使用NIO（Java）在EOF之后重新读取文件

问题描述

我使用MemoryMapped缓冲区来读取文件。最初我得到的通道大小和使用相同的大小Im映射文件在内存上，这里的初始位置是0，因为我想从头开始映射文件现在另外400KB的数据被添加到该文件，现在我想映射单独的400kb，但是我的代码中的错误，我不能弄清楚，我得到这个

I'm using MemoryMapped buffer to read a file. Initially I'm getting the channel size and using the same size I"m mapping the file on memory and here the initial position is 0 as I want to map the file from the beginning. Now another 400KB of data is added to that file, now I want to map that 400kb alone. But something is wrong in my code, I'm not able to figure it out and I'm getting this

260java.io.IOException: Channel not open for writing - cannot extend file to required size
at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:812)
at trailreader.main(trailreader.java:55

这里是我的代码

    BufferedWriter bw;

    FileInputStream fileinput = null;

    try {
        fileinput = new FileInputStream("simple.csv");
    } catch (FileNotFoundException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }


    FileChannel channel = fileinput.getChannel();




    MappedByteBuffer ByteBuffer;

    try {
        ByteBuffer = fileinput.getChannel().map(FileChannel.MapMode.READ_ONLY, 0, channel.size());
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    /*
    * Add some 400 bytes to simple.csv. outside of this program...
    */

                 //following line throw exception.
    try {
        ByteBuffer = fileinput.getChannel().map(FileChannel.MapMode.READ_ONLY, channel.size(), 400);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

在我的代码中，我试图重新读取已经添加了但它不工作，我知道的概率是channel.size（），但我不能纠正它。

So in my code I'm trying to reread the additional data that has been added but its not working, I know the prob is channel.size(), but I'm not able to rectify it.

推荐答案

channel.size（） 文件结束。您正在尝试映射400字节。它不在那里。你需要类似的东西：

channel.size() is always the current end of file. You are attempting to map 400 bytes past it. It isn't there. You need something like:

ByteBuffer = fileinput.getChannel().map(FileChannel.MapMode.READ_ONLY, channel.size()-400, 400);

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