简单demo:

import  java.security.*;
import  java.security.spec.*;

class  MD5_test {
 public   final   static  String MD5(String s) {
  char  hexDigits[] = {  '0' ,  '1' ,  '2' ,  '3' ,  '4' ,  '5' ,  '6' ,  '7' ,  '8' ,  '9' ,
    'a' ,  'b' ,  'c' ,  'd' ,  'e' ,  'f'  };
  try  {
   byte [] strTemp = s.getBytes();
   MessageDigest mdTemp = MessageDigest.getInstance("MD5" );
   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) {
  // MD5_Test aa = new MD5_Test();
  System.out.print(MD5_test.MD5("b" ));
 }
}

import java.security.*;
import java.security.spec.*;

class MD5_test {
 public final static String MD5(String s) {
  char hexDigits[] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    'a', 'b', 'c', 'd', 'e', 'f' };
  try {
   byte[] strTemp = s.getBytes();
   MessageDigest mdTemp = MessageDigest.getInstance("MD5");
   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) {
  // MD5_Test aa = new MD5_Test();
  System.out.print(MD5_test.MD5("b"));
 }
}

 

import  java.security.MessageDigest;


public   class  MD5andKL {
 // MD5加码。32位
 public   static  String MD5(String inStr) {
  MessageDigest md5 = null ;
  try  {
   md5 = MessageDigest.getInstance("MD5" );
  } catch  (Exception e) {
   System.out.println(e.toString());
   e.printStackTrace();
   return   "" ;
  }
  char [] charArray = inStr.toCharArray();
  byte [] byteArray =  new   byte [charArray.length];

  for  ( int  i =  0 ; i < charArray.length; i++)
   byteArray[i] = (byte ) charArray[i];

  byte [] md5Bytes = md5.digest(byteArray);

  StringBuffer hexValue = new  StringBuffer();

  for  ( int  i =  0 ; i < md5Bytes.length; i++) {
   int  val = (( int ) md5Bytes[i]) &  0xff ;
   if  (val <  16 )
    hexValue.append("0" );
   hexValue.append(Integer.toHexString(val));
  }

  return  hexValue.toString();
 }

 // 可逆的加密算法
 public   static  String KL(String inStr) {
  // String s = new String(inStr);
  char [] a = inStr.toCharArray();
  for  ( int  i =  0 ; i < a.length; i++) {
   a[i] = (char ) (a[i] ^  't' );
  }
  String s = new  String(a);
  return  s;
 }

 // 加密后解密
 public   static  String JM(String inStr) {
  char [] a = inStr.toCharArray();
  for  ( int  i =  0 ; i < a.length; i++) {
   a[i] = (char ) (a[i] ^  't' );
  }
  String k = new  String(a);
  return  k;
 }

 // 测试主函数
 public   static   void  main(String args[]) {
  String s = new  String( "a" );
  System.out.println("原始："  + s);
  System.out.println("MD5后："  + MD5(s));
  System.out.println("MD5后再加密："  + KL(MD5(s)));
  System.out.println("解密为MD5后的："  + JM(KL(MD5(s))));
 }
}

import java.security.MessageDigest;


public class MD5andKL {
 // MD5加码。32位
 public static String MD5(String inStr) {
  MessageDigest md5 = null;
  try {
   md5 = MessageDigest.getInstance("MD5");
  } catch (Exception e) {
   System.out.println(e.toString());
   e.printStackTrace();
   return "";
  }
  char[] charArray = inStr.toCharArray();
  byte[] byteArray = new byte[charArray.length];

  for (int i = 0; i < charArray.length; i++)
   byteArray[i] = (byte) charArray[i];

  byte[] md5Bytes = md5.digest(byteArray);

  StringBuffer hexValue = new StringBuffer();

  for (int i = 0; i < md5Bytes.length; i++) {
   int val = ((int) md5Bytes[i]) & 0xff;
   if (val < 16)
    hexValue.append("0");
   hexValue.append(Integer.toHexString(val));
  }

  return hexValue.toString();
 }

 // 可逆的加密算法
 public static String KL(String inStr) {
  // String s = new String(inStr);
  char[] a = inStr.toCharArray();
  for (int i = 0; i < a.length; i++) {
   a[i] = (char) (a[i] ^ 't');
  }
  String s = new String(a);
  return s;
 }

 // 加密后解密
 public static String JM(String inStr) {
  char[] a = inStr.toCharArray();
  for (int i = 0; i < a.length; i++) {
   a[i] = (char) (a[i] ^ 't');
  }
  String k = new String(a);
  return k;
 }

 // 测试主函数
 public static void main(String args[]) {
  String s = new String("a");
  System.out.println("原始：" + s);
  System.out.println("MD5后：" + MD5(s));
  System.out.println("MD5后再加密：" + KL(MD5(s)));
  System.out.println("解密为MD5后的：" + JM(KL(MD5(s))));
 }
}