code: private Manifest getJarManifest(File container) throws IOException {
          if (container.isDirectory()) {
              return null;
          }
          JarFile jarFile = (JarFile) jarFiles.get(container);
          if (jarFile == null) {
              return null;
          }
          return jarFile.getManifest();
      }
docs: Get the manifest from the given jar, if it is indeed a jar and it has a
  manifest
  
  @param container the File from which a manifest is required.
  
  @return the jar's manifest or null is the container is not a jar or it
  has no manifest.
  
  @exception IOException if the manifest cannot be read.