3、netty第二个例子，使用netty建立客户端，与服务端通讯

第一个例子中，建立了http的服务器端，可以直接使用curl命令，或者浏览器直接访问。

在第二个例子中，建立一个netty的客户端来主动发送请求，模拟浏览器发送请求。

这里先启动服务端，再启动客户端，启动客户端后，在 channelActive 方法中，主动向服务器端发送消息，服务器端channelRead0 方法中，接收到客户端的消息后，会再向客户端返回消息。客户端channelRead0方法中接收到消息再向客户端发送消息，依次往复。

同样的，按照顺序

client

 import io.netty.bootstrap.Bootstrap;
 import io.netty.channel.ChannelFuture;
 import io.netty.channel.EventLoopGroup;
 import io.netty.channel.nio.NioEventLoopGroup;
 import io.netty.channel.socket.nio.NioServerSocketChannel;
 import io.netty.channel.socket.nio.NioSocketChannel;

 public class MyClient {
     public static void main(String[] args) throws InterruptedException {
         //客户端只需要一个
         EventLoopGroup eventLoopGroup = new NioEventLoopGroup();

         try{

             Bootstrap bootstrap = new Bootstrap();
             bootstrap.group(eventLoopGroup)
                     .channel(NioSocketChannel.class)
                     .handler(new MyClientInitlalizer());
             ChannelFuture channelFuture = bootstrap.connect("localhost", 8899).sync();

             //channelFuture.channel().writeAndFlush("first msg");//发送数据，其实应该写到handler的active方法中

             channelFuture.channel().closeFuture().sync();

         }finally {
             eventLoopGroup.shutdownGracefully();
         }

     }
 }

Initlalizer

import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.socket.SocketChannel;
import io.netty.handler.codec.LengthFieldBasedFrameDecoder;
import io.netty.handler.codec.LengthFieldPrepender;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;
import io.netty.util.CharsetUtil;

public class MyClientInitlalizer extends ChannelInitializer<SocketChannel> {

    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline channelPipeline = ch.pipeline();

        //添加解码器的handler
        channelPipeline.addLast(new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE,0,4,0,4));

        channelPipeline.addLast(new LengthFieldPrepender(4));
        //字符串的解码器
        channelPipeline.addLast(new StringDecoder(CharsetUtil.UTF_8));
        //字符串的编码器
        channelPipeline.addLast( new StringEncoder(CharsetUtil.UTF_8));
        //自定义的处理器
        channelPipeline.addLast(new MyClientHandler());
    }
}

handler

 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.SimpleChannelInboundHandler;

 import java.time.LocalDateTime;

 public class MyClientHandler  extends SimpleChannelInboundHandler<String> {
     @Override
     protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
         System.out.println( ctx.channel().remoteAddress() + "," + msg);
         System.out.println( "client output:" + msg);

         ctx.writeAndFlush("from client" + LocalDateTime.now());
     }

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

     @Override
     public void channelActive(ChannelHandlerContext ctx) throws Exception {
         ctx.channel().writeAndFlush("来自客户端的问候！");//通道连接之后，发送数据
         super.channelActive(ctx);
     }
 }

sever中的handle

server中的server代码和第一个例子类似，initlializer和client中的一致，就不贴代码了。

public class MyServerHandler extends SimpleChannelInboundHandler<String> {
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
        System.out.println(ctx.channel().remoteAddress() + "," + msg);

        ctx.writeAndFlush("from serevber" + UUID.randomUUID());//消息返回
    }

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