Fix logback appender on android

[laurit]

Hopefully resolves #13230

[bencehornak]

Thank you so much for addressing my issue, @laurit! I would like to help to develop a test case to check, what happens, if the ClassLoader throws an exception when trying to load ClassValue.

[bencehornak]

I wasn't able to find a simple solution to un-define ClassValue. This was my attempt, but it didn't throw the expected ClassNotFoundError on the main branch, so it's not a good test:

class LoggingEventMapperTest {
  // ...

  @Test
  void testClassValueAbsent() throws Exception {
    ClassLoader classLoaderWithoutClassValue = new ClassLoaderWithoutClassValue();

    assertThatNoException()
        .isThrownBy(
            () -> {
              LoggingEventMapper.Builder builder =
                  (LoggingEventMapper.Builder)
                      classLoaderWithoutClassValue
                          .loadClass(LoggingEventMapper.class.getName())
                          .getDeclaredMethod("builder")
                          .invoke(null);
              LoggingEventMapper mapper = builder.build();
              mapper.captureMdcAttributes(Attributes.builder(), new HashMap<>());
            });
  }

  private static class ClassLoaderWithoutClassValue extends ClassLoader {
    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
      // On Android API Level <34 java.lang.ClassValue is absent, see
      // https://developer.android.com/reference/java/lang/ClassValue
      if (name.equals("java.lang.ClassValue")) {
        throw new ClassNotFoundException();
      }
      return super.loadClass(name);
    }
  }
}

It just propagates the classloading of LoggingEventMapper.class to the parent classloader, which has ClassValue in its classpath, so my ClassNotFoundException is never thrown.

Please feel free to proceed with the merging.

[laurit]

@bencehornak to make this work you'd need to load LoggingEventMapper in your custom class loader, currently it gets loaded by the parent loader. One way would be https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/instrumentation/jdbc/javaagent/src/test/java/io/opentelemetry/javaagent/instrumentation/jdbc/test/TestClassLoader.java an other typical way is to test in loadClass whether it is the class you wish to override and if it is use one of the resource finders on the parent loader to find the class bytes and call defineClass. Of course in your test you'd have to use reflection all the way to avoid class cast exceptions. Note that this is just FYI you don't need to do anything.