MessageDigest 類爲應用程序提供信息摘要算法的功能，如 MD5 或 SHA 算法。信息摘要是安全的單向哈希函數，它接收任意大小的數據，並輸出固定長度的哈希值。
md = MessageDigest.getInstance("SHA-1");
md.update(text.getBytes());//text要轉換字符串
md.digest()//返回hash串algorithm類型:
MD2: The MD2 message digest algorithm as defined in RFC 1319.
MD5: The MD5 message digest algorithm as defined in RFC 1321.
SHA-1: The Secure Hash Algorithm, as defined in Secure Hash Standard, NIST FIPS 180-1.
SHA-256, SHA-384, and SHA-512: SHA-256 is a 256-bit hash function intended to provide 128 bits of security against collision attacks, while SHA-512 is a 512-bit hash function intended to provide 256 bits of security. A 384-bit hash may be obtained by truncating the SHA-512 output.