Netty是一个高性能事件驱动的异步的非阻塞的IO（NIO）框架，用于建立TCP等底层的连接，基于Netty可以建立高性能的Http服务器。支持HTTP、WebSocket、Protobuf、Binary TCP和UDP，Netty已经被很多高性能项目作为其Socket底层基础，如PlayFamework和Cassandra。其对手是：Apache Mina和Grizzly。
传统的阻塞IO读取入下：
InputStream is=new FileInputStream("data.bin");
int byte=is.read();//当前线程等待结果直接读取错误
而使用NIO如下：
while (true) {
selector.select(); // 从多个通道请求事件
Iterator it = selector.selectedKeys().iterator();
while (it.hasNext()) {
SelectorKey key = (SelectionKey) it.next();
handleKey(key);
it.remove();
}
｝
堵塞与非堵塞原理
传统硬件的堵塞如下，从内存中读取数据，然后写到磁盘，而CPU一直等到磁盘写完成，磁盘的写操作是慢的，这段时间CPU被堵塞不能发挥效率。
堵塞与非堵塞原理
使用非堵塞的DMA如下图：CPU只是发出写操作这样的指令，做一些初始化工作，DMA具体执行，从内存中读取数据，然后写到磁盘，当完成写后发出一个中断事件给CPU。这段时间CPU是空闲的，可以做别的事情。这个原理称为Zero.copy零拷贝。
非堵塞的DMA
Netty底层基于上述Java NIO的零拷贝原理实现：
Java NIO的零拷贝原理实现
比较
Tomcat是一个Web服务器，它是采取一个请求一个线程，当有1000客户端时，会耗费很多内存。通常一个线程将花费 256kb到1mb的stack空间。
Node.js是一个线程服务于所有请求，在错误处理上有限制
Netty是一个线程服务于很多请求，如下图，当从Java NIO获得一个Selector事件，将激活通道Channel。
netty
Netty的使用代码如下：
Channel channel = ...
ChannelFuture cf = channel.write(data);
cf.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if(!future.isSuccess() {
future.cause().printStacktrace();
...
}
...
}
}
);
...
cf.sync();
通过引入观察者监听，当有数据时，将自动激活监听者中的代码运行。
我们使用Netty建立一个服务器代码：
package com.veione;
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;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
public class EchoServer {
private final int port;
public EchoServer(int port) {
this.port = port;
}
public void run() throws Exception {
// Configure the server.
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 100)
.handler(new LoggingHandler(LogLevel.INFO))
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch)
throws Exception {
ch.pipeline().addLast(
// new LoggingHandler(LogLevel.INFO),
new EchoServerHandler());
}
});
// Start the server.
ChannelFuture f = b.bind(port).sync();
// Wait until the server socket is closed.
f.channel().closeFuture().sync();
} finally {
// Shut down all event loops to terminate all threads.
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}
这段代码调用：在9999端口启动
new EchoServer(9999).run();
我们需要完成的代码是EchoServerHandler：
package com.veione;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import java.util.logging.Level;
import java.util.logging.Logger;
public class EchoServerHandler extends ChannelInboundHandlerAdapter {
private static final Logger logger = Logger.getLogger(EchoServerHandler.class.getName());
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
ctx.write(msg);
}
@Override
public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
ctx.flush();
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
// Close the connection when an exception is raised.
logger.log(Level.WARNING, "Unexpected exception from downstream.",cause);
ctx.close();
}
}
原理
一个Netty服务器的原理如下：
Netty服务器的原理
图中每次请求的读取是通过UpStream来实现，然后激活我们的服务逻辑如EchoServerHandler，而服务器向外写数据，也就是响应是通过DownStream实现的。每个通道Channel包含一对UpStream和DownStream，以及我们的handlers（EchoServerHandler），如下图，这些都是通过channel pipeline封装起来的，数据流在管道里流动，每个Socket对应一个ChannelPipeline。
channelpiple
CHANNELPIPELINE是关键，它类似Unix的管道，有以下作用：
为每个Channel 保留 ChannelHandlers ，如EchoServerHandler
所有的事件都要通过它
不断地修改：类似unix的SH管道： echo "Netty is shit...." | sed -e 's/is /is the /'
一个Channel对应一个 ChannelPipeline
包含协议编码解码 安全验证SSL/TLS和应用逻辑
客户端代码
前面我们演示了服务器端代码，下面是客户端代码：
package com.veione;
import io.netty.bootstrap.Bootstrap;
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.NioSocketChannel;
public class EchoClient {
private final String host;
private final int port;
private final int firstMessageSize;
public EchoClient(String host, int port, int firstMessageSize) {
this.host = host;
this.port = port;
this.firstMessageSize = firstMessageSize;
}
public void run() throws Exception {
// Configure the client.
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
public void initChannel(SocketChannel ch)
throws Exception {
ch.pipeline().addLast(
// new LoggingHandler(LogLevel.INFO),
new EchoClientHandler(firstMessageSize));
}
});
// Start the client.
ChannelFuture f = b.connect(host, port).sync();
// Wait until the connection is closed.
f.channel().closeFuture().sync();
} finally {
// Shut down the event loop to terminate all threads.
group.shutdownGracefully();
}
}
}
客户端的应用逻辑EchoClientHandler：
package com.veione;
import java.util.logging.Level;
import java.util.logging.Logger;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
public class EchoClientHandler extends ChannelInboundHandlerAdapter {
private static final Logger logger = Logger.getLogger(EchoClientHandler.class.getName());
private final ByteBuf firstMessage;
/**
* Creates a client-side handler.
*/
public EchoClientHandler(int firstMessageSize) {
if (firstMessageSize <= 0) {
throw new IllegalArgumentException("firstMessageSize: "+ firstMessageSize);
}
firstMessage = Unpooled.buffer(firstMessageSize);
for (int i = 0; i < firstMessage.capacity(); i++) {
firstMessage.writeByte((byte) i);
}
}
@Override
public void channelActive(ChannelHandlerContext ctx) {
ctx.writeAndFlush(firstMessage);
System.out.print("active");
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
ctx.write(msg);
System.out.print("read");
}
@Override
public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
ctx.flush();
System.out.print("readok");
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
// Close the connection when an exception is raised.
logger.log(Level.WARNING, "Unexpected exception from downstream.",cause);
ctx.close();
}
}
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