java MD5文件校验工具类

package util;

import java.io.*;
import java.nio.*;
import java.nio.channels.FileChannel;
import java.security.*;

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 nsaex) {
			System.err.println(MD5Util.class.getName() 
					+ "初始化失败，MessageDigest不支持MD5Util。");
			nsaex.printStackTrace();
			
		}
	}
	/** 
     * 生成字符串的md5校验值 
     *  
     * @param s 
     * @return 
     */  
	public static String getMD5String(String s) {
		return getMD5String(s.getBytes());
	}
	/** 
     * 判断字符串的md5校验码是否与一个已知的md5码相匹配 
     *  
     * @param password 要校验的字符串 
     * @param md5PwdStr 已知的md5校验码 
     * @return 
     */  
	public static boolean checkPassword(String password, String md5PwdStr) {
		String s = getMD5String(password);
		return s.equals(md5PwdStr);
	}
	 /** 
     * 生成文件的md5校验值 
     *  
     * @param file 
     * @return 
     * @throws IOException 
     */  
	public static String getFileMD5String(File file) throws IOException {
		InputStream fis;
		fis = new FileInputStream(file);
		byte[] buffer = new byte[1024];
		int numRead = 0;
		while((numRead = fis.read(buffer)) > 0) {
			messageDigest.update(buffer, 0, numRead);
		}
		fis.close();
		return bufferToHex(messageDigest.digest());
	}
	/**
	 * * @param file 
     * @return 
     * @throws IOException 
     */  
	public static String getFileMD5String_old(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(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[1], stringBuffer);
		}
		return stringBuffer.toString();
	}
	
	private static void  appendHexPair(byte bt, StringBuffer stringBuffer) {
		char c0 = hexDigits[(bt & 0xf0) >> 4];
		// 取字节中高 4 位的数字转换, >>> 为逻辑右移，将符号位一起右移,此处未发现两种符号有何不同 
		char c1 = hexDigits[bt & 0xf];// 取字节中低 4 位的数字转换 
		stringBuffer.append(c0);
		stringBuffer.append(c1);
	}
	
	public static void main(String[] args) throws IOException {
		long begin = System.currentTimeMillis();
		File file = new File("D:/in.txt");
		String md5 = getFileMD5String(file);
		
		//String md5 = getMD5String("a");
		long end = System.currentTimeMillis();
		System.out.println("md5: " + md5 + ((end - begin) / 1000) + "s");
	}
}