获取文件的md5(java & php)

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.