一、简介
ChannelPipeline是事件(event)的通道，它用于组织事件拦截器ChannelHandler。所以本质上，它只是ChannelHandler的一个链表。从I/O端获取的数据将以事件的方式通过ChannelPipeline，并被ChannelUpstreamHandler拦截，最终被程序的业务逻辑处理。要发送的数据通过pipeline被ChannelDowmstreamHandler拦截，并最终到达ChannelSink，由底层nio处理。
二、ChannelHandlerContext
ChannelHandlerContext是ChannelHandler的包装，使其能以链表的方式存储在ChannelPipeline中：
public class DefaultChannelPipeline implements ChannelPipeline {
private class DefaultChannelHandlerContext implements ChannelHandlerContext {
volatile DefaultChannelHandlerContext next;
volatile DefaultChannelHandlerContext prev;
private final String name;
private final ChannelHandler handler;
public void sendDownstream(ChannelEvent e) {
// 找到从prev往前第一个canHandleDownstream的handler
DefaultChannelHandlerContext prev = getActualDownstreamContext(this.prev);
if (prev == null) {
try {
// 若已到达pipeline的底端，则交由ChannelSink处理
getSink().eventSunk(DefaultChannelPipeline.this, e);
} catch (Throwable t) {
notifyHandlerException(e, t);
}
} else {
DefaultChannelPipeline.this.sendDownstream(prev, e);
}
}
public void sendUpstream(ChannelEvent e) {
DefaultChannelHandlerContext next = getActualUpstreamContext(this.next);
if (next != null) {
DefaultChannelPipeline.this.sendUpstream(next, e);
}
}
....
}
....
}三、ChannelPipeline
ChannelPipeline以链表的方式组织ChannelHandler，提供事件的上下行传输：
public class DefaultChannelPipeline implements ChannelPipeline {
private volatile Channel channel;
private volatile ChannelSink sink;
private volatile DefaultChannelHandlerContext head;
private volatile DefaultChannelHandlerContext tail;
public synchronized void addLast(String name, ChannelHandler handler) {
if (name2ctx.isEmpty()) {
init(name, handler);
} else {
checkDuplicateName(name);
// 将新加入的handler放置在链表的底端
DefaultChannelHandlerContext oldTail = tail;
DefaultChannelHandlerContext newTail = new DefaultChannelHandlerContext(oldTail,
null, name, handler);
oldTail.next = newTail;
tail = newTail;
name2ctx.put(name, newTail);
}
}
public void sendUpstream(ChannelEvent e) {
//找到从head开始第一个canHandleUpStream的handler
DefaultChannelHandlerContext head = getActualUpstreamContext(this.head);
....
sendUpstream(head, e);
}
void sendUpstream(DefaultChannelHandlerContext ctx, ChannelEvent e) {
try {
((ChannelUpstreamHandler) ctx.getHandler()).handleUpstream(ctx, e);
} catch (Throwable t) {
notifyHandlerException(e, t);
}
}
// 以类似方式实现sendDownstream
....
}四、ChannelHandler
ChannelHandler是事件的拦截器，也是系统主要的扩展点，用户一般通过继承SimpleChannelHandler来实现自己的协议解析方案：
public class SimpleChannelHandler implements
ChannelUpstreamHandler, ChannelDownstreamHandler {
// 上行事件的分发函数，根据事件的不同，调用不同的方法
public void handleUpstream(
ChannelHandlerContext ctx, ChannelEvent e) throws Exception {
if (e instanceof MessageEvent) {
messageReceived(ctx, (MessageEvent) e);
} else if (e instanceof ChildChannelStateEvent) {
ChildChannelStateEvent evt = (ChildChannelStateEvent) e;
if (evt.getChildChannel().isOpen()) {
childChannelOpen(ctx, evt);
} else {
childChannelClosed(ctx, evt);
}
} else if (e instanceof ChannelStateEvent) {
ChannelStateEvent evt = (ChannelStateEvent) e;
switch (evt.getState()) {
case OPEN:
if (Boolean.TRUE.equals(evt.getValue())) {
channelOpen(ctx, evt);
} else {
channelClosed(ctx, evt);
}
break;
case BOUND:
if (evt.getValue() != null) {
channelBound(ctx, evt);
} else {
channelUnbound(ctx, evt);
}
break;
case CONNECTED:
if (evt.getValue() != null) {
channelConnected(ctx, evt);
} else {
channelDisconnected(ctx, evt);
}
break;
case INTEREST_OPS:
channelInterestChanged(ctx, evt);
break;
default:
ctx.sendDownstream(e);
}
} else if (e instanceof ExceptionEvent) {
exceptionCaught(ctx, (ExceptionEvent) e);
} else {
ctx.sendUpstream(e);
}
}
....
}