java nio ：ByteBuffer

// 分配了一个能存储8字节的ByteBuffer，并且只能写入
ByteBuffer buffer = ByteBuffer.allocate(8);

// 由写切换成读
buffer.flip();

// 读转写
buffer.clear()；

1. capcity：容量
2. position：读和写的起始指针
3. limit：读写位置限制
   1. limit-position表示可读/写的容量

rewind():
// 将position设为0，表示会从0开始读

put() # 读数据，position会移动
put(int index) # position不会移动

mark() # 记录position位置
reset() #将position重置到mark()那里

FileChannel channel = new FileInputStream("data.txt").getChannel();
ByteBuffer buffer1 = ByteBuffer.allocate(4);
ByteBuffer buffer2 = ByteBuffer.allocate(5);
ByteBuffer buffer3 = ByteBuffer.allocate(6);
channel.read(new ByteBuffer[]{buffer1, buffer2, buffer3});
buffer1.flip();
buffer2.flip();
buffer3.flip();

ByteBuffer a = StandardCharsets.UTF_8.encode("hello");
ByteBuffer b = StandardCharsets.UTF_8.encode("world");
ByteBuffer c = StandardCharsets.UTF_8.encode("你好");
try {
    FileChannel channel = new RandomAccessFile("data2.txt", "rw").getChannel();
    channel.write(new ByteBuffer[]{a, b, c});
} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}