如何使用java.nio.channels.FileChannel将byte []写入文件-基础

Question

I do not have experience using Java channels. I would like to write a byte array to a file. Currently, I have the following code:

String outFileString = DEFAULT_DECODED_FILE; // Valid file pathname
FileSystem fs = FileSystems.getDefault();
Path fp = fs.getPath(outFileString);

FileChannel outChannel = FileChannel.open(fp, EnumSet.of(StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING, StandardOpenOption.WRITE));

// Please note: result.getRawBytes() returns a byte[]
ByteBuffer buffer = ByteBuffer.allocate(result.getRawBytes().length);
buffer.put(result.getRawBytes());

outChannel.write(buffer); // File successfully created/truncated, but no data

With this code, the output file is created, and truncated if it exists. Also, in the IntelliJ debugger, I can see that buffer contains data. Also, the line outChannel.write() is successfully called without throwing an exception. However, after the program exits, the data does not appear in the output file.

Can somebody (a) tell me if the FileChannel API is an acceptable choice for writing a byte array to a file, and (b) if so, how should the above code be modified to get it to work?

Answer 1

As gulyan points out, you need to flip() your byte buffer before writing it. Alternately, you could wrap your original byte array:

ByteBuffer buffer = ByteBuffer.wrap(result.getRawBytes());

To guarantee the write is on disk, you need to use force() :

outChannel.force(false);

Or you could close the channel:

outChannel.close();

Answer 2

You should call:

buffer.flip();

before the write.

This prepares the buffer for reading. Also, you should call

buffer.clear();

before putting data into it.

Answer 3

To answer your first question

tell me if the FileChannel API is an acceptable choice for writing a byte array to a file

It's ok but there's simpler ways. Try using a FileOutputStream . Typically this would be wrapped by a BufferedOutputStream for performance but the key is both of these extend OutputStream which has a simple write(byte[]) method. This is much easier to work with than the channel/buffer API.

Answer 4

Here is a complete example of FileChannel.

    import java.io.File;
    import java.io.FileNotFoundException;
    import java.io.IOException;
    import java.io.RandomAccessFile;
    import java.nio.ByteBuffer;
    import java.nio.channels.FileChannel;
    import java.nio.channels.WritableByteChannel;


    public class FileChannelTest {
        // This is a Filer location where write operation to be done.
        private static final String FILER_LOCATION = "C:\\documents\\test";
        // This is a text message that to be written in filer location file.
        private static final String MESSAGE_WRITE_ON_FILER = "Operation has been committed.";

        public static void main(String[] args) throws FileNotFoundException {
            // Initialized the File and File Channel
            RandomAccessFile randomAccessFileOutputFile = null;
            FileChannel outputFileChannel = null;
            try {
                // Create a random access file with 'rw' permission..
                randomAccessFileOutputFile = new RandomAccessFile(FILER_LOCATION + File.separator + "readme.txt", "rw");
                outputFileChannel = randomAccessFileOutputFile.getChannel();
                //Read line of code one by one and converted it into byte array to write into FileChannel.
                final byte[] bytes = (MESSAGE_WRITE_ON_FILER + System.lineSeparator()).getBytes();
                // Defined a new buffer capacity.
                ByteBuffer buffer = ByteBuffer.allocate(bytes.length);
                // Put byte array into butter array.
                buffer.put(bytes);
                // its flip the buffer and set the position to zero for next write operation.
                buffer.flip();
                /**
                 * Writes a sequence of bytes to this channel from the given buffer.
                 */
                outputFileChannel.write(buffer);
                System.out.println("File Write Operation is done!!");

            } catch (IOException ex) {
                System.out.println("Oops Unable to proceed file write Operation due to ->" + ex.getMessage());
            } finally {
                try {
                    outputFileChannel.close();
                    randomAccessFileOutputFile.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }

        }

    }