Elastic APM Java agent cannot automatically infer the class name / object name for a span when using Scala and @CaptureSpan

[georgms]

Describe the bug

According to the documentation @CaptureSpan will automatically set the name of the span to Classname#methodname. When using Scala 2, this works fine for classes but not for objects. For objects, only the #methodname is inferred but the Classname is just missing.

Steps to reproduce

1. Use Scala 2.12.18 and co.elastic.apm:apm-agent-api:1.50.0.
2. Create a Scala class and object:

import co.elastic.apm.api.{CaptureSpan, ElasticApm, Transaction}

class Example {
 @CaptureSpan
 def myClassMethod() = println("Example#myClassMethod")
}

object Example {
 @CaptureSpan
 def myObjectMethod() = println("Example#myObjectMethod")
}

ElasticApm.startTransaction()

(new Example).myClassMethod()

Example.myObjectMethod()

ElasticApm.stopTransaction()

3. Run the code.
4. Check your Elastic APM instance for the captured spans.

Expected behavior

Two spans are captured with the names:

1. Example#myClassMethod
2. Example#myObjectMethod

Debug logs

n/a

[jackshirazi]

Hi, thanks for reporting this. Unfortunately we're short of Scala experience at this time, so we're unlikely to priortize working on this soon.

[wolframhaussig]

I haven't used Scala, but here is what I think happens:
This article explains in the chapter "Function as Value" what is being generated by the Scala compiler:

• an Example class as a wrapper
• an Example$ class for the object
• an Example$anonfun$1 for the method within

If I am not mistaken, the issue is with the code in SimpleMethodSignatureOffsetMappingFactory:

final String className = instrumentedMethod.getDeclaringType().getTypeName();
String simpleClassName = className.substring(className.lastIndexOf('$') + 1);

This code was made for nested classes where the text after the dollar sign denotes the class name of the nested class. In the case of scala, this string is empty, therefore the class name in APM is empty.

Maybe it would be possible to check the class name for an empty string and use the parent class name in this case?

UPDATE

The same issue exists with the discouraged (but still possible) way of naming a class with a dollar sign:

public class Hello$ {
    @CaptureSpan
    public void sayHello() {
        System.out.println("Hello World!");
    }
}

This leads to:

The same is true for anonymous classes:

public class HelloBack {
    @CaptureSpan
    public void sayHello() {
        System.out.println(new Object() {

            @CaptureSpan
            @Override
            public String toString() {
                return "Hello, back!";
            }
        });
    }
}

Example attached:
demo.zip

[wolframhaussig]

@jackshirazi I added a draft PR - maybe you could check sometime if this is the correct direction?