ChannelHandlerContext

October 8, 2021 · View on GitHub

ChannelHandlerContext

Summary

ChannelHandlerContext 在 Netty 中负责包装 ChannelHandler

被 DefaultChannelPipeline 引用 head 和 tail

方便 pipeline 操作 AbstractChannelHandlerContext 链

DefaultChannelPipeline

final AbstractChannelHandlerContext head;
final AbstractChannelHandlerContext tail;

DefaultChannelPipeline 提供了 addLast addFirst 等方法,使 AbstractChannelHandlerContext 形成链表

代码如下：

private void addFirst0(AbstractChannelHandlerContext newCtx) {
    AbstractChannelHandlerContext nextCtx = head.next;
    newCtx.prev = head;
    newCtx.next = nextCtx;
    head.next = newCtx;
    nextCtx.prev = newCtx;
}

addLast

以 addLast 方法为例子:

// addLast 会把 channelHadler 调用 newContext
// 方法包装成 AbstractChannelHandlerContext
// 通过 addLast0 方法形成 ctx 链
@Override
public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
    final AbstractChannelHandlerContext newCtx;
    synchronized (this) {
        checkMultiplicity(handler);
        newCtx = newContext(group, filterName(name, handler), handler);
        addLast0(newCtx);
        // If the registered is false it means that the channel was not registered on an eventloop yet.
        // In this case we add the context to the pipeline and add a task that will call
        // ChannelHandler.handlerAdded(...) once the channel is registered.
        if (!registered) {
            newCtx.setAddPending();
            callHandlerCallbackLater(newCtx, true);
            return this;
        }
        EventExecutor executor = newCtx.executor();
        if (!executor.inEventLoop()) {
            newCtx.setAddPending();
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    callHandlerAdded0(newCtx);
                }
            });
            return this;
        }
    }
    callHandlerAdded0(newCtx);
    return this;
}

DefaultChannelHandlerContext

ChannelPipeline and Channel

ChannelPipeline 与 Channel 进行关联

// AbstractChannel 的构造方法
protected AbstractChannel(Channel parent) {
    this.parent = parent;
    id = newId();
    unsafe = newUnsafe();
    pipeline = newChannelPipeline();
}

Channel 转发 IO 事件

当 IO 事件发生的时候，Channel 把读取到的 byte 数据交给 pipeline 处理

具体的代码如下：

// AbstractNioByteChannel#NioByteUnsafe#read
pipeline.fireChannelRead(byteBuf);

看个 demo

pipeline and channel

从上图可以看下目前这个 ctx 链

HeadContext
↓
DeafultChannelHandlerContext(ObjectDecoder)
↓
DeafultChannelHandlerContext(NettyHandler)
↓
TailContext

相关代码

/**
 * 负责pipeline的组装
 */
final class ChannelInitializerImpl extends ChannelInitializer<Channel> {
    @Override
    protected void initChannel(Channel ch) throws Exception {
        ChannelPipeline pipeline = ch.pipeline();
        // ObjectDecoder 负责把 byte 转化成 java 对象
        pipeline.addLast(new ObjectDecoder(new MyClassResolver()));
        // NettyHandler 处理业务逻辑
        pipeline.addLast(new NettyHandler());
    }
}

Netty in action

下面的图片来自 Netty in action

ChannelHandlerContext