code: public void initialize(URI name, Configuration conf) throws IOException {
      // if an underlying fs has not already been specified,
      // create and initialize one
      if (this.fs == null) {
        Class<?> clazz = conf.getClass("fs.apitrace.underlyingfs.impl",
                                       DistributedFileSystem.class);
        if (clazz == null) {
          throw new IOException("No FileSystem for fs.apitrace.underlyingfs.impl.");
        }
  
        this.fs = (FileSystem)ReflectionUtils.newInstance(clazz, null);
        super.initialize(name, conf);
      }
  
      // trace conf options
      for (String opt : traceConfOpts) {
        APITrace.CallEvent ce = new APITrace.CallEvent();
        ce.logCall(APITrace.COMMENT_msg, null,
                   new Object[] {opt, conf.get(opt, "")});
      }
    }
docs: /*
  Initialize an API-Trace File System