code: public static final File getValidDocumentRoot() {
          // If document root not explicitly set see if we are running from IDE
          File file = getIDEDocumentRoot();
          // If document root not explicitly set see if we are running from a war archive
          file = file != null ? file : getWarOrJarFileDocumentRoot();
          // If not a war archive maybe it is an exploded war
          file = file != null ? file : getExplodedWarFileDocumentRoot();
          if (file == null) {
              log.debug("None of the document roots " + Arrays.asList(COMMON_DOC_ROOTS)
                        + " point to a directory and will be ignored.");
          } else {
              log.debug("Document root: " + file);
          }
          return file;
      }
docs: 获取当前系统的doc-root
  
  @return 当前系统的doc-root