Netty入门(1) - 简介

太过爱你忘了你带给我的痛 2021-12-09 05:29 303阅读 0赞

### 什么是Netty？ ###

Netty 是一个利用 Java 的高级网络的能力，隐藏其背后的复杂性而提供一个易于使用的 API 的客户端/服务器框架。

### Tomcat和Netty有什么区别？ ###

Netty和Tomcat最大的区别就在于通信协议，Tomcat是基于Http协议的，他的实质是一个基于http协议的web容器，但是Netty不一样，他能通过编程自定义各种协议，因为netty能够通过codec自己来编码/解码字节流，完成类似redis访问的功能，这就是netty和tomcat最大的不同。

### Netty有什么优点？ ###

并发高

![639336-20180921142620472-2051024090.png][]![639336-20180921143744571-520628241.png][]

传输快

Java的内存有堆内存、栈内存和字符串常量池等等，其中堆内存是占用内存空间最大的一块，也是Java对象存放的地方，一般我们的数据如果需要从IO读取到堆内存，中间需要经过Socket缓冲区，也就是说一个数据会被拷贝两次才能到达他的的终点，如果数据量大，就会造成不必要的资源浪费。Netty针对这种情况，使用了NIO中的另一大特性——**零拷贝**，当他需要接收数据的时候，他会在堆内存之外开辟一块内存，数据就直接从IO读到了那块内存中去，在netty里面通过**ByteBuf**可以直接对这些数据进行直接操作，从而加快了传输速度。

![639336-20180921145453771-657867381.png][]

public class NettyNioServer {
    
        public void server(int port) throws Exception {
            final ByteBuf buf = Unpooled.unreleasableBuffer(
                    Unpooled.copiedBuffer("Hi!\r\n", Charset.forName("UTF-8")));
            EventLoopGroup group = new NioEventLoopGroup();
            try {
                ServerBootstrap b = new ServerBootstrap();       
    
                b.group(group)                                    
                 .channel(NioServerSocketChannel.class)
                 .localAddress(new InetSocketAddress(port))
                 .childHandler(new ChannelInitializer<SocketChannel>() {
                     @Override
                     public void initChannel(SocketChannel ch) 
                         throws Exception {
                         ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
                             @Override
                             public void channelActive(ChannelHandlerContext ctx) throws Exception {
                                 ctx.writeAndFlush(buf.duplicate()).addListener(ChannelFutureListener.CLOSE);
                             }
                         });
                     }
                 });
                ChannelFuture f = b.bind().sync();
                f.channel().closeFuture().sync();
            } finally {
                group.shutdownGracefully().sync();
            }
        }
    }

相关概念

**Channel**，表示一个连接，可以理解为每一个请求，就是一个Channel。

**ChannelHandler**，核心处理业务就在这里，用于处理业务请求。

**ChannelHandlerContext**，用于传输业务数据。

**ChannelPipeline**，用于保存处理过程需要用到的ChannelHandler和ChannelHandlerContext。

**ByteBuf**，是一个存储字节的容器，最大特点就是使用方便，**它既有自己的读索引和写索引**，方便你对整段字节缓存进行读写，也支持get/set，方便你对其中每一个字节进行读写，他的数据结构如下图所示：

![639336-20180921171040756-402122946.png][]

Heap Buffer 堆缓冲区  
　　堆缓冲区是ByteBuf最常用的模式，他将数据存储在堆空间。

Direct Buffer 直接缓冲区  
　　直接缓冲区是ByteBuf的另外一种常用模式，他的内存分配都不发生在堆，jdk1.4引入的nio的ByteBuffer类允许jvm通过本地方法调用分配内存，这样做有两个好处

通过免去中间交换的内存拷贝, 提升IO处理速度; 直接缓冲区的内容可以驻留在垃圾回收扫描的堆区以外。

DirectBuffer 在 -XX:MaxDirectMemorySize=xxM大小限制下, 使用 Heap 之外的内存, GC对此”无能为力”,也就意味着规避了在高负载下频繁的GC过程对应用线程的中断影响.

Composite Buffer 复合缓冲区  
　　复合缓冲区相当于多个不同ByteBuf的视图，这是netty提供的，jdk不提供这样的功能。

**Codec**，Netty中的编码/解码器，通过他你能完成字节与pojo、pojo与pojo的相互转换，从而达到自定义协议的目的。在Netty里面最有名的就是HttpRequestDecoder和HttpResponseEncoder了。

下面用一个简单的例子来入门：

核心业务开发：

package io.netty.example.discard;
    
    import io.netty.buffer.ByteBuf;
    
    import io.netty.channel.ChannelHandlerContext;
    import io.netty.channel.ChannelInboundHandlerAdapter;
    
    /**
     * Handles a server-side channel.
     */
    public class DiscardServerHandler extends ChannelInboundHandlerAdapter { // (1)
    
