dubbo:Channel

public interface Channel extends Endpoint {
    InetSocketAddress getRemoteAddress();//获取远程地址
    boolean isConnected();//是否已连接
    boolean hasAttribute(String key);//根据key判断属性值是否存在
    Object getAttribute(String key);//根据key获取属性值
    void setAttribute(String key, Object value);//设置属性值
    void removeAttribute(String key);//移除属性值
}

继承了Endpoint，Endpoint表示端点的意思，客户端，服务端都可以认为是一个端点，Endpoint主要抽象了端点的关闭，发送消息，获取本端点的地址等。

public abstract class AbstractChannel extends AbstractPeer implements Channel {
    public AbstractChannel(URL url, ChannelHandler handler) {
        super(url, handler);
    }
    @Override
    public void send(Object message, boolean sent) throws RemotingException {
        if (isClosed()) {
            throw new RemotingException(this, "Failed to send message "
                    + (message == null ? "" : message.getClass().getName()) + ":" + PayloadDropper.getRequestWithoutData(message)
                    + ", cause: Channel closed. channel: " + getLocalAddress() + " -> " + getRemoteAddress());
        }
    }
 
    @Override
    public String toString() {
        return getLocalAddress() + " -> " + getRemoteAddress();
    }
}

AbstractChannel 继承了AbstractPeer，AbstractPeer实现了ChannelHandler接口，然后将connected，disconnected，sent，received，caught 方法给成员变量
ChannelHandler handler来维护。

在NettyChannel中，构造方法是私有的，提供给getOrAddChannel方法使用，

在getOrAddChannel方法中，netty的Channel从channelMap 这个缓存获取NettyChannel 对象，如果没有的话就创建，然后塞到这个缓存中，最后将这个NettyChannel返回

static NettyChannel getOrAddChannel(org.jboss.netty.channel.Channel ch, URL url, ChannelHandler handler) {
    if (ch == null) {
        return null;
    }
    NettyChannel ret = CHANNEL_MAP.get(ch);
    if (ret == null) {
        NettyChannel nc = new NettyChannel(ch, url, handler);
        if (ch.isConnected()) {
            ret = CHANNEL_MAP.putIfAbsent(ch, nc);
        }
        if (ret == null) {
            ret = nc;
        }
    }
    return ret;
}

在send方法中，先调用父类的send方法检查channel是否关闭，然后调用netty channel 进行发送消息，如果是需要sent反馈的话，就获取配置的timeout 超时时间，缺省是1s，等待timeout超时时间获取发送状态，如果发送状态是false，或者出现了异常，则会抛出异常。

@Override
public void send(Object message, boolean sent) throws RemotingException {
    super.send(message, sent);
 
    boolean success = true;
    int timeout = 0;
    try {
        ChannelFuture future = channel.write(message);
        if (sent) {
            timeout = getUrl().getPositiveParameter(TIMEOUT_KEY, DEFAULT_TIMEOUT);
            success = future.await(timeout);
        }
        Throwable cause = future.getCause();
        if (cause != null) {
            throw cause;
        }
    } catch (Throwable e) {
        throw new RemotingException(this, "Failed to send message " + PayloadDropper.getRequestWithoutData(message) + " to " + getRemoteAddress() + ", cause: " + e.getMessage(), e);
    }
 
    if (!success) {
        throw new RemotingException(this, "Failed to send message " + PayloadDropper.getRequestWithoutData(message) + " to " + getRemoteAddress()
                + "in timeout(" + timeout + "ms) limit");
    }
}