Netty原理和使用
Netty是一個高性能 事件驅動的異步的非堵塞的IO(NIO)框架，用于建立TCP等底層的連接，基于Netty可以建立高性能的Http服務器。支持HTTP、 WebSocket 、Protobuf、 Binary TCP |和UDP，Netty已經被很多高性能項目作為其Socket底層基礎，如HornetQ Infinispan Vert.x
Play Framework Finangle和 Cassandra。其競爭對手是：Apache MINA和 Grizzly。
傳統堵塞的IO讀取如下：
InputStream is = new FileInputStream("input.bin");
int byte = is.read(); // 當前線程等待結果到達直至錯誤
而使用NIO如下：
while (true) {
selector.select(); // 從多個通道請求事件
Iterator it = selector.selectedKeys().iterator();
while (it.hasNext()) {
SelectorKey key = (SelectionKey) it.next();
handleKey(key);
it.remove();
}
｝
堵塞與非堵塞原理
傳統硬件的堵塞如下，從內存中讀取數據，然后寫到磁盤，而CPU一直等到磁盤寫完成，磁盤的寫操作是慢的，這段時間CPU被堵塞不能發揮效率。
使用非堵塞的DMA如下圖：CPU只是發出寫操作這樣的指令，做一些初始化工作，DMA具體執行，從內存中讀取數據，然后寫到磁盤，當完成寫后發出一個中斷事件給CPU。這段時間CPU是空閑的，可以做別的事情。這個原理稱為Zero.copy零拷貝。
Netty底層基于上述Java NIO的零拷貝原理實現：
比較
- Tomcat是一個Web服務器，它是采取一個請求一個線程，當有1000客戶端時，會耗費很多內存。通常一個線程將花費 256kb到1mb的stack空間。
- Node.js是一個線程服務于所有請求，在錯誤處理上有限制
- Netty是一個線程服務于很多請求，如下圖，當從Java NIO獲得一個Selector事件，將激活通道Channel。
演示
Netty的使用代碼如下：
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();
通過引入觀察者監聽，當有數據時，將自動激活監聽者中的代碼運行。
我們使用Netty建立一個服務器代碼：
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(newChannelInitializer<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();?
??????? }?
??? }
???
}
這段代碼調用：在9999端口啟動
new?EchoServer(9999).run();
我們需要完成的代碼是EchoServerHandler：
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
一個Netty服務器的原理如下：
圖中每次請求的讀取是通過UpStream來實現，然后激活我們的服務邏輯如EchoServerHandler，而服務器向外寫數據，也就是響應是通過DownStream實現的。每個通道Channel包含一對UpStream和DownStream，以及我們的handlers（EchoServerHandler），如下圖，這些都是通過channel pipeline封裝起來的，數據流在管道里流動，每個Socket對應一個ChannelPipeline。
?
CHANNELPIPELINE是關鍵，它類似Unix的管道，有以下作用：
- 為每個Channel 保留 ChannelHandlers ，如EchoServerHandler
- 所有的事件都要通過它
- 不斷地修改：類似unix的SH管道： echo "Netty is shit...." | sed -e 's/is /is the /'
- 一個Channel對應一個 ChannelPipeline
- 包含協議編碼解碼 安全驗證SSL/TLS和應用邏輯
?
客戶端代碼
前面我們演示了服務器端代碼，下面是客戶端代碼：
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();?
??????? }?
??? }?
}
客戶端的應用邏輯EchoClientHandler：
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();?
??? }
}
?
from:?https://www.jdon.com/concurrent/netty.html
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