        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) { // (2)
            // Discard the received data silently.
            ((ByteBuf) msg).release(); // (3)
        }
    
        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { // (4)
            // Close the connection when an exception is raised.
            cause.printStackTrace();
            ctx.close();
        }
    }

注意，Bytebuf是一个引用计数的对象，使用完后需要finally块手动释放，如：

ReferenceCountUtil.release(msg);
    
    或者

((ByteBuf) msg).release();

启动Server：

package io.netty.example.discard;
        
    import io.netty.bootstrap.ServerBootstrap;
    
    import io.netty.channel.ChannelFuture;
    import io.netty.channel.ChannelInitializer;
    import io.netty.channel.ChannelOption;
    import io.netty.channel.EventLoopGroup;
    import io.netty.channel.nio.NioEventLoopGroup;
    import io.netty.channel.socket.SocketChannel;
    import io.netty.channel.socket.nio.NioServerSocketChannel;
        
    /**
     * Discards any incoming data.
     */
    public class DiscardServer {
        
        private int port;
        
        public DiscardServer(int port) {
            this.port = port;
        }
        
        public void run() throws Exception {
            EventLoopGroup bossGroup = new NioEventLoopGroup(); // (1)
            EventLoopGroup workerGroup = new NioEventLoopGroup();
            try {
                ServerBootstrap b = new ServerBootstrap(); // (2)
                b.group(bossGroup, workerGroup)
                 .channel(NioServerSocketChannel.class) // (3)
                 .childHandler(new ChannelInitializer<SocketChannel>() { // (4)
                     @Override
                     public void initChannel(SocketChannel ch) throws Exception {
                         ch.pipeline().addLast(new DiscardServerHandler());
                     }
                 })
                 .option(ChannelOption.SO_BACKLOG, 128)          // (5)
                 .childOption(ChannelOption.SO_KEEPALIVE, true); // (6)
        
                // Bind and start to accept incoming connections.
                ChannelFuture f = b.bind(port).sync(); // (7)
        
                // Wait until the server socket is closed.
                // In this example, this does not happen, but you can do that to gracefully
                // shut down your server.
                f.channel().closeFuture().sync();
            } finally {
                workerGroup.shutdownGracefully();
                bossGroup.shutdownGracefully();
            }
        }
        
        public static void main(String[] args) throws Exception {
            int port;
            if (args.length > 0) {
                port = Integer.parseInt(args[0]);
            } else {
                port = 8080;
            }
            new DiscardServer(port).run();
        }
    }

`option()` is for the `NioServerSocketChannel` that accepts incoming connections.

`childOption()` is for the `Channel`s accepted by the parent `ServerChannel`, which is `NioServerSocketChannel` in this case.

我们来打印一下内容：

public class BussinessHandler extends ChannelInboundHandlerAdapter {
    
        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            super.channelRead(ctx, msg);
            try {
                ByteBuf in = (ByteBuf) msg;
                System.out.println(in.toString(CharsetUtil.US_ASCII));
            } finally {
                ReferenceCountUtil.release(msg);
            }
            ((ByteBuf) msg).release();  // ReferenceCountUtil.release(msg);
        }
    
        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            super.exceptionCaught(ctx, cause);
            cause.printStackTrace();
            ctx.close();
        }
    }

回应消息：

@Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) {
            ctx.write(msg); // (1)
            ctx.flush(); // (2)
    　　　　　// ctx.writeAndFlush(msg);
        }

package io.netty.example.time;
    
    public class TimeServerHandler extends ChannelInboundHandlerAdapter {
    
        @Override
        public void channelActive(final ChannelHandlerContext ctx) { // (1)
            final ByteBuf time = ctx.alloc().buffer(4); // (2)
           time.writeInt((int) (System.currentTimeMillis() / 1000L + 2208988800L)); final ChannelFuture f = ctx.writeAndFlush(time); // (3)
            f.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) {
                    assert f == future;
                    ctx.close(); // it returns a ChannelFuture.
                }
            }); // (4)
        }
        
        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
            cause.printStackTrace();
            ctx.close();
        }
    }

写一个客户端：

public class TimeClientHandler extends ChannelInboundHandlerAdapter { @Override public void channelRead(ChannelHandlerContext ctx, Object msg) { ByteBuf m = (ByteBuf) msg; // (1) try { long currentTimeMillis = (m.readUnsignedInt() - 2208988800L) * 1000L; System.out.println(new Date(currentTimeMillis)); ctx.close(); } finally { m.release(); } } @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { cause.printStackTrace(); ctx.close(); } }

public class TimeClient {
        public static void main(String[] args) throws Exception {
            String host = args[0];
            int port = Integer.parseInt(args[1]);
            EventLoopGroup workerGroup = new NioEventLoopGroup();
            
            try {
                Bootstrap b = new Bootstrap(); // (1)
                b.group(workerGroup); // (2)
                b.channel(NioSocketChannel.class); // (3)
                b.option(ChannelOption.SO_KEEPALIVE, true); // (4)
                b.handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new TimeClientHandler());
                    }
                });
                
