请不要实施您当前的计划，而应该使用MessageDigest来完成此任务。对用户密码（例如SHA-256，SHA-384和SHA-512
[还有其他]中的一种）应用单向加密哈希函数和SALT，以防止基于彩虹表的攻击。最后，对于密码重置，只需替换当前的密码哈希即可。

举个例子，

// We need a bytesToHex method first. So, from -
// http://stackoverflow.com/a/9855338/2970947
final protected static char[] hexArray = "0123456789ABCDEF"
    .toCharArray();

public static String bytesToHex(byte[] bytes) {
  char[] hexChars = new char[bytes.length * 2];
  int v;
  for (int j = 0; j < bytes.length; j++) {
    v = bytes[j] & 0xFF;
    hexChars[j * 2] = hexArray[v >>> 4];
    hexChars[j * 2 + 1] = hexArray[v & 0x0F];
  }
  return new String(hexChars);
}

// Change this to something else.
private static String SALT = "123456";

// A password hashing method.
public static String hashPassword(String in) {
  try {
    MessageDigest md = MessageDigest
        .getInstance("SHA-256");
    md.update(SALT.getBytes());        // <-- Prepend SALT.
    md.update(in.getBytes());
    // md.update(SALT.getBytes());     // <-- Or, append SALT.

    byte[] out = md.digest();
    return bytesToHex(out);            // <-- Return the Hex Hash.
  } catch (NoSuchAlgorithmException e) {
    e.printStackTrace();
  }
  return "";
}

public static void main(String[] args) {
  System.out.println(hashPassword("Hello"));
  System.out.println(hashPassword("Hello"));
  System.out.println(hashPassword("Hello1"));
  System.out.println(hashPassword("Hello2"));
}

哪个应该输出

60C1E22D18D022F01EEF0CAF999E52FD44C0C8EFD2161E9F4D24120AB0AFC84D
60C1E22D18D022F01EEF0CAF999E52FD44C0C8EFD2161E9F4D24120AB0AFC84D
CAAC2288692DD57BADFAE0225A42E59E1979E0116D009EEF01912E8C75529515
E0A3963BFAF209A17422918CB1FC950A62858993CA9A7BA6F760B8D4688306FD

演示一个字符产生的哈希值有多大差异。