Java FileChannel

1.在文件特定位置进行读写操作
2.将文件一部分直接加载到内存，这样效率更高
3.以更快的速度将文件数据从一个通道传输到另一个通道
4.锁定文件的某一部分来限制其他线程访问
5.为了避免数据丢失，强制立即将更新写入文件并存储

  try (RandomAccessFile reader = new RandomAccessFile("./comm.txt", "r");FileChannel channel = reader.getChannel();ByteArrayOutputStream out = new ByteArrayOutputStream()) {int bufferSize = 1024;if (bufferSize > channel.size()) {bufferSize = (int) channel.size();}ByteBuffer buff = ByteBuffer.allocate(bufferSize);while (channel.read(buff) > 0) {out.write(buff.array(), 0, buff.position());buff.clear();}String fileContent = new String(out.toByteArray(), StandardCharsets.UTF_8);System.out.println(fileContent);} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}

读方法中获得channel的方法：

//RandomAccessFile获得
RandomAccessFile reader = new RandomAccessFile(file, "r");
FileChannel channel = reader.getChannel();//FileInputStream获得
FileInputStream fin= new FileInputStream(file);
FileChannel channel = fin.getChannel();

  String file = "./test.txt";try (RandomAccessFile writer = new RandomAccessFile(file, "rw");FileChannel channel = writer.getChannel()) {ByteBuffer buff = ByteBuffer.wrap("Hello world".getBytes(StandardCharsets.UTF_8));channel.write(buff);} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}

写方法中获得channel的方法：

//RandomAccessFile获得
RandomAccessFile writer = new RandomAccessFile(file, "rw");
FileChannel channel = writer.getChannel();//FileOutputStream获得
FileOutputStream fout = new FileOutputStream(file);
//FileOutputStream fout = new FileOutputStream(file，true);接着文件尾部进行写入
FileChannel channel = fout.getChannel();

channel.position(n);

channel.size(n);

//方法将文件截断为给定的大小(以字节为单位)
channel = channel.truncate(n);

channel.force(true);

try (RandomAccessFile reader = new RandomAccessFile("./test.txt", "r");FileChannel channel = reader.getChannel();ByteArrayOutputStream out = new ByteArrayOutputStream()) {MappedByteBuffer buff = channel.map(FileChannel.MapMode.READ_ONLY, 6, 5);if(buff.hasRemaining()) {byte[] data = new byte[buff.remaining()];buff.get(data);}
} catch (FileNotFoundException e) {e.printStackTrace();
} catch (IOException e) {e.printStackTrace();
}

类似地，可以使用FileChannel.MapMode.READ_WRITE以读写模式打开文件。还可以使用FileChannel.MapMode.PRIVATE模式，该模式下，更改不应用于原始文件。

FileLock fileLock = channel.tryLock(long position, long size, booean  shared)