code: public boolean ifExists(String pathStr,Configuration conf) {
      AllocatorPerContext context = obtainContext(contextCfgItemName);
      return context.ifExists(pathStr, conf);
    }
docs: We search through all the configured dirs for the file's existence
  and return true when we find
  @param pathStr the requested file (this will be searched)
  @param conf the Configuration object
  @return true if files exist. false otherwise
  @throws IOException