java计算文件和字符串的md5码

import java.io.File;
import java.io.FileInputStream;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class MD5Util
{
 private static char md5Chars[] =
 { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd',
   'e', 'f' };
 private static MessageDigest messagedigest;
 
 /*获取一个文件的md5码 */
 public static String getFileMD5String(File file) throws Exception
 {
  messagedigest = MessageDigest.getInstance("MD5");
  FileInputStream in = new FileInputStream(file);
  FileChannel ch = in.getChannel();
  MappedByteBuffer byteBuffer = ch.map(FileChannel.MapMode.READ_ONLY, 0,
    file.length());
  messagedigest.update(byteBuffer);
  return bufferToHex(messagedigest.digest());
 }
 /*获取一个字符串的md5码 */
 public static String getStringMD5String(String str) throws Exception
 {
  messagedigest = MessageDigest.getInstance("MD5");
  messagedigest.update(str.getBytes()); 
  return bufferToHex(messagedigest.digest());
 }
 /*验证一个字符串和一个MD5码是否相等 */
 public static boolean check(String str,String md5) throws Exception
 {
  if(getStringMD5String(str).equals(md5))
   return true;
  else
   return false;
 }
 /*验证一个文件和一个MD5码是否相等 */
 public static boolean check(File f,String md5) throws Exception
 {
  if(getFileMD5String(f).equals(md5))
   return true;
  else
   return false;
 }
 
 private static String bufferToHex(byte bytes[])
 {
  return bufferToHex(bytes, 0, bytes.length);
 }
 private static String bufferToHex(byte b