Java NIO与非NIO性能

Question

我花了相当多的时间尝试优化文件散列算法，以尽可能地检测出性能的每一个下降。

请看我之前的SO线程：

Get File Hash Performance/Optimization

FileChannel ByteBuffer and Hashing Files

Determining Appropriate Buffer Size

有人多次建议使用Java NIO来获得本机性能提升(通过将缓冲区保留在系统中，而不是将它们引入到JVM中)。然而，我的NIO代码运行得相当慢，不是基准测试(用每种算法一遍又一遍地散列相同的文件，以否定任何可能扭曲结果的OS/Drive“魔术”。

我现在有两个方法可以做同样的事情：

This one runs faster almost every time:

/**
 * 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
 * @param BUFFER Buffer size in bytes. Recommended to stay in<br/>
 *          multiples of 2 such as 1024, 2048, <br/>
 *          4096, 8192, 16384, 32768, 65536, etc.
 * @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, int BUFFER) throws IOException, HasherException {
    StringBuffer hexString = null;
    try {
        MessageDigest md = MessageDigest.getInstance(validateHashType(hashAlgo));
        FileInputStream fis = new FileInputStream(file);

        byte[] dataBytes = new byte[BUFFER];

        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 | HasherException e) {
        throw new HasherException("Unsuppored Hash Algorithm.", e);
    }
}

My Java NIO method that runs considerably slower most of the time:

/**
 * Gets Hash of file using java.nio File Channels and ByteBuffer 
 * <br/>for native system calls where possible. This may improve <br/>
 * performance in some circumstances.
 * 
 * @param fileStr 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
 * @param BUFFER Buffer size in bytes. Recommended to stay in<br/>
 *          multiples of 2 such as 1024, 2048, <br/>
 *          4096, 8192, 16384, 32768, 65536, etc.
 * @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 getHashNIO(String fileStr, String hashAlgo, int BUFFER) 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(BUFFER); // 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);
    }
}

我的想法是，Java NIO尝试使用本机系统调用等来保持处理和存储(缓冲区)在系统中和在JVM之外-这(从理论上)防止了程序必须在JVM和系统之间来回移动。理论上这应该更快..。但是，也许我的MessageDigest强制JVM引入缓冲区，从而否定了本机缓冲区/系统调用所能带来的任何性能改进？我在这个逻辑上是正确的，还是我错了？

Please help me understand why Java NIO is not better in this scenario.

Answer 1

两件事可能会让你的NIO方法变得更好：

1. 尝试使用memory-mapped file而不是将数据读入堆内存。
2. 将数据传递到摘要using a ByteBuffer而不是byte[]数组。

前者应避免在文件缓存和应用程序堆之间复制数据，而后者应避免在缓冲区和字节数组之间复制数据。如果没有这些优化，您可能会得到比单纯的非NIO方法更多的复制。