以较小长度的字节缓冲区块数读取和写入文件

Question

我正在尝试以 ByteBuffer 固定长度的块读取文件，然后将其存储到 ByteBuffer 的列表中，然后在一些操作之后按顺序读取这些 ByteBuffer 块为了重建文件。问题是在写入输出文件时通道位置没有增加。我不想使用字节数组，因为它们是固定长度的，文件重建不能正常工作。所以我想知道如何增加文件写入通道位置的大小，或者任何其他方式来执行此操作。示例代码将不胜感激。这是我的代码片段，

file = new File(fileName);  // hello.txt - 20 MB size
fis = new FileInputStream(file);
inChannel = fis.getChannel();
double maxChunkSequenceNoFloat = ((int)inChannel.size()) / chunkSize;
int maxChunkSequenceNo = 1;
if(maxChunkSequenceNoFloat%10 > 0) {
    maxChunkSequenceNo = ((int)maxChunkSequenceNoFloat)+1;
} else if(maxChunkSequenceNoFloat%10 < 0) {
    maxChunkSequenceNo = 1;
} else {
    maxChunkSequenceNo = (int)maxChunkSequenceNoFloat;
}
maxChunkSequenceNo = (maxChunkSequenceNo == 0) ? 1 : maxChunkSequenceNo;            
ByteBuffer buffer = ByteBuffer.allocate(chunkSize);
buffer.clear();

while(inChannel.read(buffer) > 0) {
    buffer.flip();
    bufferList.add(buffer);
    buffer.clear();
    chunkSequenceNo++;
}
maxChunkSequenceNo = chunkSequenceNo;

// write
File file1 = new File("hello2.txt") ; 
buffer.clear();
FileOutputStream fos = new FileOutputStream(file1);
FileChannel outChannel = fos.getChannel();
chunkSequenceNo = 1;
for(ByteBuffer test : bufferList) {
    writeByteCount += outChannel.write(test);
    //outChannel.position() += writeByteCount;'
    System.out.println("main - channelPosition: "+outChannel.position()
                        +" tempBuffer.Position: "+test.position()
                        +" limit: "+test.limit()
                        +" remaining: "+test.remaining()
                        +" capacity: "+test.capacity());              
}
BufferedReader br = new BufferedReader(new FileReader(file1));
String line = null;
while ((line = br.readLine()) != null) {
    System.out.println(line);
}
outChannel.close();
fos.close();

字节缓冲区位置正确，但 outChannel 位置仍为块大小 1048。

Answer 1

下面确实会按要求维护一个 ByteBuffer 列表。

String fileName = "hello.txt";
final int chunkSize = 256;
List<ByteBuffer> bufferList = new ArrayList<>();
Path path = Paths.get(fileName);
try (SeekableByteChannel inChannel = Files.newByteChannel(path,
        EnumSet.of(StandardOpenOption.READ))) {
    long size = inChannel.size();
    while (size > 0) {
        ByteBuffer buffer = ByteBuffer.allocate(chunkSize);

        int nread = inChannel.read(buffer);
        if (nread <= 0) {
            break;
        }
        buffer.flip();
        bufferList.add(buffer);
        size -= nread;
    }
}

// write
Path file1 = Paths.get("hello2.txt") ;
try (SeekableByteChannel outChannel = Files.newByteChannel(file1,
        EnumSet.of(StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE))) {
    for (ByteBuffer buffer : bufferList) {
        int nwritten = outChannel.write(buffer);
    }
}

Try-with-resources 负责关闭 channels/files。文件（实用函数）和路径（比文件更通用）很有用。

当有 chunkSize 限制时，需要添加新的 ByteBuffer 实例。（所以也可能添加了底层字节数组。）

最好不要使用浮点数，即使在这里也是如此。

以较小长度的字节缓冲区块数读取和写入文件

Reading and writing file in number of Byte Buffer chunks of smaller length

java

bytebuffer

filechannel