Java 复制大文件方式FileChannel 用法

目前为止，我们已经学习了很多 Java 拷贝文件的方式，除了 FileChannel 提供的方法外，还包括使用 Files.copy() 或使用字节数组的缓冲/非缓冲流。那个才是最好的选择呢？这个问题很难回答，因为答案基于很多因素。本文将目光集中到一个因素，那就是速度，因为拷贝任务 越快将会提高效率，在有些情况下，这是成功的关键。因此，本文将使用一个应用程序来比较下面这些拷贝方式的具体时间：

• FileChannel 和非直接模式的 ByteBuffer
• FileChannel 和直接模式的 ByteBuffer
• FileChannel.transferTo()
• FileChannel.transferFrom()
• FileChannel.map()
• 使用字节数组和缓冲流
• 使用字节数组和非缓冲流
• File.copy()（Path 到 Path，InputStream 到 Path 和 Path 到 OutputStream）

应用程序基于下面的条件：

• 拷贝文件类型 MP4 视频（文件名为 Rafa Best Shots.mp4，所在目录为 C:\rafaelnadal\tournaments\2009\videos）
• 文件大小：58.3MB
• 测试的缓冲区大小：4KB, 16KB, 32KB, 64KB, 128KB, 256KB, and 1024KB
• 机器配置：Mobile AMD Sempron Processor 3400 + 1.80 GHz, 1.00GB RAM, 32-bit
  OS, Windows 7 Ultimate
• 测量类型：使用 System.nanoTime() 方法
• 连续运行三次后再获取时间；前三次运行将会被忽略。开始运行的时间总会比后面运行的时间要长一些。

下面将列出完整的应用程序：

import  java.nio.MappedByteBuffer;
import  java.io.OutputStream;
import  java.io.InputStream;
import  java.io.BufferedInputStream;
import  java.io.BufferedOutputStream;
import  java.io.File;
import  java.io.FileInputStream;
import  java.io.FileOutputStream;
import  java.io.IOException;
import  java.nio.ByteBuffer;
import  java.nio.channels.FileChannel;
import  java.nio.file.Files;
import  java.nio.file.Path;
import  java.nio.file.Paths;
import  java.nio.file.StandardOpenOption;
import  java.util.EnumSet;
import  static  java.nio.file.LinkOption.NOFOLLOW_LINKS;

public  class  Main {

 public  static  void  deleteCopied(Path path){

  try  {
      Files.deleteIfExists(path);
  }  catch  (IOException ex) {
    System.err.println(ex);
  }

 }

 public  static  void  main(String[] args) {

 final  Path copy_from = Paths.get("C:/rafaelnadal/tournaments/ 2009 /videos/
                                                                        Rafa Best Shots.mp4");
 final  Path copy_to = Paths.get( "C:/Rafa Best Shots.mp4" );
 long  startTime, elapsedTime;
 int  bufferSizeKB =  4 ;  //also tested for 16, 32, 64, 128, 256 and 1024
 int  bufferSize = bufferSizeKB *  1024 ;

 deleteCopied(copy_to);

 //FileChannel and non-direct buffer
 System.out.println( "Using FileChannel and non-direct buffer ..." );
 try  (FileChannel fileChannel_from = (FileChannel.open(copy_from,
                      EnumSet.of(StandardOpenOption.READ)));
      FileChannel fileChannel_to = (FileChannel.open(copy_to,
                      EnumSet.of(StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE)))) {

      startTime = System.nanoTime();

      // Allocate a non-direct ByteBuffer
      ByteBuffer bytebuffer = ByteBuffer.allocate(bufferSize);

      // Read data from file into ByteBuffer
      int  bytesCount;
      while  ((bytesCount = fileChannel_from.read(bytebuffer)) >  0 ) {
       //flip the buffer which set the limit to current position, and position to 0
       bytebuffer.flip();
       //write data from ByteBuffer to file
       fileChannel_to.write(bytebuffer);
       //for the next read
       bytebuffer.clear();
      }

      elapsedTime = System.nanoTime() - startTime;
      System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
 }  catch  (IOException ex) {
   System.err.println(ex);
 }

 deleteCopied(copy_to);

 //FileChannel and direct buffer
 System.out.println( "Using FileChannel and direct buffer ..." );
 try  (FileChannel fileChannel_from = (FileChannel.open(copy_from,
                      EnumSet.of(StandardOpenOption.READ)));
      FileChannel fileChannel_to = (FileChannel.open(copy_to,
                      EnumSet.of(StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE)))) {

      startTime = System.nanoTime();

      // Allocate a direct ByteBuffer
      ByteBuffer bytebuffer = ByteBuffer.allocateDirect(bufferSize);

      // Read data from file into ByteBuffer
      int  bytesCount;
      while  ((bytesCount = fileChannel_from.read(bytebuffer)) >  0 ) {
       //flip the buffer which set the limit to current position, and position to 0
       bytebuffer.flip();
       //write data from ByteBuffer to file
       fileChannel_to.write(bytebuffer);
       //for the next read
       bytebuffer.clear();
      }

      elapsedTime = System.nanoTime() - startTime;
      System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
 }  catch  (IOException ex) {
   System.err.println(ex);
 }

 deleteCopied(copy_to);

 //FileChannel.transferTo()
 System.out.println( "Using FileChannel.transferTo method ..." );
 try  (FileChannel fileChannel_from = (FileChannel.open(copy_from,
                      EnumSet.of(StandardOpenOption.READ)));
      FileChannel fileChannel_to = (FileChannel.open(copy_to,
                      EnumSet.of(StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE)))) {

      startTime = System.nanoTime();

      fileChannel_from.transferTo(0L, fileChannel_from.size(), fileChannel_to);

      elapsedTime = System.nanoTime() - startTime;
      System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
 }  catch  (IOException ex) {
   System.err.println(ex);
 }

 deleteCopied(copy_to);

