FileChannel字节缓冲区和散列文件(FileChannel ByteBuffer and H

我建了一个文件散列中的Java方法，它接受的输入字符串表示filepath+filename ，然后计算该文件的哈希值。哈希可以是任何原生的支持的Java散列算法中的诸如MD2通过SHA-512 。

我想伊克出的性能的每一个最后一滴，因为这种方法是一个项目，我工作的一个组成部分。有人建议我用尝试FileChannel ，而不是一个普通的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);
        }
    }

都返回正确的哈希值，但重构的方法似乎只对小文件进行合作。当我在一个大的文件传递，它完全扼流圈了，我想不通为什么。我是新来的NIO所以请各位指教。

编辑：忘了提我要把SHA-512的通过它进行测试。

UPDATE:我现在目前的方法更新。

    /**
     * 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);
        }
    }

所以，我试图基准使用我原来的例子，我最近更新的例子哈希了2.92GB文件的MD5。当然，因为有操作系统和磁盘缓存等“神奇”的事情，这将歪斜重复相同文件的读取......但这里的一些基准的射门任何基准是相对的。我装每种方法并编制新鲜后被解雇其关闭的5倍。基准从最后（第5）运行作为这将是“最热”运行该算法，任何“神奇”（在我的理论反正）。

Here's the benchmarks so far: 

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

这是一个25.03% decrease在时间散列相同2.92GB文件拍摄。非常好。

Answer 1:

3点建议：

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）不要关闭FC和FIS，它是多余的，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）如果你想与FileChannel的使用性能改进

ByteBuffer.allocateDirect(1024);

Answer 2:

另一个可能的改善可能来，如果代码只分配临时缓冲区一次。

如

        int bufsize = 8192;
        ByteBuffer buffer = ByteBuffer.allocateDirect(bufsize); 
        byte[] temp = new byte[bufsize];
        int b = channel.read(buffer);

        while (b > 0) {
            buffer.flip();

            buffer.get(temp, 0, b);
            md.update(temp, 0, b);
            buffer.clear();

            b = channel.read(buffer);
        }

附录

注意：在字符串建设代码中的错误。它打印零作为一个单一的数字。这很容易固定。例如

hexString.append(mdbytes[i] == 0 ? "00" : Integer.toHexString((0xFF & mdbytes[i])));

此外，作为一个实验，我重写了代码，使用映射的字节的缓冲区。它运行快30％（6-7米利斯VS 9-11米利斯FWIW）。我希望你能获得更多的它，如果你写了直接操作的字节的缓冲区码散列码。

我试图通过启动定时器之前用散列算法每一个不同的文件占JVM初始化和文件系统缓存。通过代码在第一次运行是比正常运行速度较慢的约25倍。这似乎是由于JVM初始化，因为在定时环路所有运行大致相同的长度。它们不会出现从高速缓存受益。我用MD5算法进行测试。此外，时序部分期间，只有一个算法运行为测试程序的持续时间。

在循环中的代码越短，所以可能更容易理解。我不是100％确定什么样的压力内存映射许多文件下的高容量将发挥在JVM上，所以这可能是东西，如果你想运行就需要研究和考虑，如果你想考虑这种解决方案这个负载下。

public static byte[] hash(File file, String hashAlgo) throws IOException {

    FileInputStream inputStream = null;

    try {
        MessageDigest md = MessageDigest.getInstance(hashAlgo);
        inputStream = new FileInputStream(file);
        FileChannel channel = inputStream.getChannel();

        long length = file.length();
        if(length > Integer.MAX_VALUE) {
            // you could make this work with some care,
            // but this code does not bother.
            throw new IOException("File "+file.getAbsolutePath()+" is too large.");
        }

        ByteBuffer buffer = channel.map(MapMode.READ_ONLY, 0, length);

        int bufsize = 1024 * 8;          
        byte[] temp = new byte[bufsize];
        int bytesRead = 0;

        while (bytesRead < length) {
            int numBytes = (int)length - bytesRead >= bufsize ? 
                                         bufsize : 
                                         (int)length - bytesRead;
            buffer.get(temp, 0, numBytes);
            md.update(temp, 0, numBytes);
            bytesRead += numBytes;
        }

        byte[] mdbytes = md.digest();
        return mdbytes;

    } catch (NoSuchAlgorithmException e) {
        throw new IllegalArgumentException("Unsupported Hash Algorithm.", e);
    }
    finally {
        if(inputStream != null) {
            inputStream.close();
        }
    }
}

Answer 3:

下面是使用NIO文件散列的例子

路径
FileChanngel
MappedByteBuffer

并避免使用字节[]的。所以这个我认为应该上述的改进版本。并且其中所述散列值被存储在用户的第二NIO示例属性。这可用于HTML ETAG代的其他样品存在的文件不会更改。

    public static final byte[] getFileHash(final File src, final String hashAlgo) throws IOException, NoSuchAlgorithmException {
    final int         BUFFER = 32 * 1024;
    final Path        file = src.toPath();
    try(final FileChannel fc   = FileChannel.open(file)) {
        final long        size = fc.size();
        final MessageDigest hash = MessageDigest.getInstance(hashAlgo);
        long position = 0;
        while(position < size) {
            final MappedByteBuffer data = fc.map(FileChannel.MapMode.READ_ONLY, 0, Math.min(size, BUFFER));
            if(!data.isLoaded()) data.load();
            System.out.println("POS:"+position);
            hash.update(data);
            position += data.limit();
            if(position >= size) break;
        }
        return hash.digest();
    }
}

public static final byte[] getCachedFileHash(final File src, final String hashAlgo) throws NoSuchAlgorithmException, FileNotFoundException, IOException{
    final Path path = src.toPath();
    if(!Files.isReadable(path)) return null;
    final UserDefinedFileAttributeView view = Files.getFileAttributeView(path, UserDefinedFileAttributeView.class);
    final String name = "user.hash."+hashAlgo;
    final ByteBuffer bb = ByteBuffer.allocate(64);
    try { view.read(name, bb); return ((ByteBuffer)bb.flip()).array();
    } catch(final NoSuchFileException t) { // Not yet calculated
    } catch(final Throwable t) { t.printStackTrace(); }
    System.out.println("Hash not found calculation");
    final byte[] hash = getFileHash(src, hashAlgo);
    view.write(name, ByteBuffer.wrap(hash));
    return hash;
}

文章来源: FileChannel ByteBuffer and Hashing Files