Netty ChannelHandler使用报错

最近在研究和学习Netty，按照书上的教程写了一个很简单的demo，但是运行却报错了，代码如下：

package com.fan3cn.netty;

import java.net.InetSocketAddress;

import io.netty.bootstrap.ServerBootstrap;

import io.netty.buffer.ByteBuf;

import io.netty.channel.ChannelFuture;

import io.netty.channel.ChannelFutureListener;

import io.netty.channel.ChannelHandlerContext;

import io.netty.channel.SimpleChannelInboundHandler;

import io.netty.channel.nio.NioEventLoopGroup;

import io.netty.channel.socket.nio.NioServerSocketChannel;

public class TestDemo {

//端口号

private static int port = 7777;

public static void main(String[] args) {

// TODO Auto-generated method stub

TestDemo testDemo = new TestDemo();

testDemo.startup();

}

public void startup(){

ServerBootstrap sb = new ServerBootstrap();

sb.group(new NioEventLoopGroup(), new NioEventLoopGroup());

sb.channel(NioServerSocketChannel.class);

sb.childHandler(new MySimpleChannelInboundHandler());

ChannelFuture future = sb.bind(new InetSocketAddress(port));

future.addListener(new ChannelFutureListener() {

@Override

public void operationComplete(ChannelFuture f) throws Exception {

// TODO Auto-generated method stub

if(f.isSuccess()){

System.out.println("bind success,listening on "+port);

}else{

System.out.println("bind failed");

}

});

}

/**

* handler

* @author Eric

public class MySimpleChannelInboundHandler extends SimpleChannelInboundHandler<ByteBuf>{

@Override

protected void channelRead0(ChannelHandlerContext ctx, ByteBuf data) throws Exception {

// TODO Auto-generated method stub

int readIdx = data.readableBytes();

byte[] bytes = data.readBytes(readIdx).array();

String msg = new String(bytes);

System.out.println("recevied data:\n"+msg);

}

启动好了之后，浏览器输入：http://127.0.0.1:7777?x=1&y=2 ,但是令人奇怪的是这个服务器只能接受一次请求，后面的请求均会产生如下的报错：

Oct 24, 2016 9:52:41 PM io.netty.channel.DefaultChannelPipeline$TailHandler exceptionCaught

WARNING: 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: com.fan3cn.netty.TestDemo$MySimpleChannelInboundHandler is not a @Sharable handler, so can't be added or removed multiple times.

at io.netty.channel.DefaultChannelPipeline.checkMultiplicity(DefaultChannelPipeline.java:461)

at io.netty.channel.DefaultChannelPipeline.addLast0(DefaultChannelPipeline.java:138)

at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:131)

at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:258)

at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:245)

at io.netty.bootstrap.ServerBootstrap$ServerBootstrapAcceptor.channelRead(ServerBootstrap.java:232)

at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:372)

at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:357)

at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:780)

at io.netty.channel.nio.AbstractNioMessageChannel$NioMessageUnsafe.read(AbstractNioMessageChannel.java:87)

at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:497)

at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:465)

at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:359)

at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:101)

at java.lang.Thread.run(Thread.java:745)

按照报错的提示，原因就是这个channelHandler没有使用@Shareable注解，加上之后果然问题解决了。但是到这里我的疑问并没有得到解决，为什么这个handler必须要加上@Shareable注解？这一步步到底是怎么回事？经过几次断点，再结合阅读源码，总算是搞清楚为什么了。
首先来看抛出异常的方法DefaultChannelPipeline->checkMultiplicity，代码如下：

private static void checkMultiplicity(ChannelHandlerContext ctx) {

ChannelHandler handler = ctx.handler();

if (handler instanceof ChannelHandlerAdapter) {

ChannelHandlerAdapter h = (ChannelHandlerAdapter) handler;

if (!h.isSharable() && h.added) {

throw new ChannelPipelineException(

h.getClass().getName() +

" is not a @Sharable handler, so can't be added or removed multiple times.");

}

h.added = true;

}

这段代码很简单，如果这个handler被加过了，也就是h.adder=true，并且不是shareable的，就会抛出异常。但是为很么会重复加呢？
再来看ServerBootstrap.java 第232行：

@Override

@SuppressWarnings("unchecked")

public void channelRead(ChannelHandlerContext ctx, Object msg) {

Channel child = (Channel) msg;

child.pipeline().addLast(childHandler);//源代码232行

for (Entry<ChannelOption<?>, Object> e: childOptions) {

try {

if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {

logger.warn("Unknown channel option: " + e);

}

} catch (Throwable t) {

logger.warn("Failed to set a channel option: " + child, t);

}

for (Entry<AttributeKey<?>, Object> e: childAttrs) {

child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());

}

try {

childGroup.register(child);

} catch (Throwable t) {

child.unsafe().closeForcibly();

logger.warn("Failed to register an accepted channel: " + child, t);

}

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。

final class ChannelInitializerImpl extends ChannelInitializer<Channel> {

@Override

protected void initChannel(Channel ch) throws Exception {

ChannelPipeline pipeline = ch.pipeline();

pipeline.addLast(new MySimpleChannelInboundHandler());

//pipeline.addLast(new InboundHandler1());

//pipeline.addLast(new InboundHandler2());

//...

}

和之前那个handler一样，虽然所有的channel都共享了ChannelInitializerImpl这个实例，但是这个实例却能为每一个channel new出新的handler实例，这就是区别。

发表评论 取消回复

发表评论取消回复