FileChannel ByteBuffer和散列文件

Question

我在Java中构建了一个文件哈希方法，该方法采用a的输入字符串表示形式filepath+filename，然后计算该文件的哈希值。哈希可以是任何本机支持的Java哈希算法，例如MD2through
SHA-512。

我试图找出每一个性能下降的原因，因为这种方法是我正在研究的项目不可或缺的一部分。建议我尝试使用FileChannel而不是常规FileInputStream。

我原来的方法：

    /**
     * Gets Hash of file.
     * 
     * @param file String path + filename of file to get hash.
     * @param hashAlgo Hash algorithm to use. <br/>
     *     Supported algorithms are: <br/>
     *     MD2, MD5 <br/>
     *     SHA-1 <br/>
     *     SHA-256, SHA-384, SHA-512
     * @return String value of hash. (Variable length dependent on hash algorithm used)
     * @throws IOException If file is invalid.
     * @throws HashTypeException If no supported or valid hash algorithm was found.
     */
    public String getHash(String file, String hashAlgo) throws IOException, HashTypeException {
        StringBuffer hexString = null;
        try {
            MessageDigest md = MessageDigest.getInstance(validateHashType(hashAlgo));
            FileInputStream fis = new FileInputStream(file);

            byte[] dataBytes = new byte[1024];

            int nread = 0;
            while ((nread = fis.read(dataBytes)) != -1) {
                md.update(dataBytes, 0, nread);
            }
            fis.close();
            byte[] mdbytes = md.digest();

            hexString = new StringBuffer();
            for (int i = 0; i < mdbytes.length; i++) {
                hexString.append(Integer.toHexString((0xFF & mdbytes[i])));
            }

            return hexString.toString();

        } catch (NoSuchAlgorithmException | HashTypeException e) {
            throw new HashTypeException("Unsuppored Hash Algorithm.", e);
        }
    }

重构方法：

    /**
     * Gets Hash of file.
     * 
     * @param file String path + filename of file to get hash.
     * @param hashAlgo Hash algorithm to use. <br/>
     *     Supported algorithms are: <br/>
     *     MD2, MD5 <br/>
     *     SHA-1 <br/>
     *     SHA-256, SHA-384, SHA-512
     * @return String value of hash. (Variable length dependent on hash algorithm used)
     * @throws IOException If file is invalid.
     * @throws HashTypeException If no supported or valid hash algorithm was found.
     */
    public String getHash(String fileStr, String hashAlgo) throws IOException, HasherException {

        File file = new File(fileStr);

        MessageDigest md = null;
        FileInputStream fis = null;
        FileChannel fc = null;
        ByteBuffer bbf = null;
        StringBuilder hexString = null;

        try {
            md = MessageDigest.getInstance(hashAlgo);
            fis = new FileInputStream(file);
            fc = fis.getChannel();
            bbf = ByteBuffer.allocate(1024); // allocation in bytes

            int bytes;

            while ((bytes = fc.read(bbf)) != -1) {
                md.update(bbf.array(), 0, bytes);
            }

            fc.close();
            fis.close();

            byte[] mdbytes = md.digest();

            hexString = new StringBuilder();

            for (int i = 0; i < mdbytes.length; i++) {
                hexString.append(Integer.toHexString((0xFF & mdbytes[i])));
            }

            return hexString.toString();

        } catch (NoSuchAlgorithmException e) {
            throw new HasherException("Unsupported Hash Algorithm.", e);
        }
    }

两者都返回正确的哈希，但是重构的方法似乎只在小文件上起作用。当我传入一个大文件时，它完全阻塞了，我不知道为什么。我是新手，NIO所以请提出建议。

编辑：忘了提我要通过它抛出SHA-512进行测试。

UPDATE: 用我现在的方法更新。

    /**
     * Gets Hash of file.
     * 
     * @param file String path + filename of file to get hash.
     * @param hashAlgo Hash algorithm to use. <br/>
     *     Supported algorithms are: <br/>
     *     MD2, MD5 <br/>
     *     SHA-1 <br/>
     *     SHA-256, SHA-384, SHA-512
     * @return String value of hash. (Variable length dependent on hash algorithm used)
     * @throws IOException If file is invalid.
     * @throws HashTypeException If no supported or valid hash algorithm was found.
     */
    public String getHash(String fileStr, String hashAlgo) throws IOException, HasherException {

        File file = new File(fileStr);

        MessageDigest md = null;
        FileInputStream fis = null;
        FileChannel fc = null;
        ByteBuffer bbf = null;
        StringBuilder hexString = null;

        try {
            md = MessageDigest.getInstance(hashAlgo);
            fis = new FileInputStream(file);
            fc = fis.getChannel();
            bbf = ByteBuffer.allocateDirect(8192); // allocation in bytes - 1024, 2048, 4096, 8192

            int b;

            b = fc.read(bbf);

            while ((b != -1) && (b != 0)) {
                bbf.flip();

                byte[] bytes = new byte[b];
                bbf.get(bytes);

                md.update(bytes, 0, b);

                bbf.clear();
                b = fc.read(bbf);
            }

            fis.close();

            byte[] mdbytes = md.digest();

            hexString = new StringBuilder();

            for (int i = 0; i < mdbytes.length; i++) {
                hexString.append(Integer.toHexString((0xFF & mdbytes[i])));
            }

            return hexString.toString();

        } catch (NoSuchAlgorithmException e) {
            throw new HasherException("Unsupported Hash Algorithm.", e);
        }
    }

因此，我尝试使用我的原始示例和最新更新的示例对一个2.92GB文件的MD5进行散列测试。当然，任何基准测试都是相对的，因为存在操作系统和磁盘缓存以及其他“魔术”，它们会使相同文件的重复读取产生偏差……但这是一些基准测试的结果。我重新加载每种方法，并在重新编译后将其触发了5次。该基准是从上次（第5次）运行中获取的，因为这将是该算法的“最热”运行，以及任何“魔术”运行（无论如何，按照我的理论）。

Here's the benchmarks so far:

    Original Method - 14.987909 (s) 
    Latest Method - 11.236802 (s)

这是25.03% decrease散列相同的2.92GB文件所花费的时间。非常好。

Answer 1

3条建议：

1）每次读取后清除缓冲区

while (fc.read(bbf) != -1) {
    md.update(bbf.array(), 0, bytes);
    bbf.clear();
}

2）不要同时关闭fc和fis，这是多余的，关闭fis就足够了。FileInputStream.close API说：

If this stream has an associated channel then the channel is closed as well.

3）如果您想通过使用FileChannel来提高性能

ByteBuffer.allocateDirect(1024);