 //FileChannel.transferFrom()
 System.out.println( "Using FileChannel.transferFrom method ..." );
 try  (FileChannel fileChannel_from = (FileChannel.open(copy_from,
                      EnumSet.of(StandardOpenOption.READ)));
      FileChannel fileChannel_to = (FileChannel.open(copy_to,
                      EnumSet.of(StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE)))) {

      startTime = System.nanoTime();

      fileChannel_to.transferFrom(fileChannel_from, 0L, ( int ) fileChannel_from.size());

      elapsedTime = System.nanoTime() - startTime;
      System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
 }  catch  (IOException ex) {
   System.err.println(ex);
 }

 deleteCopied(copy_to);

 //FileChannel.map
 System.out.println( "Using FileChannel.map method ..." );
 try  (FileChannel fileChannel_from = (FileChannel.open(copy_from,
                      EnumSet.of(StandardOpenOption.READ)));
      FileChannel fileChannel_to = (FileChannel.open(copy_to,
                      EnumSet.of(StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE)))) {

      startTime = System.nanoTime();
      MappedByteBuffer buffer = fileChannel_from.map(FileChannel.MapMode.READ_ONLY,
                                                                  0 , fileChannel_from.size());

      fileChannel_to.write(buffer);
      buffer.clear();

      elapsedTime = System.nanoTime() - startTime;
      System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
}  catch  (IOException ex) {
  System.err.println(ex);
}

deleteCopied(copy_to);

//Buffered Stream I/O
System.out.println( "Using buffered streams and byte array ..." );
File inFileStr = copy_from.toFile();
File outFileStr = copy_to.toFile();
try  (BufferedInputStream in =  new  BufferedInputStream( new  FileInputStream(inFileStr));
     BufferedOutputStream out =  new  BufferedOutputStream( new  FileOutputStream(outFileStr))) {

     startTime = System.nanoTime();

     byte [] byteArray =  new  byte [bufferSize];
     int  bytesCount;
     while  ((bytesCount = in.read(byteArray)) != - 1 ) {
             out.write(byteArray,  0 , bytesCount);
     }

     elapsedTime = System.nanoTime() - startTime;
     System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
}  catch  (IOException ex) {
  System.err.println(ex);
}

deleteCopied(copy_to);

System.out.println( "Using un-buffered streams and byte array ..." );
try  (FileInputStream in =  new  FileInputStream(inFileStr);
     FileOutputStream out =  new  FileOutputStream(outFileStr)) {

     startTime = System.nanoTime();

     byte [] byteArray =  new  byte [bufferSize];
     int  bytesCount;
     while  ((bytesCount = in.read(byteArray)) != - 1 ) {
             out.write(byteArray,  0 , bytesCount);
     }

     elapsedTime = System.nanoTime() - startTime;
     System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
}  catch  (IOException ex) {
  System.err.println(ex);
}

deleteCopied(copy_to);

System.out.println( "Using Files.copy (Path to Path) method ..." );
try  {
    startTime = System.nanoTime();

    Files.copy(copy_from, copy_to, NOFOLLOW_LINKS);

    elapsedTime = System.nanoTime() - startTime;
    System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
}  catch  (IOException e) {
  System.err.println(e);
}

deleteCopied(copy_to);

System.out.println( "Using Files.copy (InputStream to Path) ..." );
try  (InputStream is =  new  FileInputStream(copy_from.toFile())) {

    startTime = System.nanoTime();

    Files.copy(is, copy_to);

    elapsedTime = System.nanoTime() - startTime;
    System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
}  catch  (IOException e) {
  System.err.println(e);
}

deleteCopied(copy_to);

System.out.println( "Using Files.copy (Path to OutputStream) ..." );
try  (OutputStream os =  new  FileOutputStream(copy_to.toFile())) {

     startTime = System.nanoTime();

     Files.copy(copy_from, os);

     elapsedTime = System.nanoTime() - startTime;
     System.out.println( "Elapsed Time is "  + (elapsedTime /  1000000000.0 ) +  " seconds" );
 }  catch  (IOException e) {
   System.err.println(e);
 }
 }
}

 

输出结果排序比较复杂，其中包含了很多数据。下面我将主要的对比用图形的方式展示出来。图形中 Y 坐标表示消耗的时间（单位：秒），X 坐标表示缓冲的大小（或运行次数，跳过了前三次运行）。

FileChannel 和非直接模式 Buffer vs. FileChannel 和直接模式 Buffer

从下图看来，如果缓存小于 256KB，那么非直接模式的 Buffer 快一点，而缓存大于 256KB 后，直接模式的 Buffer 快一点：

FileChannel.transferTo() vs. FileChannel.transferFrom() vs. FileChannel.map()

从下图看来，FileChannel.transferTo() 和 FileChannel.transferFrom 运行七次的速度都差不多，而 FileChannel.map 的速度就要差很多：

三种 Files.copy() 方法

从下图看来，最快的是 Path 到 Path，其次是 Path 到 OutputStream，最慢的是 InputStream 到 Path：

FileChannel 和非直接模式 Buffer vs. FileChannel.transferTo() vs. Path 到 Path

最后，我们将前面最快的三种方式综合起来比较。从比较的结果来看，似乎 Path 到 Path 是最快的解决方案：