How to instrument an abstract function inside a class ?

[meirtolpin11]

Hello everyone,

I am trying to instrument the newDocumentBuilder function inside javax.xml.parsers.DocumentBuilderFactory -

    /**
     * Creates a new instance of a {@link javax.xml.parsers.DocumentBuilder}
     * using the currently configured parameters.
     *
     * @return A new instance of a DocumentBuilder.
     *
     * @throws ParserConfigurationException if a DocumentBuilder
     *   cannot be created which satisfies the configuration requested.
     */

    public abstract DocumentBuilder newDocumentBuilder()
        throws ParserConfigurationException;

This is my code snippet -

public class DocumentBuilderFactoryInstrumentation implements TypeInstrumentation {
    @Override
    public ElementMatcher<TypeDescription> typeMatcher() {
        return extendsClass(named("javax.xml.parsers.DocumentBuilderFactory"));
    }

    @Override
    public void transform(TypeTransformer typeTransformer) {

        // DocumentBuilderFactory.newDocumentBuilder()
        typeTransformer.applyAdviceToMethod(
                named("newDocumentBuilder").and(takesArguments(0)),
                this.getClass().getName() + "$NewDocumentBuilder"
        );
    }

But unfortunately it's not working and $NewDocumentBuilder is never called.
What am I doing wrong? it's working with all the other function which are part of DocumentBuilderFactory (there are not abstract).
So I believe that the reason it's not working is because it's abstract, but I have no idea how to solve that.

Thanks!

[mateuszrzeszutek]

Hey @meirtolpin11 ,

The fragment you posted looks correct; can you also post the advice class? You could replace the advice enter/exit methods with some System.out.println() statements in case you don't want to share your code (that's also a good way to test that).

Instrumenting abstract methods definitely works in general, here`s a sample instrumentation that does that in the agent.

[laurit]

You aren't actually instrumenting the abstract method, as abstract methods don't have a body, but rather the newDocumentBuilder method in a class that extends the DocumentBuilderFactory. Most likely this doesn't work because agent by default excludes some classes from instrumentation. For example you probably want to instrument newDocumentBuilder in com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl but agent by default excludes classes under com.sun in

opentelemetry-java-instrumentation/javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/ignore/GlobalIgnoredTypesConfigurer.java

Line 87 in fe56784

.ignoreClass("com.sun.")

If that is the case you can use your own IgnoredTypesConfigurer to override the exclude.