue( - java.lang.NoSuchFieldError if CONST field more complicated and requires initialization in static block.

edited Mar 17, 2023 at 8:35

answered Mar 17, 2023 at 7:03

Valeriy K.

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