实现MD5加密的代码

package com.maxtech.util;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class MD5Util {
protected static char hexDigits[] = { '0', '1', '2', '3', '4', '5', '6',
        '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
protected static MessageDigest messageDigest = null;
static {
   try {
       messageDigest = MessageDigest.getInstance("MD5");
   } catch (NoSuchAlgorithmException e) {
       e.printStackTrace();
   }
}




public static String MD5Encode(String origin) {
String resultString = null;

try {
resultString = new String(origin);
resultString = bufferToHex(messageDigest.digest(resultString
.getBytes()));
} catch (Exception ex) {

}
return resultString;
}
    public static String getFileMD5String(String fileName) throws IOException {
        File f = new File(fileName);
        return getFileMD5String(f);
    }


    public static String getFileMD5String(File file) throws IOException {
        FileInputStream in = new FileInputStream(file);
        FileChannel ch = in.getChannel();
        MappedByteBuffer byteBuffer = ch.map(FileChannel.MapMode.READ_ONLY, 0,
                file.length());
        messageDigest.update(byteBuffer);
        String str = bufferToHex(messageDigest.digest());
        in.close();
        return str;
    }


    public static String getMD5String(byte[] bytes) { 
    messageDigest.update(bytes);   
        return bufferToHex(messageDigest.digest());   
    } 


    private static String bufferToHex(byte bytes[]) {
        return bufferToHex(bytes, 0, bytes.length);
    }

    private static String bufferToHex(byte bytes[], int m, int n) {
        StringBuffer stringbuffer = new StringBuffer(2 * n);
        int k = m + n;
        for (int l = m; l < k; l++) {
            appendHexPair(bytes[l], stringbuffer);
        }
        return stringbuffer.toString();
    }

    private static void appendHexPair(byte bt, StringBuffer stringbuffer) {
        char c0 = hexDigits[(bt & 0xf0) >> 4];
        char c1 = hexDigits[bt & 0xf];
        stringbuffer.append(c0);
        stringbuffer.append(c1);
    }


    public static void main(String[] args) throws IOException {
        String fileName = "/Users/wdong/Downloads/2013.pptx"; // 更改文件名不会导致文件md5值变化

        System.out.println(getFileMD5String(fileName));
    }

}