MessageDigest是Java自带的类

比如我们需要实现md5,或者sha加密

public class MD5AndSHA {  public final static String MD5_SHA(String s, String method) {  char hexDigits[] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',  'a', 'b', 'c', 'd', 'e', 'f' };  try {  byte[] strTemp = s.getBytes();  //如果输入“SHA”，就是实现SHA加密。  MessageDigest mdTemp = MessageDigest.getInstance(method);   mdTemp.update(strTemp);  byte[] md = mdTemp.digest();  int j = md.length;  char str[] = new char[j * 2];  int k = 0;  for (int i = 0; i < j; i++) {  byte byte0 = md[i];  str[k++] = hexDigits[byte0 >>> 4 & 0xf];  str[k++] = hexDigits[byte0 & 0xf];  }  return new String(str);  } catch (Exception e) {  return null;  }  }  public static void main(String[] args) {  //System.out.print(MD5AndSHA.MD5_SHA("123456"), "MD5");  //System.out.print(MD5AndSHA.MD5_SHA("123456"), "SHA");  }  
}