Java NIO 文件通道 FileChannel 用法及原理

FileOutputStream ous = new FileOutputStream(new File("a.txt"));
FileChannel out = ous.getChannel(); // 获取一个只读通道
FileInputStream ins = new FileInputStream(new File("a.txt"));
FileChannel in = ins.getChannel(); // 获取一个只写通道

RandomAccessFile file = new RandomAccessFile("a.txt", "rw");
FileChannel channel = file.getChannel(); // 获取一个可读写文件通道

FileChannel channel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.READ); // 以只读的方式打开一个文件 a.txt 的通道

FileChannel channel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.READ);
ByteBuffer buf = ByteBuffer.allocate(5);
while(channel.read(buf)!=-1){
 buf.flip();
 System.out.print(new String(buf.array()));
 buf.clear();
}
channel.close();

ScatteringByteChannel channel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.READ);
ByteBuffer key = ByteBuffer.allocate(5), value=ByteBuffer.allocate(10);
ByteBuffer[] buffers = new ByteBuffer[]{key, value};
while(channel.read(buffers)!=-1){
 key.flip();
 value.flip();
 System.out.println(new String(key.array()));
 System.out.println(new String(value.array()));
 key.clear();
 value.clear();
}
channel.close();

FileChannel channel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.WRITE);
ByteBuffer buf = ByteBuffer.allocate(5);
byte[] data = "Hello, Java NIO.".getBytes();
for (int i = 0; i < data.length; ) {
 buf.put(data, i, Math.min(data.length - i, buf.limit() - buf.position()));
 buf.flip();
 i += channel.write(buf);
 buf.compact();
}
channel.force(false);
channel.close();

FileChannel channel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.WRITE);
ByteBuffer key = ByteBuffer.allocate(10), value = ByteBuffer.allocate(10);
byte[] data = "017 Robothy".getBytes();
key.put(data, 0, 3);
value.put(data, 4, data.length-4);
ByteBuffer[] buffers = new ByteBuffer[]{key, value};
key.flip();
value.flip();
channel.write(buffers);
channel.force(false); // 将数据刷出到磁盘
channel.close();

channel.force(false); // 将数据刷出到磁盘，但不包括元数据

FileLock lock = channel.lock(0, Long.MAX_VALUE, false);// 排它锁，此时同一操作系统下的其它进程不能访问 a.txt
System.out.println("Channel locked in exclusive mode.");
Thread.sleep(30 * 1000L); // 锁住 30 s
lock.release(); // 释放锁

lock = channel.lock(0, Long.MAX_VALUE, true); // 共享锁，此时文件可以被其它文件访问
System.out.println("Channel locked in shared mode.");
Thread.sleep(30 * 1000L); // 锁住 30 s
lock.release();

public static void copy(File source, File target) throws IOException {
 FileInputStream sourceOutStream = new FileInputStream(source);
 FileOutputStream targetOutStream = new FileOutputStream(target);
 FileChannel sourceChannel = sourceOutStream.getChannel();
 FileChannel targetChannel = targetOutStream.getChannel();
 sourceChannel.transferTo(0, sourceChannel.size(), targetChannel);
 sourceChannel.close();
 targetChannel.close();
 sourceOutStream.close();
 targetOutStream.close();
}

/** 服务端 **/
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open(); // 打开服务通道
serverSocketChannel.bind(new InetSocketAddress(9090)); // 绑定端口号
SocketChannel clientChannel = serverSocketChannel.accept(); // 等待客户端连接，获取 SocketChannel
FileChannel fileChannel = FileChannel.open(Paths.get("video.mp4"), StandardOpenOption.READ); // 打开文件通道
fileChannel.transferTo(0, fileChannel.size(), clientChannel); // 【可能出错位置】文件通道数据输出转化到 socket 通道，输出范围为整个文件。文件太大将导致输出不完整

/** 客户端 **/
SocketChannel socketChannel = SocketChannel.open(new InetSocketAddress("localhost", 9090)); // 打卡 socket 通道并连接到服务端
FileChannel fileChannel = FileChannel.open(Paths.get("video-downloaded.mp4"), StandardOpenOption.TRUNCATE_EXISTING, StandardOpenOption.WRITE, StandardOpenOption.CREATE); // 打开文件通道
fileChannel.transferFrom(socketChannel, 0, Long.MAX_VALUE); // 【非阻塞模式下可能出错】
fileChannel.force(false); // 确保数据刷出到磁盘

long transfered = 0;
while (transfered < fileChannel.size()){
 transfered += fileChannel.transferTo(transfered, fileChannel.size(), clientChannel);
}

FileChannel fileChannel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.WRITE);
fileChannel.truncate(1);
System.out.println(fileChannel.size()); // 输出 1
fileChannel.write(ByteBuffer.wrap("Hello".getBytes()));
System.out.println(fileChannel.size()); // 输出 5
fileChannel.force(true);
fileChannel.close();

FileChannel fileChannel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.READ, StandardOpenOption.WRITE); // 以读写的方式打开文件通道
MappedByteBuffer buf = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, fileChannel.size()); // 将整个文件映射到内存

FileChannel fileChannel = FileChannel.open(Paths.get("a.txt"), StandardOpenOption.WRITE, StandardOpenOption.READ);
MappedByteBuffer buf = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, fileChannel.size());
fileChannel.close(); // 关于文件通道对 buf 没有影响
System.out.println(buf.capacity()); // 输出 fileChannel.size()
System.out.println(buf.limit()); // 输出 fileChannel.size()
System.out.println(buf.position()); // 输出 0
buf.put((byte)'R'); // 写入内容
buf.compact();  // 截掉 positoin 之前的内容
buf.force();  // 将数据刷出到 I/O 设备