/**
  * Returns the file's sha1, computing it if necessary.
  *
  * @return the sha1
  * @throws Sha1Exception
  */
 private String getSha1() throws Sha1Exception {
   if (mSha1 == null) {
     mSha1 = JarListSanitizer.getSha1(mFile);
   }
   return mSha1;
 }