JAVA netty 简单使用

实现一个功能，客户端和服务器 轮流对一个数加+1

服务器

public class Server {
	public static void main(String[] args) {
		NioEventLoopGroup boss=new NioEventLoopGroup(1);
		NioEventLoopGroup worker=new NioEventLoopGroup(3);
		try {
			final ServerBootstrap server=new ServerBootstrap();
			server.group(boss,worker).channel(NioServerSocketChannel.class)
					.childHandler(new ChannelInitializer<SocketChannel>() {
						@Override
						protected void initChannel(SocketChannel ch) throws Exception {
							ChannelPipeline pipeline = ch.pipeline();
							pipeline.addLast(new ServerHandler());
						}
					});
			ChannelFuture future = server.bind(8881).sync();
			future.channel().closeFuture().sync();
		}catch (Exception e){
			e.printStackTrace();
		}
		finally {
			boss.shutdownGracefully();
			worker.shutdownGracefully();
		}
	}
}

服务器handler

netty ChannelHandler 类似 spring mvc的filter，使用的是责任链模式，可以对客户端传来的数据进行层层解析，解码等操作。

在没有任何特殊操作下，默认传递在责任中的对象是ByteBuf

public class ServerHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {//msg 其实就是个ByteBuf 对象
        ByteBuf buf=(ByteBuf) msg;
        int i = buf.readInt();
        System.out.println("服务器收到客户端消息"+ctx.channel().remoteAddress()+" "+i);
        ByteBuf newbuf=ctx.alloc().buffer(1024);
        newbuf.writeInt(i+1);
        ctx.writeAndFlush(newbuf);
        buf.release();
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channel active");
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }
}

客户端

public class Client {
    public static void main(String[] args) {
        NioEventLoopGroup boss=new NioEventLoopGroup(1);
        try {
            final Bootstrap client=new Bootstrap();
            client.group(boss).channel(NioSocketChannel.class)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast(new ClientHandler());
                        }
                    });
            ChannelFuture future = client.connect("127.0.0.1",8881).sync();
            future.channel().closeFuture().sync();
        }catch (Exception e){
            e.printStackTrace();
        }
        finally {
            boss.shutdownGracefully();
        }
    }
}

客户端handlr

public class ClientHandler extends SimpleChannelInboundHandler {
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception {
        ByteBuf buf=(ByteBuf)msg;
        int i=buf.readInt();
        System.out.println("客服端收到服务器"+ctx.channel().remoteAddress()+"---> "+i);
        Thread.sleep(2000);
        ByteBuf newbuf = ctx.alloc().buffer(1024);
        newbuf.writeInt(i+1);
        ctx.writeAndFlush(newbuf);
    }

    @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        System.out.println("连接服务器成功");
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        ByteBuf buffer = ctx.alloc().buffer(1024);
        buffer.writeInt(1);
        ctx.writeAndFlush(buffer);
    }
}

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一开始很疑惑，channelRead0(ChannelHandlerContext ctx, Object msg) 方法里的msg 到底是个什么对象？
在添加其它handler之后，比如一个StringDecode，netty会自动的把ByteBuf里的字节转String，并传递在Object对象中。这是Object就是一个String。
翻了下源码，还是很清晰的
在ChannelHandlerContext 接口中有一个fireChannelRead 方法，这个方法的解释如下
> ChannelHandlerContext fireChannelRead(java.lang.Object msg)
Description copied from interface: ChannelInboundInvoker
A Channel received a message. This will result in having the ChannelInboundHandler.channelRead(ChannelHandlerContext, Object) method called of the next ChannelInboundHandler contained in the ChannelPipeline of the Channel.
Specified by:
fireChannelRead in interface ChannelInboundInvoker

简单来就是会调用handler 责任链的下一个处理类。同过fireChannelRead方法，可以给下一个责任链传递一个对象msg，这个msg会传递到下一个处理类的 ChannelRead方法上。例如，StringDecode把ByteBuf转为一个String对象msg，在调用fireChannelRead(msg),就把这个String传到下一个handler去了。

channelRead0也是同样的意思

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netty是基于nio的框架，要是理解了nio，其实操作起来也不是很费劲。