获取文件的md5(java & php)

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每个文件都可以用MD5验证程序算出一个固定的md5码来，它是一个文件的文件签名，可用于验证自己下载的软件是否为原版。百度网盘的“秒传”应该也用到了这个，即上传之前首先计算该文件的md5值，然后看服务器上有没有该文件，有的话从服务端直接拷贝，节省上传时间和降低网络负担。

java的MD5util类：

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;import org.slf4j.Logger;import org.slf4j.LoggerFactory;public class MD5FileUtil {private static final Logger logger = LoggerFactory.getLogger(MD5FileUtil.class);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) {logger.error("MD5FileUtil messagedigest初始化失败", e);}}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());}public static String getMD5String(String s) {return getMD5String(s.getBytes());}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 boolean checkPassword(String password, String md5PwdStr) {String s = getMD5String(password);return s.equals(md5PwdStr);}public static void main(String[] args) throws IOException {long begin = System.currentTimeMillis();File big = new File("D:\\temp\\jre-7u11-linux-i586.tar.gz");String md5 = getFileMD5String(big);long end = System.currentTimeMillis();System.out.println("md5:" + md5);System.out.println("time:" + ((end - begin) / 1000) + "s");}}

php

例子1：

<?php$filename = "test.txt";$md5file = md5_file($filename);echo $md5file;?>

会输出类似下面一串字符：
5d41402abc4b2a76b9719d911017c592

例子 2 存储 "test.txt" 文件的 MD5 散列：

<?php$md5file = md5_file("test.txt");file_put_contents("md5file.txt",$md5file);?>

//在本例中，我们将检测 "test.txt" 是否已被更改（即是否 MD5 散列已被更改）：

<?php$md5file = file_get_contents("md5file.txt");if (md5_file("test.txt") == $md5file)  {  echo "The file is ok.";  }else  {  echo "The file has been changed.";  }?>

输出：
The file is ok.

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