private String getMD5Str(String str) {
    MessageDigest messageDigest = null;
    try {
      messageDigest = MessageDigest.getInstance("MD5");
      messageDigest.reset();
      messageDigest.update(str.getBytes("UTF-8"));
    } catch (NoSuchAlgorithmException e) {
      System.out.println("NoSuchAlgorithmException caught!");
      System.exit(-1);
    } catch (UnsupportedEncodingException e) {
      e.printStackTrace();
    }
    byte[] byteArray = messageDigest.digest();
    StringBuffer md5StrBuff = new StringBuffer();
    for (int i = 0; i < byteArray.length; i++) {
      if (Integer.toHexString(0xFF & byteArray[i]).length() == 1)
        md5StrBuff.append("0").append(Integer.toHexString(0xFF & byteArray[i]));
      else
        md5StrBuff.append(Integer.toHexString(0xFF & byteArray[i]));
    }
    return md5StrBuff.toString();
}

import java.security.MessageDigest;
public class Test_MD5{
  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();
      //使用MD5创建MessageDigest对象
      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 b = md[i];
        //System.out.println((int)b);
        //将没个数(int)b进行双字节加密
        str[k++] = hexDigits[b >> 4 & 0xf];
        str[k++] = hexDigits[b & 0xf];
      }
      return new String(str);
    } catch (Exception e) {
      return null;
    }
  }
  //测试
  public static void main(String[] args) {
    System.out.println("caidao的MD5加密后：/n"+Test_MD5.MD5("caidao"));
    System.out.println("http://www.baidu.com/的MD5加密后：/n"+Test_MD5.MD5("http://www.baidu.com/"));
  }
}