Java实现MD5加密以及解密类，附带测试类，具体见代码。

MD5加密解密类——MyMD5Util,代码如下

package com.zyg.security.md5;import java.io.UnsupportedEncodingException;import java.security.MessageDigest;import java.security.NoSuchAlgorithmException;import java.security.SecureRandom;import java.util.Arrays;public class MyMD5Util {  private static final String HEX_NUMS_STR="0123456789ABCDEF"; private static final Integer SALT_LENGTH = 12;  /**   * 将16进制字符串转换成字节数组   * @param hex   * @return   */ public static byte[] hexStringToByte(String hex) {  int len = (hex.length() / 2);  byte[] result = new byte[len];  char[] hexChars = hex.toCharArray();  for (int i = 0; i < len; i++) {   int pos = i * 2;   result[i] = (byte) (HEX_NUMS_STR.indexOf(hexChars[pos]) << 4        | HEX_NUMS_STR.indexOf(hexChars[pos + 1]));  }  return result; }  /**  * 将指定byte数组转换成16进制字符串  * @param b  * @return  */ public static String byteToHexString(byte[] b) {  StringBuffer hexString = new StringBuffer();  for (int i = 0; i < b.length; i++) {   String hex = Integer.toHexString(b[i] & 0xFF);   if (hex.length() == 1) {    hex = '0' + hex;   }   hexString.append(hex.toUpperCase());  }  return hexString.toString(); }  /**  * 验证口令是否合法  * @param password  * @param passwordInDb  * @return  * @throws NoSuchAlgorithmException  * @throws UnsupportedEncodingException  */ public static boolean validPassword(String password, String passwordInDb)   throws NoSuchAlgorithmException, UnsupportedEncodingException {  //将16进制字符串格式口令转换成字节数组  byte[] pwdInDb = hexStringToByte(passwordInDb);  //声明盐变量  byte[] salt = new byte[SALT_LENGTH];  //将盐从数据库中保存的口令字节数组中提取出来  System.arraycopy(pwdInDb, 0, salt, 0, SALT_LENGTH);  //创建消息摘要对象  MessageDigest md = MessageDigest.getInstance("MD5");  //将盐数据传入消息摘要对象  md.update(salt);  //将口令的数据传给消息摘要对象  md.update(password.getBytes("UTF-8"));  //生成输入口令的消息摘要  byte[] digest = md.digest();  //声明一个保存数据库中口令消息摘要的变量  byte[] digestInDb = new byte[pwdInDb.length - SALT_LENGTH];  //取得数据库中口令的消息摘要  System.arraycopy(pwdInDb, SALT_LENGTH, digestInDb, 0, digestInDb.length);  //比较根据输入口令生成的消息摘要和数据库中消息摘要是否相同  if (Arrays.equals(digest, digestInDb)) {   //口令正确返回口令匹配消息   return true;  } else {   //口令不正确返回口令不匹配消息   return false;  } } /**  * 获得加密后的16进制形式口令  * @param password  * @return  * @throws NoSuchAlgorithmException  * @throws UnsupportedEncodingException  */ public static String getEncryptedPwd(String password)   throws NoSuchAlgorithmException, UnsupportedEncodingException {  //声明加密后的口令数组变量  byte[] pwd = null;  //随机数生成器  SecureRandom random = new SecureRandom();  //声明盐数组变量  byte[] salt = new byte[SALT_LENGTH];  //将随机数放入盐变量中  random.nextBytes(salt);  //声明消息摘要对象  MessageDigest md = null;  //创建消息摘要  md = MessageDigest.getInstance("MD5");  //将盐数据传入消息摘要对象  md.update(salt);  //将口令的数据传给消息摘要对象  md.update(password.getBytes("UTF-8"));  //获得消息摘要的字节数组  byte[] digest = md.digest();  //因为要在口令的字节数组中存放盐，所以加上盐的字节长度  pwd = new byte[digest.length + SALT_LENGTH];  //将盐的字节拷贝到生成的加密口令字节数组的前12个字节，以便在验证口令时取出盐  System.arraycopy(salt, 0, pwd, 0, SALT_LENGTH);  //将消息摘要拷贝到加密口令字节数组从第13个字节开始的字节  System.arraycopy(digest, 0, pwd, SALT_LENGTH, digest.length);  //将字节数组格式加密后的口令转化为16进制字符串格式的口令  return byteToHexString(pwd); }}测试类——Client,代码如下：package com.zyg.security.md5;import java.io.UnsupportedEncodingException;import java.security.NoSuchAlgorithmException;import java.util.HashMap;import java.util.Map;public class Client { private static Map users = new HashMap();  public static void main(String[] args){  String userName = "zyg";  String password = "123";  registerUser(userName,password);    userName = "changong";  password = "456";  registerUser(userName,password);    String loginUserId = "zyg";  String pwd = "1232";  try {   if(loginValid(loginUserId,pwd)){    System.out.println("欢迎登陆！！！");   }else{    System.out.println("口令错误，请重新输入！！！");   }  } catch (NoSuchAlgorithmException e) {   // TODO Auto-generated catch block   e.printStackTrace();  } catch (UnsupportedEncodingException e) {   // TODO Auto-generated catch block   e.printStackTrace();  }  }  /**  * 注册用户  *   * @param userName  * @param password  */ public static void registerUser(String userName,String password){  String encryptedPwd = null;  try {   encryptedPwd = MyMD5Util.getEncryptedPwd(password);      users.put(userName, encryptedPwd);     } catch (NoSuchAlgorithmException e) {   // TODO Auto-generated catch block   e.printStackTrace();  } catch (UnsupportedEncodingException e) {   // TODO Auto-generated catch block   e.printStackTrace();  } }  /**  * 验证登陆  *   * @param userName  * @param password  * @return  * @throws UnsupportedEncodingException   * @throws NoSuchAlgorithmException   */ public static boolean loginValid(String userName,String password)     throws NoSuchAlgorithmException, UnsupportedEncodingException{  String pwdInDb = (String)users.get(userName);  if(null!=pwdInDb){ // 该用户存在    return MyMD5Util.validPassword(password, pwdInDb);  }else{   System.out.println("不存在该用户！！！");   return false;  } }}