前言

上周负责的模块中需要逐行读取文件内容, 写完之后对程序执行效率不太满意, 索性上网查了一下 Java 逐行读取文件内容的各种方法, 并且简单地比对了一下执行效率. 在此记录, 希望能够帮到有需要的人.

注意: 本文比对的项目为 逐行读取文本内容, 并不能代表其他方式的文件读取效率优劣!!!

文末有完整代码.

先放结果

1000000 行文本读取结果比对:

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BufferedReader 耗时: 49ms
Scanner 耗时: 653ms
Apache Commons IO 耗时: 44ms
InputStreamReader 耗时: 191ms
FileInputStream 耗时: 3171ms
BufferedInputStream 耗时: 70ms
FileUtils 耗时: 46ms
Files 耗时: 99ms

24488656 行文本读取结果比对:

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BufferedReader 耗时: 989ms
Scanner 耗时: 11899ms
Apache Commons IO 耗时: 568ms
InputStreamReader 耗时: 3377ms
FileInputStream 耗时: 78903ms
BufferedInputStream 耗时: 1480ms
FileUtils 耗时: 16569ms
Files 耗时: 25162ms

可见, 当文件较小时:

  • ApacheCommonsIO 流 表现最佳;
  • FileUtils, BufferedReader 居其二;
  • BufferedInputStream, Files 随其后;
  • InputStreamReader, Scanner, FileInputStream 略慢.

当文件较大时, Apache Commons IO 流, BufferedReader 依然出色, Files, FileUtils 速度开始变慢.

简要分析

使用到的工具类包括:

  • java.io.BufferedReader
  • java.util.Scanner
  • org.apache.commons.io.FileUtils
  • java.io.InputStreamReader
  • java.io.FileInputStream
  • java.io.BufferedInputStream
  • com.google.common.io.Files

其中:

Apache Commons IO 流 和 BufferedReader 使用到了缓冲区, 所以在不消耗大量内存的情况下提高了处理速度;

FileUtils 和 Files 是先把文件内容全部读入内存, 然后在进行操作, 是典型的空间换时间案例. 这种方法可能会大量消耗内存, 建议酌情使用;

其他几个工具类本来就不擅长逐行读取, 效率底下也是情理之中.

建议

在逐行读取文本内容的需求下, 建议使用 Apache Commons IO 流, 或者 BufferedReader, 既不会过多地占用内存, 也保证了优异的处理速度.

参考文献:

  • [Java]读取文件方法大全 — lovebread
  • java读取文件API速度对比 — fengxingzhe001
  • Java高效读取大文件 — Eugen Paraschiv[文] / ImportNew - 进林[译]

附录-源代码:

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import com.google.common.io.Files;
import org.apache.commons.io.Charsets;
import org.apache.commons.io.FileUtils;
import org.apache.commons.io.LineIterator;

import java.io.*;
import java.util.List;
import java.util.Random;
import java.util.Scanner;






public class ReadByLineFromFileTest {

  public static void main(String[] args) {
    ReadByLineFromFileTest test = new ReadByLineFromFileTest();
    String filePath = "./testFile.txt";
    File file = new File(filePath);
    if (!file.exists()) {
      
      test.writeRandom(filePath, 1000000);
    }

    long before, after, time;
    
    before = System.currentTimeMillis();
    test.bufferedReader(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("BufferedReader 耗时: " + time + "ms");

    
    before = System.currentTimeMillis();
    test.scanner(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("Scanner 耗时: " + time + "ms");

    
    before = System.currentTimeMillis();
    test.apacheCommonsIo(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("Apache Commons IO 耗时: " + time + "ms");

    
    before = System.currentTimeMillis();
    test.inputStreamReader(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("InputStreamReader 耗时: " + time + "ms");

    
    before = System.currentTimeMillis();
    test.fileInputStream(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("FileInputStream 耗时: " + time + "ms");


    
    before = System.currentTimeMillis();
    test.bufferedInputStream(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("BufferedInputStream 耗时: " + time + "ms");

    
    before = System.currentTimeMillis();
    test.fileUtils(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("FileUtils 耗时: " + time + "ms");

    
    before = System.currentTimeMillis();
    test.files(filePath);
    after = System.currentTimeMillis();
    time = after - before;
    System.out.println("Files 耗时: " + time + "ms");
  }

