无法用MappedByteBuffer分块读取文件。

Question

在读取操作过程中，我得到了java.io.IOException: 通道未打开，无法将文件扩展到所需大小。

我写了一个简单的程序，用MappedByteBuffer读取文件。我的想法是根据API读取带有区域的文件。但是在执行过程中，我得到了异常。我有一个测试文件，内容如下。

SimpleTestFile!

static void readFileWithChunks(final String file){

    final Path pathToFile = Paths.get(file);
    LOG.info("Path to file: {}", pathToFile.toString());

    try (FileChannel fileChannel = (FileChannel) Files.newByteChannel(pathToFile, EnumSet.of(StandardOpenOption.READ))) {

        final long fileSize = fileChannel.size();
        LOG.info("Total size of the file: {} bytes", fileSize);

        final int maxChunkSize = 4;
        long startPosition = 0;
        long endPosition = 0;

        // main cycle to read chunks of data from file
        while (startPosition < fileSize){
            if (endPosition + maxChunkSize < fileSize){
                endPosition += maxChunkSize;
            } else {
                endPosition = fileSize;
            }
            readChunk(fileChannel, startPosition, endPosition);
            startPosition = endPosition;
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}


static void readChunk(final FileChannel fileChannel, final long startPosition, final long endPosition) throws IOException {
    LOG.info("Start position: {}; End position: {}", startPosition, endPosition);

    final MappedByteBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, startPosition, endPosition);

    final int bufferSize = (int) (endPosition - startPosition);
    LOG.info("Buffer size: {}", bufferSize);
    final byte[] buffer = new byte[bufferSize];

    mappedByteBuffer.get(buffer);

    LOG.info("Content of the buffer: {}", new String(buffer, StandardCharsets.UTF_8));
}

OUTPUT。

OUTPUT
21:39:39.192 [main] com.test.FileReader INFO - Path to file: /user/test/testfile.txt
21:39:39.196 [main] com.test.FileReader INFO - Total size of the file: 15 bytes
21:39:39.196 [main] com.test.FileReader INFO - Start position: 0; End position: 4
21:39:39.198 [main] com.test.FileReader INFO - Buffer size: 4
21:39:39.198 [main] com.test.FileReader INFO - Content of the buffer: Simp
21:39:39.198 [main] com.test.FileReader INFO - Start position: 4; End position: 8
21:39:39.198 [main] com.test.FileReader INFO - Buffer size: 4
21:39:39.198 [main] com.test.FileReader INFO - Content of the buffer: leTe
21:39:39.198 [main] com.test.FileReader INFO - Start position: 8; End position: 12
java.io.IOException: Channel not open for writing - cannot extend file to required size
    at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:901)
    at com.test.FileReader.readChunk(FileReader.java:59)
    at com.test.FileReader.readFile(FileReader.java:42)
    at com.test.TestClass.main(TestClass.java:14)

Answer 1

这是你的问题。

final MappedByteBuffer mappedByteBuffer = 
    fileChannel.map(
        FileChannel.MapMode.READ_ONLY,
        startPosition,
        endPosition // <-- (endPosition - startPosition)
    );

FileChannel.map需要一个偏移量+长度，而不是一个开始+结束。

public abstract MappedByteBuffer map(MapMode mode,
                                     long position, long size)
    throws IOException;

当你把映射视图推进到超过文件长度的时候，底层文件会被扩展以适应（在文件的前一端和新的一端之间有未定义的内容）。扩展文件需要写访问，因此出现了异常。