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。

IO与NIO