Examples of java.nio.channels.FileChannel

java.nio.channels.FileChannel
An abstract channel type for interaction with a platform file.
A FileChannel defines the methods for reading, writing, memory mapping, and manipulating the logical state of a platform file. This type does not have a method for opening files, since this behaviour has been delegated to the FileInputStream, FileOutputStream, and RandomAccessFile types.

FileChannels created from a FileInputStream, or a RandomAccessFile created in mode "r", are read-only. FileChannels created from a FileOutputStream are write-only. FileChannels created from a RandomAccessFile created in mode "rw" are read/write. FileChannels created from a RandomAccessFile that was opened in append-mode will also be in append-mode -- meaning that each write will be preceeded by a seek to the end of file. Some platforms will seek and write atomically, others will not.

FileChannels has a virtual pointer into the file which is refered to as a file position. The position can be manipulated by repositioning it within the file, and its current position can be queried.

FileChannels also have an associated size. The size of the file is the number of bytes that it currently contains. The size can be manipulated by adding more bytes to the end of the file (which increases the size) or truncating the file (which decreases the size). The current size can also be queried.

FileChannels have operations beyond the simple read, write, and close. They can also:
- request that cached data be forced onto the disk
- lock ranges of bytes associated with the file
- transfer data directly to another channel in a manner that has the potential to be optimized by the platform
- memory-mapping files into NIO buffers to provide efficient manipulation of file data
- read and write to the file at absolute byte offsets in a fashion that does not modify the current position
FileChannels are thread-safe. Only one operation involving manipulation of the file position may be in-flight at once. Subsequent calls to such operations will block, and one of those blocked will be freed to continue when the first operation has completed. There is no ordered queue or fairness applied to the blocked threads.

It is undefined whether operations that do not manipulate the file position will also block when there are any other operations in-flight.

The logical view of the underlying file is consistent across all FileChannels and IO streams opened on the same file by the same JVM process. Therefore modifications performed via a channel will be visible to the stream, and vice versa; including modifications to the file position, content, size, etc.


        preFileAppend(file, append);
        final FileOutputStream dst = new FileOutputStream(file, append);
        final String fp = file.getAbsolutePath();
        final ReadWriteLock filelock = accquireLock(fp, locks);
        final FileChannel fileCh = dst.getChannel();
        final long startPos = file.length();
        try {
            NIOUtils.transferFullyFrom(inChannel, 0, len, fileCh); // REVIEWME really an atomic operation?
        } finally {
            IOUtils.closeQuietly(fileCh, dst);

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     * Create a temp file
     */


    final File temp = File.createTempFile(this.file.getName(), null);


    final FileChannel tempChannel = new RandomAccessFile(temp, "rw")
        .getChannel();


    /*
     * Copy entire edits tree, including the root file, to temp file
     */
    for (final RegionSegment<PartialLoader> segment : this.edits) {
      final PartialLoader loader = new ConstrainedPartialLoader(segment
          .getData(), segment);


      loader.transferTo(tempChannel);
    }
    this.channel.close();
    tempChannel.close();


    System.gc();


    /*
     * We're done with the original file. All changes are now in the temp file

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    if (dst.createNewFile() == false) {
      throw new FileNotFoundException("Unable to create dst file ["
          + dst.getName() + "]");
    }


    final FileChannel srcC = new RandomAccessFile(src, "r").getChannel();


    final FileChannel dstC = new RandomAccessFile(dst, "rw").getChannel();


    srcC.transferTo(0, srcC.size(), dstC);


    srcC.close();
    dstC.close();
  }

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    out.close();


  }


  public static long gzipUncompressedSize(File src) throws IOException {
    FileChannel in = new RandomAccessFile(src, "r").getChannel();


    ByteBuffer b = ByteBuffer.allocate(128);
    b.order(ByteOrder.LITTLE_ENDIAN);


    in.read(b, in.size() - b.remaining());


    final long size = b.getInt(b.limit() - 4);


    in.close();


    return size;
  }

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    return size;
  }


  public static long gzipUncompressedCRC32(File src) throws IOException {
    FileChannel in = new RandomAccessFile(src, "r").getChannel();


    ByteBuffer b = ByteBuffer.allocate(128);
    b.order(ByteOrder.LITTLE_ENDIAN);


    in.read(b, in.size() - b.remaining());


    final long size = b.getInt(b.limit() - 8);


    in.close();


    return (size < 0 ? ((long) Integer.MAX_VALUE )* 2L + 1 + size : size);
  }

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        final StopWatch sw = new StopWatch();
        FileInputStream src = null;
        final long nbytes;
        try {
            src = new FileInputStream(file);
            FileChannel fc = src.getChannel();


            String fileName = file.getName();
            IOUtils.writeString(fileName, dos);
            IOUtils.writeString(writeDirPath, dos);
            long xferBytes = (count == -1L) ? fc.size() : count;
            dos.writeLong(xferBytes);
            dos.writeBoolean(append); // append=false
            dos.writeBoolean(sync);
            if(handler == null) {
                dos.writeBoolean(false);
            } else {
                dos.writeBoolean(true);
                handler.writeAdditionalHeader(dos);
            }


            // send file using zero-copy send
            nbytes = fc.transferTo(fromPos, xferBytes, channel);
            if(LOG.isDebugEnabled()) {
                LOG.debug("Sent a file '" + file.getAbsolutePath() + "' of " + nbytes
                        + " bytes to " + dstSockAddr.toString() + " in " + sw.toString());
            }

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    private int hslots;


    public ReadOnlyCDB(String filename) throws IOException {
        file = new RandomAccessFile(filename, "r");
        FileChannel channel = file.getChannel();
        map = channel.map(MapMode.READ_ONLY, 0, file.length());
        map.order(ByteOrder.LITTLE_ENDIAN);
    }

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   * @param f            The file to open that contains binary logs.
   * @throws IOException
   */
  public void read(File f) throws IOException {
    FileInputStream fis = new FileInputStream(f);
    FileChannel fc=fis.getChannel();
    buffer=fc.map(FileChannel.MapMode.READ_ONLY, 0, f.length());
    int o=0;
    do{
      int l=buffer.getInt(o);
      if(l<=STRING_OFFSET){
        throw new IOException("Corrupted file : invalid packet length");

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   * @param f                The file to write to.
   * @throws IOException
   */
  public void save(File f) throws IOException {
    FileOutputStream fos=new FileOutputStream(f);
    FileChannel fc=fos.getChannel();
    buffer.flip();
    fc.write(buffer);
    buffer.limit(buffer.capacity());
    fc.close();
    fos.flush();
    fos.close();
  }

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      if (src.length()==0) {
        dest.createNewFile();
        return;
      }
      // transfering >0 bytes
      FileChannel fc = new FileInputStream(src).getChannel();
      FileChannel dstChannel = new FileOutputStream(dest).getChannel();
      long transfered = 0;
      long totalLength = src.length();
      while (transfered < totalLength) {
        long num = fc.transferTo(transfered,totalLength-transfered,dstChannel);
        if (num==0) throw new IOException("Error while copying");
        transfered += num;
      }
      dstChannel.close();
      fc.close();
    } catch (IOException e) {
      if (os.equals("Unix")) {
        
        _logger.fine("Trying to use cp to copy file...");

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