Channel就是一个通道，用于传输数据，两端分别是缓冲区和实体（文件或者套接字），通道的特点（也是NIO的特点）：通道中的数据总是要先读到一个缓冲区，或者总是要从一个缓冲区中读入。
Channel的分类
1) FileChannel：从文件中读写数据
2) SocketChannel：通过TCP协议读写网络中的数据
3) ServerSocketChannel：在服务器端可以监听新进来的TCP连接，像WEB服务器那样，对每一个新进来的请求创建一个SocketChannel
4) DatagramChannel：通过UDP协议读写网络中的数据
上面众多的分类，是对应了不同的实体，这些通道包括了文件IO、TCP和UDP网络IO。
下面来看看Channel的源码：
public interface Channel extends Closeable {
public boolean isOpen();
public void close() throws IOException;
}从这里我们可以看到，Channel接口只提供了关闭通道和检测通道是否打开这两个方法，剩下方法的都是由子接口和实现类来定义提供。
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" alt="" />
我们选择其中几个来看看这些接口的源码：
public interface WritableByteChannel
extends Channel
{ public int write(ByteBuffer src) throws IOException; }
public interface ReadableByteChannel extends Channel
{
public int read(ByteBuffer dst) throws IOException; }
public interface ByteChannel
extends ReadableByteChannel, WritableByteChannel
{ }
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" alt="" />
前面我提到过：通道可以只读、只写或者同时读写，因为Channel类可以只实现只读接口ReadableByteChannel或者只实现只写接口WritableByteChannel，而我们常用的Channel类FileChannel、SocketChannel、DatagramChannel是双向通信的， 因为实现了ByteChannel接口。
Channel的获取
IO在广义上可以分为：文件IO和网络IO。文件IO对应的通道为FileChannel，而网络IO对应的通道则有三个：SocketChannel、ServerSoketChannel和DatagramChannel。
一、文件通道
FileChannel对象不能直接创建，只能通过FileInputStream、OutputStream、RandomAccessFile对象的getChannel()来获取，如：
FileInputStream fis = new FileInputStream("c:/in.txt");
FileChannel fic = fis.getChannel();FileChannel无法设置为非阻塞模式，它总是运行在阻塞模式下。
1）使用通道读取文件
public class NIOFileReadTest
{
public static void main(String[] args) throws IOException
{
RandomAccessFile raf = new RandomAccessFile("D:/in.txt","rw");
FileChannel fis = raf.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(1024);
fis.read(buffer);
buffer.flip();
while(buffer.hasRemaining())
{
System.out.print((char)buffer.get());
}
buffer.clear();
fis.close();
}
}
执行结果：
FileChannel
ByteBuffer
SelectorPicked
2）使用通道写入文件
public class NIOFileWriteTest
{
public static void main(String[] args) throws Exception
{
FileOutputStream fos = new FileOutputStream("d:/out.txt");
FileChannel fc = fos.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(10);
buffer.clear();
String str = "Channel";
buffer.put(str.getBytes());
buffer.flip();
while(buffer.hasRemaining())
{
fc.write(buffer);
}
fc.close();
fos.close();
}
}
在这里总是要记住channel是要关闭的。
ByteBuffer中的方法我在下一章再详细介绍，这里只要注意这点即可：通道只能使用ByteBuffer，不管是读还是写，通道都要对接缓冲区。
3）通道的常用方法
position();返回通道的文件位置
position(long newPosition):设置通道的文件位置
将上面读文件的程序修改下，来观察这几个方法：
public class NIOFileReadTest
{
public static void main(String[] args) throws IOException
{
RandomAccessFile raf = new RandomAccessFile("D:/in.txt","rw");
FileChannel fis = raf.getChannel();
System.out.println("此通道文件的总长度:" +fis.size());
//当前通道的文件位置
long position = fis.position();
System.out.println("通道当前的位置:" + position);
//设置新的通道文件位置，从这个位置开始读取
fis.position(position + 8);
