NIO简单来讲就是new IO，核心是一个接口两个抽象类，interface Channel、abstract class Buffer、abstract class Charset 。
1.Buffer最常用的两个子类是ByteBuffer、charBuffer。Buffer中有三个重要的概念：capacity(容量)、limit(界限)、position(位置)：
capacity，缓冲区的容量表示该Buffer的最大数据容量，即最多可以存储多少数据，Buffer对象创建后，capacity不能改变，例如 ByteBuffer bbuff=ByteBuffer.allocate(8); CharBuffer cbuff=CharBuffer.allocate(8);
另外两个概念limit、position 则参考Buffer的flip()方法，clear()方法的源码与注释理解：
/**
* Flips this buffer. The limit is set to the current position and then the
* position is set to zero. If the mark is defined then it is discarded.
*
* <p>
* After a sequence of channel-read or <i>put</i> operations, invoke this
* method to prepare for a sequence of channel-write or relative <i>get</i>
*/
public final Buffer flip() {
limit = position;
position = 0;
mark = -1;
return this;
}
flip()方法为从Buffer中取数据做好准备，把limit设到原来position所在位置，这样相当于把Buffer中没有数据的存储空间挡住了，从而避免读到null。
/**
* Clears this buffer. The position is set to zero, the limit is set to the
* capacity, and the mark is discarded.
*
* <p>
* Invoke this method before using a sequence of channel-read or <i>put</i>
*
* <p>
* This method does not actually erase the data in the buffer, but it is
* named as if it did because it will most often be used in situations in
* which that might as well be the case.
* </p>
*/
public final Buffer clear() {
position = 0;
limit = capacity;
mark = -1;
return this;
}
clear()方法为再次向Buffer中装入数据做好准备，Buffer中的原来的数据还在，没有被删除。
2.Channel的常用实现类为FileChannel，此实现类最常用的方法有MappedByteBuffer map(MapMode mode, long position, long size)、read()的一系列重载、write()的一系列重载方法：
FileChannel对象不是由构造器产生的，而是由FileInputStream、FileOutputStream的getChannel()产生的：
@Test
public void testFileChannel() throws IOException, InterruptedException {
File f = new File("src/main/resources/b.txt");
// 通过fileInputStream.getChannel()方法得到FileChannel对象
FileChannel inChannel = new FileInputStream(f).getChannel();
/**
* 程序不能直接访问FileChannel中的数据，包括读取、写入都不行，FileChannel只能与MappedByteBuffer(
* ByteBuffer的子类)进行交互。
*/
// FileChannel可以将指定文件的部分或全部直接映射成ByteBuffer
ByteBuffer bbuff = inChannel.map(MapMode.READ_ONLY, 0, f.length());
// 通过fileOutputStream.getChannel()方法得到FileChannel对象
FileChannel outChannel = new FileOutputStream("src/main/resources/b2.txt").getChannel();
outChannel.write(bbuff);
}
3.Charset主要来处理ByteBuffer和CharBuffer(或字符串，CharBuffer对象的toString()方法可以获取对应的字符串)之间的转换。利用Charset的静态方法forName(String charsetName)来获取Charset对象，之后，就可以起通过该对象的newDecoder()、newEncoder()这两个方法分别返回CharsetDecoder和CharsetEncoder对象，代表该Charset的解码器和编码器。调用CharsetDecoder的decode()方法可以将ByteBuffer转换成CharBuffer，调用CharsetEncoder的encode()方法可以将CharBuffer或者String转换成byteBuffer。
Charset charset = Charset.forName("UTF-8");
CharBuffer cbuff = charset.decode(bbuff);
System.out.println(cbuff);
decode()方法的参数是ByteBuffer对象，怎么获得呢？由FileChannel对象的map(MapMode mode, long position, long size)方法获得。FileChannel对象怎么获得呢？FileInputStream对象的getChannel()方法获得，而FileInputStream与最终处理的文件有关系。