很简单的方法

    public static String md5(String string) {

        byte[] hash;

        try {

            hash = MessageDigest.getInstance("MD5").digest(string.getBytes("UTF-8"));

        } catch (NoSuchAlgorithmException e) {

            throw new RuntimeException("Huh, MD5 should be supported?", e);

        } catch (UnsupportedEncodingException e) {

            throw new RuntimeException("Huh, UTF-8 should be supported?", e);

        }

        StringBuilder hex = new StringBuilder(hash.length * );

        for (byte b : hash) {

            if ((b & 0xFF) < 0x10) hex.append("");

            hex.append(Integer.toHexString(b & 0xFF));

        }

        return hex.toString();

    }