import java.security.MessageDigest;
public class MD5 {
	//公盐
	private static final String PUBLIC_SALT = "demo" ;
	//十六进制下数字到字符的映射数组 
	private final static String[] hexDigits = {"0", "1", "2", "3", "4", 
	  "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f"};
	/**
  * 用户密码加密，盐值为 ：私盐+公盐
  * @param password 密码
  * @param salt 私盐
  * @return MD5加密字符串
  */
	public static String encryptPassword(String password,String salt){
		return encodeByMD5(PUBLIC_SALT+password+salt);
	}
	/**
  * md5加密算法
  * @param originString
  * @return 
  */
	private static String encodeByMD5(String originString){
		if (originString != null){
			try{
				//创建具有指定算法名称的信息摘要 
				MessageDigest md = MessageDigest.getInstance("MD5");
				//使用指定的字节数组对摘要进行最后更新，然后完成摘要计算 
				byte[] results = md.digest(originString.getBytes());
				//将得到的字节数组变成字符串返回 
				String resultString = byteArrayToHexString(results);
				return resultString.toUpperCase();
			}
			catch(Exception ex){
				ex.printStackTrace();
			}
		}
		return null;
	}
	/** 
  * 转换字节数组为十六进制字符串 
  * @param  字节数组 
  * @return 十六进制字符串 
  */
	private static String byteArrayToHexString(byte[] b){
		StringBuffer resultSb = new StringBuffer();
		for (int i = 0; i < b.length; i++){
			resultSb.append(byteToHexString(b[i]));
		}
		return resultSb.toString();
	}
	/** 将一个字节转化成十六进制形式的字符串  */
	private static String byteToHexString(byte b){
		int n = b;
		if (n < 0) 
		   n = 256 + n;
		int d1 = n / 16;
		int d2 = n % 16;
		return hexDigits[d1] + hexDigits[d2];
	}
}