                // Start the client.
                ChannelFuture f = b.connect(host, port).sync(); // (5)
    
                // Wait until the connection is closed.
                f.channel().closeFuture().sync();
            } finally {
                workerGroup.shutdownGracefully();
            }
        }
    }

关于拆包和黏包的解决：

public class TimeClientHandler extends ChannelInboundHandlerAdapter {
        private ByteBuf buf;
        
        @Override
        public void handlerAdded(ChannelHandlerContext ctx) {
            buf = ctx.alloc().buffer(4); // (1)
        }
        
        @Override
        public void handlerRemoved(ChannelHandlerContext ctx) {
            buf.release(); // (1)
            buf = null;
        }
        
        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) {
            ByteBuf m = (ByteBuf) msg;
            buf.writeBytes(m); // (2)
            m.release();
            
            if (buf.readableBytes() >= 4) { // (3)
                long currentTimeMillis = (buf.readUnsignedInt() - 2208988800L) * 1000L;
                System.out.println(new Date(currentTimeMillis));
                ctx.close();
            }
        }
        
        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
            cause.printStackTrace();
            ctx.close();
        }
    }

package io.netty.example.time;
    
    public class TimeDecoder extends ByteToMessageDecoder { // (1)
        @Override
        protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) { // (2)
            if (in.readableBytes() < 4) {
                return; // (3)
            }
            
            out.add(in.readBytes(4)); // (4)
        }
    }

public class TimeDecoder extends ReplayingDecoder<Void> {
        @Override
        protected void decode(
                ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
            out.add(in.readBytes(4));
        }
    }

传输POJO对象：

定义对象

public class UnixTime {
    
        private final long value;
        
        public UnixTime() {
            this(System.currentTimeMillis() / 1000L + 2208988800L);
        }
        
        public UnixTime(long value) {
            this.value = value;
        }
            
        public long value() {
            return value;
        }
            
        @Override
        public String toString() {
            return new Date((value() - 2208988800L) * 1000L).toString();
        }
    }

client定义decoder：

@Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
        if (in.readableBytes() < 4) {
            return;
        }
        out.add(new UnixTime(in.readUnsignedInt()));
    }

client定义handler：

@Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        UnixTime m = (UnixTime) msg;
        System.out.println(m);
        ctx.close();
    }

server定义handler：

@Override
    public void channelActive(ChannelHandlerContext ctx) {
        ChannelFuture f = ctx.writeAndFlush(new UnixTime());
        f.addListener(ChannelFutureListener.CLOSE);
    }

server定义encoder：

public class TimeEncoder extends MessageToByteEncoder<UnixTime> {
        @Override
        protected void encode(ChannelHandlerContext ctx, UnixTime msg, ByteBuf out) {
            out.writeInt((int)msg.value());
        }
    }

停止应用：

EventLoopGroup bossGroup = new NioEventLoopGroup(); // (1)
            EventLoopGroup workerGroup = new NioEventLoopGroup();
            try {
                ServerBootstrap b = new ServerBootstrap(); // (2)
                b.group(bossGroup, workerGroup)
                 .channel(NioServerSocketChannel.class) // (3)
                 .childHandler(new ChannelInitializer<SocketChannel>() { // (4)
                     @Override
                     public void initChannel(SocketChannel ch) throws Exception {
                         ch.pipeline().addLast(new DiscardServerHandler());
                     }
                 })
                 .option(ChannelOption.SO_BACKLOG, 128)          // (5)
                 .childOption(ChannelOption.SO_KEEPALIVE, true); // (6)
        
                // Bind and start to accept incoming connections.
                ChannelFuture f = b.bind(port).sync(); // (7)
        
                // Wait until the server socket is closed.
                // In this example, this does not happen, but you can do that to gracefully
                // shut down your server.
                f.channel().closeFuture().sync();
            } finally {
         
    　　　　　　　// 异步的，返回future
     workerGroup.shutdownGracefully(); bossGroup.shutdownGracefully();
            }

转载于:https://www.cnblogs.com/ijavanese/p/5175913.html

[639336-20180921142620472-2051024090.png]: https://img2018.cnblogs.com/blog/639336/201809/639336-20180921142620472-2051024090.png
[639336-20180921143744571-520628241.png]: /images/20211209/998d5a56575b45ad92165bbde7963ac0.png
[639336-20180921145453771-657867381.png]: /images/20211209/bdb1977e92874b8d8d0cab0c9d0befb9.png
[639336-20180921171040756-402122946.png]: /images/20211209/4882daf3c4ae4755bdc5252f536b8b6a.png