code: protected ClassMapping readClassMapping ()
          throws IOException, ClassNotFoundException
      {
          // create our classmap if necessary
          if (_classmap == null) {
              _classmap = Lists.newArrayList();
              // insert a zeroth element
              _classmap.add(null);
          }
  
          // read in the class code for this instance
          short code = readShort();
  
          // a zero code indicates a null value
          if (code == 0) {
              return null;
  
          // if the code is negative, that means that we've never seen it before and class
          // metadata follows
          } else if (code < 0) {
              // first swap the code into positive-land
              code *= -1;
  
              // read in the class metadata
              String cname = readUTF();
              // if we have a translation (used to cope when serialized classes are renamed) use
              // it
              if (_translations != null) {
                  String tname = _translations.get(cname);
                  if (tname != null) {
                      cname = tname;
                  }
              }
  
              // create the class mapping
              return mapClass(code, cname);
  
          } else {
              ClassMapping cmap = (code < _classmap.size()) ? _classmap.get(code) : null;
  
              // sanity check
              if (cmap == null) {
                  // this will help with debugging
                  log.warning("Internal stream error, no class metadata", "code", code,
                              "ois", this, new Exception());
                  log.warning("ObjectInputStream mappings", "map", _classmap);
                  String errmsg = "Read object code for which we have no registered class " +
                      "metadata [code=" + code + "]";
                  throw new RuntimeException(errmsg);
              }
              return cmap;
          }
      }
docs: Reads a class mapping from the stream.
  
  @return the class mapping, or <code>null</code> to represent a null value.