Question about how java.net.http.HttpClient#sendAsync instrumentation works

[fmterrorf]

Hello 👋 I'm very curious on how otel is able to hook into sendAsync

opentelemetry-java-instrumentation/instrumentation/java-http-client/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/httpclient/HttpClientInstrumentation.java

Line 58 in d919f84

.and(named("sendAsync"))

.

I'm using bytebuddy to create an agent but for some reason the binconsumer that is supplied in future.whenComplete

opentelemetry-java-instrumentation/instrumentation/java-http-client/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/httpclient/HttpClientInstrumentation.java

Line 136 in d919f84

future = future.whenComplete(new ResponseConsumer(context, httpRequest));

throws a java.lang.NoClassDefFoundError.

[laurit]

You need to ensure that the class you instrument (in this case jdk.internal.net.http.HttpClientImpl) has access to the class that you use in your advice (lets call it ResponseConsumer). The first hurdle is that the javaagent is loaded with app class loader (system class loader in jdk8) and HttpClientImpl is loaded with platform class loader (it is not present in jdk8, but on jdk8 jdk classes are loaded by boot loader). Platform loader is parent of app loader so HttpClientImpl won't have access to classes in agent jar. The way you get around this is adding your agent to boot loader with Instrumentation.appendToBootstrapClassLoaderSearch. Typically you'd do this as the verify first thing in the agent to ensure that all agent classes (except the premain class) are in boot loader. We do this in https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/javaagent-bootstrap/src/main/java/io/opentelemetry/javaagent/OpenTelemetryAgent.java
The next problem that you'll probably hit is that your agent classes are in unnamed module of boot loader but HttpClientImpl doesn't read that module and can't use your class. You should be able to get around this with --add-reads=java.net.http=ALL-UNNAMED (didn't try this). Same can be done programmatically with Instrumentation.redefineModule or with bytebuddy. We do this in https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/javaagent-tooling/javaagent-tooling-java9/src/main/java/io/opentelemetry/javaagent/tooling/ExposeAgentBootstrapListener.java
In our agent the classes that are used in advice are packaged in a subdirectory inside the agent jar and they can't be directly loaded so we need to define them in the appropriate loader ourself. This is handled in https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/muzzle/src/main/java/io/opentelemetry/javaagent/tooling/HelperInjector.java

[fmterrorf]

Thank you very much for the answer