Java NIO read/write file through FileChannel

referee:  Java NIO FileChannel

A java nio FileChannel is an channel that is connected to a file. Using a file channel you can read data from a file, and write data to a file. NIO is an alternative to reading files with the standard Java IO API.

A FileChannel cannot be set into non-blocking mode, and it's always runs in blocks mode.

Opening a FileChannel

To use an FileChannel, we should firstly open it. And we should obtain a FileChannel via in InputStream, OutputStream, or an RandomAccessFile. Here is how you can open a FileChannel via RandomChannel:

 RandomAccessFile aFile = new RandomAccessFile("./testNewFile.txt", "rw");
 FileChannel inChannel =  aFile.getChannel();

Reading Data from FileChannel

we use  read() methods to read  data from FileChannel. Here is an example:

        ByteBuffer buf = ByteBuffer.allocate(48);
        int bytesRead = inChannel.read(buf);

Firstly we allocated a buffer, and read data from channel into buffer. And the method FileChannel.read() return an integer tells how many bytes written into the Buffer. If -1 is returned, then it means end-of-file is reached.

Write Data into a FileChannel

We call method FileChannel.write() to write data into the Buffer. Here is an example:

        String newData = "New String to write file..." + System.currentTimeMillis();
        ByteBuffer newBuf = ByteBuffer.allocate(48);
        newBuf.clear();
        newBuf.put(newData.getBytes());
        String v = new String(newBuf.array());
        System.out.println("content of new byte buffer: " + v);

FileChannel Position

When reading or writing to a FileChannel at an position, we can get the current of the position of the FileChannel object by calling the position() method.

We can set the position value by calling method position(long pos), and here are the examples:

        long pos = fileChannel.position();
        System.out.println(pos);
        fileChannel.position(pos - 10);
        System.out.println(fileChannel.position());

Note:

The the position value we set bigger than length of channel, and then try to read data from channel,  then we will get minus 1 showing end-of-file.

And if we try to write data to an position bigger than the length of channel, the file will be extended to fit the position and the writing method can run successfully. But this action will cause an "file hole" meaning file on the disk has graps.

FileChannel Size

We can call method size() to get the length of channel

FileChannel Truncate

We can truncate a file by calling the FileChannel.truncate() method. When you truncate a file, we cut it off at a given length. 

FileChannel Force

The force method will flush all unwritten data from the channel to the disk. An operating system may cache data in memory for high performance, so we should take flush to keep data consisentency.

        String content = new String("Before flush: " + Files.readAllBytes(Paths.get("testNewFile.txt")));
        System.out.println(content);

        inChannel.force(true);

        String contentAfterFlush = new String("After flush: " + Files.readAllBytes(Paths.get("testNewFile.txt")));
        System.out.println(contentAfterFlush);

程序源代码：

        String newData = "New String to write file..." + System.currentTimeMillis();
        ByteBuffer newBuf = ByteBuffer.allocate(48);
        newBuf.clear();
        newBuf.put(newData.getBytes());
        String v = new String(newBuf.array());
        System.out.println("content of new byte buffer: " + v);

        newBuf.flip();
        while(newBuf.hasRemaining()){
            inChannel.write(newBuf);
        }
        long pos = inChannel.position();
        System.out.println(pos);
        inChannel.position(pos - 10);
        System.out.println("position of channel: " + inChannel.position());

        // size of the file channel
        System.out.println("size of channel: "+inChannel.size());

        String content = new String(Files.readAllBytes(Paths.get("testNewFile.txt")));
        System.out.println(content);

        inChannel.force(true);

        String contentAfterFlush = new String(Files.readAllBytes(Paths.get("testNewFile.txt")));
        System.out.println(contentAfterFlush);


        //close file channel
        inChannel.close();