Netty In Action中文版 - 第六章：ChannelHandler

• addFirst(...)，加入ChannelHandler在ChannelPipeline的第一个位置
  
• addBefore(...)，在ChannelPipeline中指定的ChannelHandler名称之前加入ChannelHandler
  
• addAfter(...)，在ChannelPipeline中指定的ChannelHandler名称之后加入ChannelHandler
  
• addLast(ChannelHandler...)，在ChannelPipeline的末尾加入ChannelHandler
  
• remove(...)，删除ChannelPipeline中指定的ChannelHandler
  
• replace(...)，替换ChannelPipeline中指定的ChannelHandler
  

ChannelPipeline pipeline = ch.pipeline();
FirstHandler firstHandler = new FirstHandler();
pipeline.addLast("handler1", firstHandler);
pipeline.addFirst("handler2", new SecondHandler());
pipeline.addLast("handler3", new ThirdHandler());
pipeline.remove("?handler3?");
pipeline.remove(firstHandler);
pipeline.replace("handler2", "handler4", new FourthHandler());

• 调用Channel的方法
  
• 调用ChannelPipeline的方法
  

@Override
protected void initChannel(SocketChannel ch) throws Exception {
	ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
		@Override
		public void channelActive(ChannelHandlerContext ctx) throws Exception {
			//Event via Channel
			Channel channel = ctx.channel();
			channel.write(Unpooled.copiedBuffer("netty in action", CharsetUtil.UTF_8));
			//Event via ChannelPipeline
			ChannelPipeline pipeline = ctx.pipeline();
			pipeline.write(Unpooled.copiedBuffer("netty in action", CharsetUtil.UTF_8));
		}
	});
}

• 为了节省开销，不感兴趣的ChannelHandler不让通过
  
• 排除一些ChannelHandler
  

// Get reference of ChannelHandlerContext
ChannelHandlerContext ctx = ..;
// Write buffer via ChannelHandlerContext
ctx.write(Unpooled.copiedBuffer("Netty in Action", CharsetUtil.UTF_8));

public class WriteHandler extends ChannelHandlerAdapter {
	private ChannelHandlerContext ctx;

	@Override
	public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
		this.ctx = ctx;
	}
	
	public void send(String msg){
		ctx.write(msg);
	}
}

@Sharable
public class NotSharableHandler extends ChannelInboundHandlerAdapter {

	private int count;

	@Override
	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
		count++;
		System.out.println("channelRead(...) called the " + count + " time?");
		ctx.fireChannelRead(msg);
	}
	
}

• channelUnregistered
  
• channelRegistered
  
• channelActive
  
• channelInactive
  

• handlerAdded，ChannelHandler加入到实际上下文中准备处理事件
  
• handlerRemoved，将ChannelHandler从实际上下文中删除，不再处理事件
  
• exceptionCaught，处理抛出的异常
  

• channelRegistered，ChannelHandlerContext的Channel被注冊到EventLoop；
  
• channelUnregistered，ChannelHandlerContext的Channel从EventLoop中注销
  
• channelActive，ChannelHandlerContext的Channel已激活
  
• channelInactive，ChannelHanderContxt的Channel结束生命周期
  
• channelRead，从当前Channel的对端读取消息
  
• channelReadComplete，消息读取完毕后运行
  
• userEventTriggered，一个用户事件被处罚
  
• channelWritabilityChanged，改变通道的可写状态，能够使用Channel.isWritable()检查
  
• exceptionCaught，重写父类ChannelHandler的方法，处理异常
  

/**
 * 实现ChannelInboundHandlerAdapter的Handler，不会自己主动释放接收的消息对象
 * @author c.k
 *
 */
public class DiscardHandler extends ChannelInboundHandlerAdapter {
	@Override
	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
		//手动释放消息
		ReferenceCountUtil.release(msg);
	}
}

/**
 * 继承SimpleChannelInboundHandler，会自己主动释放消息对象
 * @author c.k
 *
 */
public class SimpleDiscardHandler extends SimpleChannelInboundHandler<Object> {
	@Override
	protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception {
		//不须要手动释放
	}
}

6.4.3 ChannelOutboundHandler

• bind，Channel绑定本地地址
  
• connect，Channel连接操作
  
• disconnect，Channel断开连接
  
• close，关闭Channel
  
• deregister，注销Channel
  
• read，读取消息，实际是截获ChannelHandlerContext.read()
  
• write，写操作，实际是通过ChannelPipeline写消息，Channel.flush()属性到实际通道
  
• flush，刷新消息到通道
  

public class DiscardOutboundHandler extends ChannelOutboundHandlerAdapter {
	@Override
	public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
		ReferenceCountUtil.release(msg);
		promise.setSuccess();
	}
}