java - 并发读取文件(首选java)

Question

我有一个需要几个小时才能处理的大文件。所以我正在考虑尝试估计 block 并并行读取 block 。

是否可以同时读取单个文件？我查看了 RandomAccessFile 和 nio.FileChannel 但根据其他帖子不确定这种方法是否可行。

Answer 1

这里最重要的问题是您的情况的瓶颈是什么。

如果瓶颈是您的磁盘 IO，那么您在软件部分无能为力。并行计算只会让事情变得更糟，因为同时从不同部分读取文件会降低磁盘性能。

如果瓶颈是处理能力，并且您有多个 CPU 内核，那么您可以利用启动多个线程来处理文件的不同部分。您可以安全地创建多个 InputStream 或 Reader 来并行读取文件的不同部分(只要您不超过操作系统的数量限制)打开的文件)。您可以将工作分成任务并并行运行，如下例所示:

import java.io.*;
import java.util.*;
import java.util.concurrent.*;

public class Split {
    private File file;

    public Split(File file) {
        this.file = file;
    }

    // Processes the given portion of the file.
    // Called simultaneously from several threads.
    // Use your custom return type as needed, I used String just to give an example.
    public String processPart(long start, long end)
        throws Exception
    {
        InputStream is = new FileInputStream(file);
        is.skip(start);
        // do a computation using the input stream,
        // checking that we don't read more than (end-start) bytes
        System.out.println("Computing the part from " + start + " to " + end);
        Thread.sleep(1000);
        System.out.println("Finished the part from " + start + " to " + end);

        is.close();
        return "Some result";
    }

    // Creates a task that will process the given portion of the file,
    // when executed.
    public Callable<String> processPartTask(final long start, final long end) {
        return new Callable<String>() {
            public String call()
                throws Exception
            {
                return processPart(start, end);
            }
        };
    }

    // Splits the computation into chunks of the given size,
    // creates appropriate tasks and runs them using a 
    // given number of threads.
    public void processAll(int noOfThreads, int chunkSize)
        throws Exception
    {
        int count = (int)((file.length() + chunkSize - 1) / chunkSize);
        java.util.List<Callable<String>> tasks = new ArrayList<Callable<String>>(count);
        for(int i = 0; i < count; i++)
            tasks.add(processPartTask(i * chunkSize, Math.min(file.length(), (i+1) * chunkSize)));
        ExecutorService es = Executors.newFixedThreadPool(noOfThreads);

        java.util.List<Future<String>> results = es.invokeAll(tasks);
        es.shutdown();

        // use the results for something
        for(Future<String> result : results)
            System.out.println(result.get());
    }

    public static void main(String argv[])
        throws Exception
    {
        Split s = new Split(new File(argv[0]));
        s.processAll(8, 1000);
    }
}