使用Java查找文件中有多少非空行的最快方法是什么？

Question

使用Java查找文件中有多少非空行的最快方法是什么？

Answer 1

最简单的方法是使用BufferedReader,并检查哪些行为空.但是,这是一种相对较慢的方法,因为它需要为文件中的每一行创建一个String对象.更快的方法是使用read()将文件读入数组,然后遍历数组以计算换行符.

这是两个选项的代码; 第二个占用了我机器上大约50%的时间.

public static void timeBufferedReader () throws IOException
{
    long bef = System.currentTimeMillis ();

    // The reader buffer size is the same as the array size I use in the other function
    BufferedReader reader = new BufferedReader(new FileReader("test.txt"), 1024 * 10);
    int counter = 0;
    while (reader.ready())
    {
        if (reader.readLine().length() > 0)
            counter++;
    }

    long after = System.currentTimeMillis() - bef;

    System.out.println("Time: " + after + " Result: " + counter);

}

public static void timeFileReader () throws IOException
{
    long bef = System.currentTimeMillis();

    FileReader reader = new FileReader("test.txt");
    char[] buf = new char[1024 * 10];
    boolean emptyLine = true;
    int     counter = 0;
    while (reader.ready())
    {
        int len = reader.read(buf,0,buf.length);
        for (int i = 0; i < len; i++)
        {
            if (buf[i] == '\r' || buf[i] == '\n')
            {
                if (!emptyLine)
                {
                    counter += 1;
                    emptyLine = true;
                }
            }
            else emptyLine = false;
        }
    }

    long after = System.currentTimeMillis() - bef;

    System.out.println("Time: " + after + " Result: " + counter);

}

Answer 2

我在NIO推荐的Limbic系统上.我已经为Daphna的测试代码添加了一个NIO方法,并且基于他的两种方法对它进行了标记:

public static void timeNioReader () throws IOException {
    long bef = System.currentTimeMillis();

    File file = new File("/Users/stu/test.txt");
    FileChannel fc = (new FileInputStream(file)).getChannel(); 
    MappedByteBuffer buf = fc.map(MapMode.READ_ONLY, 0, file.length());
    boolean emptyLine = true;
    int     counter = 0;

    while (buf.hasRemaining())
    {
        byte element = buf.get();

        if (element == '\r' || element == '\n') {
            if (!emptyLine) {
                counter += 1;
                emptyLine = true;
            }
        } else 
            emptyLine = false;

    }

    long after = System.currentTimeMillis() - bef;

    System.out.println("timeNioReader      Time: " + after + " Result: " + counter);

}

以下是89MB文件的预热结果:

timeBufferedReader Time: 947 Result: 747656
timeFileReader     Time: 670 Result: 747656
timeNioReader      Time: 251 Result: 747656

NIO比FileReader快2.5倍,比BufferedReader快4倍!

使用6.4MB文件,结果甚至更好,尽管预热时间更长.

//jvm start, warming up
timeBufferedReader Time: 121 Result: 53404
timeFileReader     Time: 65 Result: 53404
timeNioReader      Time: 40 Result: 53404

//still warming up
timeBufferedReader Time: 107 Result: 53404
timeFileReader     Time: 60 Result: 53404
timeNioReader      Time: 20 Result: 53404

//ripping along
timeBufferedReader Time: 79 Result: 53404
timeFileReader     Time: 56 Result: 53404
timeNioReader      Time: 16 Result: 53404

按照你的意愿做到.