字符串的MD5加密

 public String hashKeyForDisk(String key) {
     String cacheKey;
     try {
         final MessageDigest mDigest = MessageDigest.getInstance("MD5");
         mDigest.update(key.getBytes());
         cacheKey = bytesToHexString(mDigest.digest());
     } catch (NoSuchAlgorithmException e) {
         cacheKey = String.valueOf(key.hashCode());
     }
     return cacheKey;
 }

 private String bytesToHexString(byte[] bytes) {
     StringBuilder sb = new StringBuilder();
     for (int i = 0; i < bytes.length; i++) {
         String hex = Integer.toHexString(0xFF & bytes[i]);
         if (hex.length() == 1) {
             sb.append('0');
         }
         sb.append(hex);
     }
     return sb.toString();
 }