ByteBuffer buffer = ByteBuffer.allocate(50);
fis.read(buffer);
buffer.flip();
while(buffer.hasRemaining())
{
System.out.print((char)buffer.get());
}
buffer.clear();
fis.close();
}
}
执行结果：
此通道文件的总长度:33
通道当前的位置:0
nel
ByteBuffer
Selector
FileChannel是线程安全的，可以多个线程在同一个实例上并发操作，但是其中有些方法（改变文件通道位置或者文件大小的方法）必须是单线程操作。
二、网络通道
SocketChannel是一个连接到TCP套接字的通道，获取的方式有两种：
1、打开一个SocketChannel并连接到互联网上某台服务器。
2、一个新连接到达ServerSocketChannel时，会创建一个SocketChannel。
上面这两种模式跟IO的Socket、ServerSocket类似，下面分别来看看客户端和服务器端：
一、SocketChannel
从通道中读取数据
public class SocketChannelTest
{
public static void main(String[] args) throws Exception
{
//获取socket通道
SocketChannel sc = SocketChannel.open();
//设置为非阻塞模式
sc.configureBlocking(false);
//建立连接，非阻塞模式下，该方法可能在连接建立之前就返回了
sc.connect(new InetSocketAddress("wap.cmread.com",80));
//判断连接是否建立
while(!sc.finishConnect())
{
System.out.println("连接未建立");
Thread.sleep(5);
}
ByteBuffer buffer = ByteBuffer.allocate(48);
int byteRead = sc.read(buffer);
System.out.println(byteRead);
sc.close();
buffer.clear();
}
}
执行结果；
连接未建立
连接未建立
0
1、第6、7行是获取一个socket通道，并且设置为非阻塞模式。
2、由于是非阻塞模式，通道在调用方法connect/read/writer这三个方法时，会出现这些情况：连接未建立，connect方法就返回了；尚未读取任何数据时，read方法就返回；尚未写出任何内容时，writer就返回。
3、在12行的循环代码中，是判断连接是否建立，从执行结果来看，循环执行了两次连接才建立（在循环里线程还有休眠）。
4、由于只是建立了连接，通道里面其实没有任何的数据。
5、第18行调用read方法，由于是非阻塞模式，所以在并未读取任何数据的情况下就返回0（尽管通道里面没有数据）。
将数据写入通道
public class SocketChannelTest
{
public static void main(String[] args) throws Exception
{
SocketChannel sc = SocketChannel.open();
String str = "non-blocking socket channel";
ByteBuffer buffer = ByteBuffer.allocate(100);
buffer.put(str.getBytes());
buffer.flip();
while(buffer.hasRemaining())
{
sc.write(buffer);
}
sc.close();
buffer.clear();
}
}
1、SocketChannel.write()方法的调用是在一个while循环中的。Write()方法无法保证能写多少字节到SocketChannel。所以，我们重复调用write()直到Buffer没有要写的字节为止。
二、ServerSocketChannel
ServerSocketChannel是一个可以监听新进来的TCP连接的通道。
public class ServerSocketChannelTest
{
public static void main(String[] args) throws Exception
{
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.socket().bind(new InetSocketAddress(80));
ssc.configureBlocking(false);
while(true)
{
SocketChannel sc = ssc.accept();
if(null != sc)
{
//do something;
}
}
}
}
1、第5、6、7行，获取一个ServerSocketChannel，并且监听80端口，设置为非阻塞模式。
2、通过accept方法监听新接入进来的连接，这个方法会返回一个包含新进来的连接的SocketChannel（服务器端的通道的获取方式）。如果是阻塞模式，该方法会一直阻塞直到有新的连接进来。如果是非阻塞模式，则accept方法会立刻返回，返回值是null。
3、第11行，是因为在非阻塞模式下，需要检查SocketChannel是否为null。
三、socket通道与socket
ServerSocketChannel ssc = ServerSocketChannel.open();
ServerSocket socket = ssc.socket();
ServerSocketChannel ssc1 = socket.getChannel();
1、从这代码片段可以大概看到这样一种关系：所有socket通道（SocketChannel/ServerSocketChanne/DatagramSocketChannel）在被实例化之后，都是伴随生成对应的socket对象，就是前面IO章节介绍的java.net类（Socket/ServerSocket/DatagramSocket）。通过通道类的socket方法来获取。
2、java.net类（Socket/ServerSocket/DatagramSocket）现在可以通过getChannel方法来获取对应的通道。前提是这些socket对象不是使用传统方式（直接实例化）创建的。否则它就没有关联的socket通道，调用getChannel方法返回总是null。