上一节中介绍的java NIO的开发，回顾下NIO开发的步骤:
1、创建ServerSocketChannel并设置为非阻塞模式
2、绑定监听端口
3、创建多路服务器Selector，将创建的ServerSocketChannel注册到Selector,监听SelectKey.Accept事件
4、创建IO线程轮询Selector.select
5、如果轮询到就绪的Channel如果是OP_ACCEPT状态说明是新的客户端接入则调用ServerSocketChannel.accept方法接受新的客户端
6、设置客户端链路SocketChannel为非阻塞模式，并且将其注册到Selector上，监听SelectKey.OP_READ操作位
7、如果轮询到Channel为OP_READ,则说明SocketChannel中有新的就绪的数据包需要读取，则通过ByteBuffer读取数据
由上来看,直接通过Java原生的类进行NIO编程非常复杂繁琐，而Netty正好解决了这个难题
下面通过一个简单的Netty程序了解Netty开发
netty服务端代码：
public class TimeServer {
public void bind(int port) throws Exception {
//创建两个线程组 一个用于服务端接收客户端的连接
EventLoopGroup bossGroup = new NioEventLoopGroup();
//一个用于网络读写
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, 1024)
.childHandler(new ChildChannelHander());
ChannelFuture future = b.bind(port).sync();
future.channel().closeFuture().sync();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
private class ChildChannelHander extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new TimeServerHandler());
}
}
public static void main(String[] args) {
int port = 8080;
if (args != null && args.length > 0) {
try {
port = Integer.valueOf(args[0]);
} catch (NumberFormatException e) {
}
}
try {
new TimeServer().bind(port);
} catch (Exception e) {
e.printStackTrace();
}
}
}
public class TimeServerHandler extends ChannelHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx,Object msg) throws Exception{
ByteBuf byteBuf = (ByteBuf)msg;
byte[] req = new byte[byteBuf.readableBytes()];
byteBuf.readBytes(req);
String body = new String(req,"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";
ByteBuf resp = Unpooled.copiedBuffer(currentTIme.getBytes());
ctx.write(resp);
}
@Override
public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
ctx.flush();
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
ctx.close();
}
}
程序刚开始创建了两个EventLoopGroup线程，一个专门监听服务端接收客户端请求，一个专门处理业务逻辑，之后创建一个ServerBootstrap类,并将线程组传递到ServerBootstrap，设置的Channel为NioServerSocketChannel并将事件处理类设置为ChildChannelHander，最后调用bind方法绑定监听端口，最后future.channel().closeFuture().sync()会阻塞直到服务端断开连接
客户端代码：
public class TimeClient {
public void connect(int port,String host) throws Exception {
//创建读写io线程组
EventLoopGroup group = new NioEventLoopGroup();
try {
Bootstrap b = new Bootstrap();
b.group(group).channel(NioSocketChannel.class)
.option(ChannelOption.TCP_NODELAY,true)
.handler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel socketChannel) throws Exception {
System.out.println("1");
socketChannel.pipeline().addLast(new TimeClientHandler());
}
});
System.out.println("2");
ChannelFuture f = b.connect(host,port).sync();
System.out.println("3");
f.channel().closeFuture().sync();
System.out.println("4");
} finally {
group.shutdownGracefully();
}
}
public static void main(String[] args) {
int port = 8080;
if (args != null && args.length >0) {
try {
port = Integer.valueOf(args[0]);
}catch (NumberFormatException e) {
}
}
try {
new TimeClient().connect(port,"127.0.0.1");
} catch (Exception e) {
e.printStackTrace();
}
}
}
Netty客户端代码更为简单，第一步创建NioEventLoopGroup线程组，跟服务端一样再创建客户端辅助类Bootstrap并将线程组和渠道作为参数出传入，并且指定handler实现initChannel方法，其作用就是当创建NioSocketChannel之后回调initChannel方法处理网络IO事件
下面是TimeChientHander代码
public class TimeClientHandler extends ChannelHandlerAdapter {
private final ByteBuf firstMsg;
public TimeClientHandler() {
byte[] req = "QUERY TIME ORDER".getBytes();
firstMsg = Unpooled.buffer(req.length);
firstMsg.writeBytes(req);
}
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
ctx.writeAndFlush(firstMsg);
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
ByteBuf buf = (ByteBuf) msg;
byte[] req = new byte[buf.readableBytes()];
buf.readBytes(req);
String body = new String(req,"UTF-8");
System.out.println("Now is : "+body);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println("Unexpected exception from downstream :"+cause.getMessage());
ctx.close();
}
}
TimeChientHander实现了channelActive,channelRead,exceptionCaught方法，
channelActive为客户端或者服务端TCP链路建立连接之后回调，当服务端写数据到客户端时channelRead回调，当发生异常时会回调exceptionCaught方法。
从上面代码可以看出Netty将IO事件的处理抽象为ChannelHandler接口,开发者可以继承ChannelHandler接口来自定义事件处理逻辑。
ChannelHandler接口主要有以下方法：
channelRegistered(ChannelHandlerContext ctx) 注册到EventLoop成功回调
channelActive(ChannelHandlerContext ctx) 当建立连接后回调
channelRead(ChannelHandlerContext ctx, Object msg) 当前channel读到数据时回调
channelReadComplete(ChannelHandlerContext ctx) 读取数据完成后回调
exceptionCaught(ChannelHandlerContext ctx, Throwable cause) 发生异常时回调
在实际Netty开发中基本都是继承ChannelHandler实现业务逻辑
在初始化Channel时需要将自定义的ChannelHandler添加到该Channel上，Netty实现是ChannelPipeline.addLast
源码如下
public interface ChannelPipeline extends Iterable<Entry<String, ChannelHandler>> {
……
}
其中继承Iterable迭代器接口，可以看出其hannelPipeline是一系列ChannelHandler的集合（可以理解为链表）自定义的ChannelHandler可以调用它的addFirst、addLast添加到IO处理pipeLine的头或者尾
主要方法有以下：
ChannelPipeline addFirst(String name, ChannelHandler handler) 将handler插入到handler链表头部
ChannelPipeline addLast(ChannelHandler... handlers) 将handler插入到handler链表尾部
自动以的ChannelHandler一般通过上述方法添加到事件处理链表中
