code: public void buildResourcePaths() {
          try {
              if (Boolean.getBoolean(Stapler.class.getName()+".noResourcePathCache")) {
                  resourcePaths = null;
                  return;
              }
  
              Map<String,URL> paths = new HashMap<String,URL>();
              Stack<String> q = new Stack<String>();
              q.push("/");
              while (!q.isEmpty()) {
                  String dir = q.pop();
                  Set<String> children = context.getResourcePaths(dir);
                  if (children!=null) {
                      for (String child : children) {
                          if (child.endsWith("/"))
                              q.push(child);
                          else {
                              URL v = context.getResource(child);
                              if (v==null) {
                                  resourcePaths = null;
                                  return; // this can't happen. abort with no cache
                              }
                              paths.put(child, v);
                          }
                      }
                  }
              }
  
              resourcePaths = Collections.unmodifiableMap(paths);
          } catch (MalformedURLException e) {
              resourcePaths = null; // abort
          }
      }
docs: Rebuild the internal cache for static resources.