java中对文件计算MD5

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;

/**
* 计算文件的MD5
*/
public class FileMD5 {
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) {
System.err.println(FileMD5.class.getName()+"初始化失败，MessageDigest不支持MD5Util.");
e.printStackTrace();
}
}

/**
* 计算文件的MD5
* @param fileName 文件的绝对路径
* @return
* @throws IOException
*/
public static String getFileMD5String(String fileName) throws IOException{
File f = new File(fileName);
return getFileMD5String(f);
}

/**
* 计算文件的MD5，重载方法
* @param file 文件对象
* @return
* @throws IOException
*/
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);
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 = "E:\\lyyy.flv";
long start = System.currentTimeMillis();
System.out.println(getFileMD5String(fileName));
long end = System.currentTimeMillis();
System.out.println("Consume " + (end - start) + "ms");
}

}

转自：http://zhouzaibao.iteye.com/blog/343870