Java NIO和非NIO性能

Question

我花了相当多的时间尝试优化文件哈希算法以消除每一次可能的性能下降。

见我以前的SO线程：

Get File Hash Performance/Optimization

FileChannel ByteBuffer and Hashing Files

Determining Appropriate Buffer Size

有人recommened多次使用Java NIO获取本机的性能提高（通过保持缓冲区的系统而不是将它们带入JVM）。但是，我的NIO代码运行速度相当慢，基准（反复散列相同的文件与每个算法，否定任何操作系统/驱动器“魔术”，可能会造成扭曲结果。

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尝试使用本地系统调用等来保持系统中的处理和存储（缓冲区）并且不在JVM之中 - 这可以防止（理论上）程序不得不在JVM和系统之间来回拖动事物。从理论上讲，这应该会更快......但是也许我的MessageDigest强制JVM带入缓冲区，否定本地缓冲区/系统调用带来的任何性能改进？我在这个逻辑中纠正了，还是我失去了方向？

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

Answer 1

两两件事很可能会令您的NIO的做法更好：

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

第一个应该避免在文件缓存和应用程序堆之间复制数据，而第二个应该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，您可能会复制一些天真的非NIO方法。