  












  public void apacheCommonsIo(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    try {
      LineIterator iterator = FileUtils.lineIterator(file, "UTf-8");
      while (iterator.hasNext()) {
        String line = iterator.nextLine();
        
        
      }
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  





  public void scanner(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    FileInputStream fileInputStream = null;
    Scanner scanner = null;
    try {
      fileInputStream = new FileInputStream(file);
      scanner = new Scanner(fileInputStream, "UTF-8");

      while (scanner.hasNextLine()) {
        
        String line = scanner.nextLine();
        
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } finally {
      if (fileInputStream != null) {
        try {
          fileInputStream.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }

      if (scanner != null) {
        scanner.close();
      }
    }
  }

  












  public void files(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    try {
      List<String> fileLines = Files.readLines(file, Charsets.toCharset("UTF-8"));
      for (String str : fileLines) {
        
        
      }
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  












  public void fileUtils(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    try {
      List<String> fileLines = FileUtils.readLines(file, Charsets.UTF_8);
      for (String str : fileLines) {
        
        
      }
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  public void bufferedInputStream(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    FileInputStream fileInputStream = null;
    BufferedInputStream bufferedInputStream = null;
    try {
      fileInputStream = new FileInputStream(file);
      bufferedInputStream = new BufferedInputStream(fileInputStream);

      int temp;
      char character;
      String line = "";
      while ((temp = bufferedInputStream.read()) != -1) {
        character = (char) temp;
        if (character != '\n') {
          line += character;
        } else {
          
          
          line = "";
        }
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (fileInputStream != null) {
        try {
          fileInputStream.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }

      if (bufferedInputStream != null) {
        try {
          bufferedInputStream.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }

  





  public void fileInputStream(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    FileInputStream fileInputStream = null;
    try {
      fileInputStream = new FileInputStream(file);
      int temp;
      char character;
      String line = "";
      while ((temp = fileInputStream.read()) != -1) {
        character = (char) temp;
        if (character != '\n') {
          line += character;
        } else {
          
          
          line = "";
        }
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (fileInputStream != null) {
        try {
          fileInputStream.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }

  





  public void inputStreamReader(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    FileInputStream fileInputStream = null;
    InputStreamReader inputStreamReader = null;
    try {
      fileInputStream = new FileInputStream(file);
      inputStreamReader = new InputStreamReader(fileInputStream);
      int temp;
      char character;
      String line = "";
      while ((temp = inputStreamReader.read()) != -1) {
        character = (char) temp;
        if (character != '\n') {
          line += character;
        } else {
          
          
          line = "";
        }
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (fileInputStream != null) {
        try {
          fileInputStream.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }

      if (inputStreamReader != null) {
        try {
          inputStreamReader.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }

  





  public void bufferedReader(String filePath) {
    File file = new File(filePath);
    if (!file.exists()) {
      return;
    }

    FileReader fileReader = null;
    BufferedReader bufferedReader = null;
    try {
      fileReader = new FileReader(file);
      bufferedReader = new BufferedReader(fileReader);

      String line = "";
      while ((line = bufferedReader.readLine()) != null) {
        
        
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (fileReader != null) {
        try {
          fileReader.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }

      if (bufferedReader != null) {
        try {
          bufferedReader.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }

  





  public void writeRandom(String filePath, int totalLines) {
    RandomAccessFile file = null;
    Random random = new Random();
    try {
      file = new RandomAccessFile(filePath, "rw");
      long length = file.length();
      for (int i = 0; i < totalLines; i++) {
        file.seek(length);
        int number = random.nextInt(1000000);
        String line = number + "\n";
        file.writeBytes(line);
        length += line.length();
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (file != null) {
        try {
          file.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }
}