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技术 2022年10月17日
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作者:Grey

原文地址:Java IO学习笔记八:Netty入门

多路复用多线程方式还是有点麻烦,Netty帮我们做了封装,大大简化了编码的复杂度,接下来熟悉一下netty的基本使用。

Netty+最朴素的阻塞的方式来实现一版客户端和服务端通信的代码,然后再重构成Netty官方推荐的写法。

第一步,引入netty依赖包。

<dependency>
<groupId>io.netty</groupId>
<artifactId>netty-all</artifactId>
<version>4.1.65.Final</version>
</dependency>

准备发送端

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioSocketChannel;import java.net.InetSocketAddress;import static java.nio.charset.StandardCharsets.UTF_8;/**
* @author <a href="mailto:410486047@qq.com" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >Grey</a>
* @since
*/
public class NettyClientSync {
public static void main(String[] args) throws Exception {
NioEventLoopGroup thread = new NioEventLoopGroup(1);
NioSocketChannel client = new NioSocketChannel();
thread.register(client);
ChannelPipeline p = client.pipeline();
p.addLast(new MyInHandler());
ChannelFuture connect = client.connect(new InetSocketAddress("192.168.205.138", 9090));
ChannelFuture sync = connect.sync();
ByteBuf buf = Unpooled.copiedBuffer("hello server".getBytes());
ChannelFuture send = client.writeAndFlush(buf);
send.sync();
sync.channel().closeFuture().sync();
System.out.println("client over....");
} static class MyInHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRegistered(ChannelHandlerContext ctx) {
System.out.println("client register...");
} @Override
public void channelActive(ChannelHandlerContext ctx) {
System.out.println("client active...");
} @Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
ByteBuf buf = (ByteBuf) msg;
CharSequence str = buf.getCharSequence(0, buf.readableBytes(), UTF_8);
System.out.println(str);
ctx.writeAndFlush(buf);
}
}
}

这个客户端主要就是给服务端(192.168.205.138:9090)发送数据, 启动一个服务端:

[root@io ~]# nc -l 192.168.205.138 9090

然后启动客户端,服务端可以接收到客户端发来的数据:

[root@io ~]# nc -l 192.168.205.138 9090
hello server

这就是netty实现的一个客户端,再来看服务端的写法:

import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;import java.net.InetSocketAddress;/**
* @author <a href="mailto:410486047@qq.com" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >Grey</a>
* @since
*/
public class NettyServerSync {
public static void main(String[] args) throws Exception {
NioEventLoopGroup thread = new NioEventLoopGroup(1);
NioServerSocketChannel server = new NioServerSocketChannel();
thread.register(server);
ChannelPipeline p = server.pipeline();
p.addLast(new MyAcceptHandler(thread, new NettyClientSync.MyInHandler()));
ChannelFuture bind = server.bind(new InetSocketAddress("192.168.205.1",9090));
bind.sync().channel().closeFuture().sync();
System.out.println("server close....");
} static class MyAcceptHandler extends ChannelInboundHandlerAdapter { private final EventLoopGroup selector;
private final ChannelHandler handler; public MyAcceptHandler(EventLoopGroup thread, ChannelHandler myInHandler) {
this.selector = thread;
this.handler = myInHandler;
} @Override
public void channelRegistered(ChannelHandlerContext ctx) {
System.out.println("server registered...");
} @Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
SocketChannel client = (SocketChannel) msg;
ChannelPipeline p = client.pipeline();
p.addLast(handler);
selector.register(client);
}
}
}

启动这个服务端,然后通过一个客户端来连接这个服务端,并且向这个服务端发送一些数据

[root@io ~]# nc 192.168.205.1 9090
hello
hello

服务端可以感知到客户端连接并接收到客户端发来的数据

client  register...
client active...
hello

但是,这样的服务端如果再接收一个客户端连接,客户端继续发送一些数据进来,服务端就会报一个错误:

An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception.
io.netty.channel.ChannelPipelineException: git.snippets.io.netty.NettyClientSync$MyInHandler is not a @Sharable handler, so can't be added or removed multiple times.

