取文件MD5的应用

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 {
/**
* 默认的密码字符串组合，apache校验下载的文件的正确性用的就是默认的这个组合
*/
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();
   }
} 

/**
* 适用于上G大的文件
* @param file
* @return
* @throws IOException
* 此方法慎用，FileChannel.map后不会释放资源，会导致文件被锁定
*/
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 md5Str = bufferToHex(messagedigest.digest());
   byteBuffer.flip();
   ch.close();
   in.close();
   return md5Str; 
} 

public static String MD5File(File file) {
    byte[] buf = new byte[4096]; //这个byte[]的长度可以是任意的。
    MessageDigest md;
    boolean fileIsNull = true;

    try {
        FileInputStream fis = new FileInputStream(file);
        int len = 0;
        md = MessageDigest.getInstance("MD5");

        len = fis.read(buf);
        if (len > 0) {
            fileIsNull = false;
            while (len > 0){
                md.update(buf, 0, len);
                len = fis.read(buf);
            }
        }
        fis.close();
    } catch (Exception e) {
        return null;
    }
    
    if (fileIsNull)
        return null;
    else
        return bufferToHex(md.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);
} 

}