首先是channel，是一个双向的全双工的通道，可同时读写，而输入输出流都是单工的，要么读要么写。Channel分为两大类，分别是用于网络数据的SelectableChannel和用于文件操作的FileChannel。
注意：在java NIO库中，所有的数据都是用缓冲区处理，常用的是ByteBuffer。
多路复用器Selector：
Selector会不断轮询注册在其上的Channel，如果某个Channel上又新的连接接入、读和写事件，这个Channel就处于就绪状态，通过SelectorKey可以获取就绪Channel的集合。底层使用了epoll()实现，没有最大连接句柄的限制。
服务端代码：
public class TimeServer {
public static void main(String[] args) throws IOException {
int port = 8080;
MultiplexerTimeServer timeServer = new MultiplexerTimeServer(port);
new Thread(timeServer, "NIO-MultiplexerTimeServer-001").start();
}
}
public class MultiplexerTimeServer implements Runnable {
private Selector selector;
private ServerSocketChannel servChannel;
private volatile boolean stop;
public MultiplexerTimeServer(int port) {
try {
selector = Selector.open();
servChannel = ServerSocketChannel.open();
servChannel.configureBlocking(false);//设置非阻塞模式
servChannel.socket().bind(new InetSocketAddress(port), 1024);
servChannel.register(selector, SelectionKey.OP_ACCEPT);//将Channel注册到selector，监听accept事件
System.out.println("The time server is start in port : " + port);
} catch (IOException e) {
e.printStackTrace();
System.exit(1);
}
}
public void stop() {
this.stop = true;
}
@Override
public void run() {
while (!stop) {
try {
selector.select(1000);//每隔一秒轮询一次
Set<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> it = selectedKeys.iterator();
SelectionKey key = null;
while (it.hasNext()) {//如果有新的客户端接入
key = it.next();
it.remove();
try {
handleInput(key);//处理请求
} catch (Exception e) {
if (key != null) {
key.cancel();
if (key.channel() != null)
key.channel().close();
}
}
}
} catch (Throwable t) {
t.printStackTrace();
}
}
// 多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源
if (selector != null)
try {
selector.close();
} catch (IOException e) {
e.printStackTrace();
}
}
private void handleInput(SelectionKey key) throws IOException {
if (key.isValid()) {
// 处理新接入的请求消息
if (key.isAcceptable()) {
// 获取客户端的连接通道
ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
SocketChannel sc = ssc.accept();
sc.configureBlocking(false);
//将新连接注册到selector，并监听读事件
sc.register(selector, SelectionKey.OP_READ);
}
if (key.isReadable()) {
//读取通道数据写入字节缓存
SocketChannel sc = (SocketChannel) key.channel();
ByteBuffer readBuffer = ByteBuffer.allocate(1024);
int readBytes = sc.read(readBuffer);
if (readBytes > 0) {
readBuffer.flip();
byte[] bytes = new byte[readBuffer.remaining()];
readBuffer.get(bytes);//字节缓存写入字节数组
String body = new String(bytes, "UTF-8");
System.out.println("The time server receive order : " + body);
String currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body)
? new java.util.Date(System.currentTimeMillis()).toString()
: "BAD ORDER";
doWrite(sc, currentTime);//向socketchannel写入数据
} else if (readBytes < 0) {
// 对端链路关闭
key.cancel();
sc.close();
}
}
}
}
private void doWrite(SocketChannel channel, String response)
throws IOException {
if (response != null && response.trim().length() > 0) {
byte[] bytes = response.getBytes();
ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
writeBuffer.put(bytes);
writeBuffer.flip();
channel.write(writeBuffer);
}
}
}
客户端：
public class TimeClient {
public static void main(String[] args) {
int port = 8080;
new Thread(new TimeClientHandle("127.0.0.1", port), "TimeClient-001").start();
}
}
public class TimeClientHandle implements Runnable {
private String host;
private int port;
private Selector selector;
private SocketChannel socketChannel;
private volatile boolean stop;
public TimeClientHandle(String host, int port) {
this.host = host == null ? "127.0.0.1" : host;
this.port = port;
try {
selector = Selector.open();
socketChannel = SocketChannel.open();
socketChannel.configureBlocking(false);
} catch (IOException e) {
e.printStackTrace();
System.exit(1);
}
}
@Override
public void run() {
try {
doConnect();
} catch (IOException e) {
e.printStackTrace();
System.exit(1);
}
while (!stop) {
try {
selector.select(1000);
Set<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> it = selectedKeys.iterator();
SelectionKey key = null;
while (it.hasNext()) {
key = it.next();
it.remove();
try {
handleInput(key);
} catch (Exception e) {
if (key != null) {
key.cancel();
if (key.channel() != null)
key.channel().close();
}
}
}
} catch (Exception e) {
e.printStackTrace();
System.exit(1);
}
}
// 多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源
if (selector != null)
try {
selector.close();
} catch (IOException e) {
e.printStackTrace();
}
}
private void doConnect() throws IOException {
// 如果直接连接成功，则注册到多路复用器上，发送请求消息，读应答
if (!socketChannel.connect(new InetSocketAddress(host, port))) {
socketChannel.register(selector, SelectionKey.OP_READ);
doWrite(socketChannel);
} else//没有直接连接成功，不代表失败，而是说明服务器还没有返回TCP握手的应答消息，所以注册OP_CONNECT事件，监听消息。
socketChannel.register(selector, SelectionKey.OP_CONNECT); }
private void handleInput(SelectionKey key) throws IOException {
if (key.isValid()) {
// 判断是否连接成功
SocketChannel sc = (SocketChannel) key.channel();
if (key.isConnectable()) {//判断是否有连接事件，是的话，说明未连接
if (sc.finishConnect()) {//再连接
sc.register(selector, SelectionKey.OP_READ);
doWrite(sc);
} else
System.exit(1);// 连接失败，进程退出
}
if (key.isReadable()) {
ByteBuffer readBuffer = ByteBuffer.allocate(1024);
int readBytes = sc.read(readBuffer);
if (readBytes > 0) {
readBuffer.flip();
byte[] bytes = new byte[readBuffer.remaining()];
readBuffer.get(bytes);
String body = new String(bytes, "UTF-8");
System.out.println("Now is : " + body);
this.stop = true;
} else if (readBytes < 0) {
// 对端链路关闭
key.cancel();
sc.close();
}
}
}
}
private void doWrite(SocketChannel sc) throws IOException {
byte[] req = "QUERY TIME ORDER".getBytes();
ByteBuffer writeBuffer = ByteBuffer.allocate(req.length);
writeBuffer.put(req);
writeBuffer.flip();
sc.write(writeBuffer);
if (!writeBuffer.hasRemaining())
System.out.println("Send order 2 server succeed.");
}
}
来源：https://www.cnblogs.com/nazhizq/p/6538708.html