code: public CharSequence readSource(JavaFileObject filename) {
          try {
              inputFiles.add(filename);
              return filename.getCharContent(false);
          } catch (IOException e) {
              log.error("error.reading.file", filename, JavacFileManager.getMessage(e));
              return null;
          }
      }
docs: Try to open input stream with given name.
  Report an error if this fails.
  @param filename   The file name of the input stream to be opened.