原因在这个博客里面说的比较清楚:Netty ChannelHandler使用报错

我们可以发现每当有新的数据可读时都会往这个channel的pipeline里加入handler,这里加的是childHander。值得注意的是,我们初始化的时候这个childHandler都是同一个实例,也就说会导致不同的channel用了同一个handler,这个从netty的设计角度来说是要避免的。因为netty的一大好处就是每一个channel都有自己绑定的eventloop和channelHandler,这样可以保证代码串行执行,不必考虑并发同步的问题。所以才会有checkMultiplicity这个方法来检查这个问题。那该怎么办呢?netty的这段代码:child.pipeline().addLast(childHandler)就是用了同一个handler啊,怎么才能为每一个channel创建不同的handler呢?

很简单,只要写个类继承ChannelInitializer就行了,ChannelInitializer这个类比较特殊,你可以把它想象成是很多channelhandler的集合体,而且这个类就是@Shareable的,继承了这个类之后你可以为每一个channel单独创建handler,甚至是多个handler。

解决方案也很简单,只需要在服务端传入的handler上加上@Sharable注解即可

@ChannelHandler.Sharable
static class MyInHandler extends ChannelInboundHandlerAdapter{
...
}

但是对于每次服务端的Handler,如果都要加@Sharable,就会非常不好扩展,Netty里面提供了一个没有任何业务功能的并且标注为@Sharable的类:ChannelInitializer, 每个业务handler只需要重写其initChannel()方法即可,我们可以改造一下NettyClientSync和NettyServerSync的代码,并用Netty推荐的写法来修改。

客户端改成:

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.Channel;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioSocketChannel;import java.net.InetSocketAddress;/**
* @author <a href="mailto:410486047@qq.com" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >Grey</a>
* @since
*/
public class NettyClient {
public static void main(String[] args) throws InterruptedException {
NioEventLoopGroup group = new NioEventLoopGroup(1);
Bootstrap bs = new Bootstrap();
ChannelFuture fu = bs
.group(group).channel(NioSocketChannel.class)
.handler(new ChannelInitializer<NioSocketChannel>() {
@Override
protected void initChannel(NioSocketChannel nioSocketChannel) throws Exception {
ChannelPipeline pipeline = nioSocketChannel.pipeline();
pipeline.addLast(new NettyClientSync.MyInHandler());
}
}).connect(new InetSocketAddress("192.168.205.138", 9090));
Channel client = fu.channel();
ByteBuf buf = Unpooled.copiedBuffer("Hello Server".getBytes());
ChannelFuture future = client.writeAndFlush(buf);
future.sync();
}
}

启动一个服务端,然后启动上述客户端代码,服务端可以收到信息

[root@io ~]# nc -l 192.168.205.138 9090
Hello Server

接下来改造服务端代码:

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;import java.net.InetSocketAddress;/**
* @author <a href="mailto:410486047@qq.com" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >Grey</a>
* @since
*/
public class NettyServer {
public static void main(String[] args) throws InterruptedException {
NioEventLoopGroup group = new NioEventLoopGroup(1);
ServerBootstrap bs = new ServerBootstrap();
ChannelFuture bind = bs
.group(group, group)
.channel(NioServerSocketChannel.class)
.childHandler(new ChannelInitializer<NioSocketChannel>() {
@Override
protected void initChannel(NioSocketChannel nioServerSocketChannel) throws Exception {
ChannelPipeline pipeline = nioServerSocketChannel.pipeline();
pipeline.addLast(new NettyClientSync.MyInHandler());
}
}).bind(new InetSocketAddress("192.168.205.1", 9090));
bind.sync().channel().closeFuture().sync();
}
}

启动服务端代码,然后通过客户端连接服务端并发送一些数据:

[root@io ~]# nc 192.168.205.1 9090
sdfasdfas
sdfasdfas

可以正常接收。

源码:Github

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