接口 ChannelHandlerContext 提供的功能是统一管理这些ChannelHandler，使其能够和这个ChannelPipeline及其他Handler进行交互，一个handler可以往上流或者下流发送一个ChannelEvent，动态修改其所属的Channelpipeline。
充当链表节点的 DefaultChannelHandlerContext 里面的重要成员有： 链表指针next , prev, 以及是否能够处理downStream，upStream事件的布尔变量。
private final class DefaultChannelHandlerContext implements ChannelHandlerContext {
volatile DefaultChannelHandlerContext next;
volatile DefaultChannelHandlerContext prev;
private final String name ;
private final ChannelHandler handler;
private final boolean canHandleUpstream ;
private final boolean canHandleDownstream ;
private volatile Object attachment ;
DefaultChannelHandlerContext(
DefaultChannelHandlerContext prev, DefaultChannelHandlerContext next,
String name, ChannelHandler handler) {
if (name
== null) {
throw new NullPointerException( "name");
}
if (handler
== null) {
throw new NullPointerException( "handler");
}
canHandleUpstream =
handler instanceof ChannelUpstreamHandler;
canHandleDownstream =
handler instanceof ChannelDownstreamHandler;
if (!canHandleUpstream &&
! canHandleDownstream) {
throw new IllegalArgumentException(
"handler must be either " +
ChannelUpstreamHandler. class.getName()
+ " or " +
ChannelDownstreamHandler. class.getName()
+ ‘.‘);
}
this .prev =
prev;
this .next =
next;
this .name =
name;
this .handler =
handler;
}
public Channel getChannel()
{
return getPipeline().getChannel();
}
public ChannelPipeline getPipeline()
{
return DefaultChannelPipeline. this ;
}
public boolean canHandleDownstream()
{
return canHandleDownstream ;
}
public boolean canHandleUpstream()
{
return canHandleUpstream ;
}
public ChannelHandler getHandler()
{
return handler ;
}
public String
getName() {
return name ;
}
public Object
getAttachment() {
return attachment ;
}
public void setAttachment(Object
attachment) {
this .attachment =
attachment;
}
public void sendDownstream(ChannelEvent e)
{
DefaultChannelHandlerContext prev = getActualDownstreamContext( this. prev);
if (prev
== null) {
try {
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);
}
}
}
