code: public JCTree.JCCompilationUnit parse(JavaFileObject filename) {
          JavaFileObject prev = log.useSource(filename);
          try {
              JCTree.JCCompilationUnit t = parse(filename, readSource(filename));
              if (t.endPositions != null)
                  log.setEndPosTable(filename, t.endPositions);
              return t;
          } finally {
              log.useSource(prev);
          }
      }
docs: Parse contents of file.
  @param filename     The name of the file to be parsed.