AbstractChannel顾名思义, 从Channel的继承结构上也可以看到, 在排除ServerChannel/ServerSocketChannel/SocketChannel三个接口定义之后, Channen的类层次就是从Channel -> AbstractChannel -> AbstractNioChannel 一路继承下去.

下面详细看代码实现.

类定义和外围实现

类定义

package io.netty.channel

public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {}

外围接口实现

在Channel接口定义中申明的需要继承的两个接口AttributeMap/Comparable

public interface Channel extends AttributeMap, Comparable<Channel> {

通过继承DefaultAttributeMap实现了对接口AttributeMap的实现. Comparable接口的实现则是通过实现compareTo()方法来实现, compareTo()方法的代码:

    @Override
    public final int compareTo(Channel o) {
        if (this == o) {
            return 0;
        }

        return id().compareTo(o.id());
    }

Channel接口实现

AbstractChannel实现了Channel接口定义的部分方法,作为所有Channel子类的基本实现.

下面是AbstractChannel中定义的几个基本属性和构造函数, 注意这几个基本属性都是final:

    private final Channel parent;
    private final ChannelId id;
    private final Unsafe unsafe;
    private final DefaultChannelPipeline pipeline;

    protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = DefaultChannelId.newInstance();
        unsafe = newUnsafe();
        pipeline = new DefaultChannelPipeline(this);
    }

    protected AbstractChannel(Channel parent, ChannelId id) {
        this.parent = parent;
        this.id = id;
        unsafe = newUnsafe();
        pipeline = new DefaultChannelPipeline(this);
    }

id

每个Channel的实例都应该有一个唯一的id,这个id一旦设置就不能被修改. 默认通过DefaultChannelId.newInstance()方法来获取:

private final ChannelId id;

protected AbstractChannel(Channel parent) {
    id = DefaultChannelId.newInstance();
    ......
}

protected AbstractChannel(Channel parent, ChannelId id) {
    this.id = id;
    ......
}

@Override
public final ChannelId id() {
    return id;
}

parent

每个Channel的parent在构造函数中指定:

private final Channel parent;

protected AbstractChannel(Channel parent) {
    this.parent = parent;
    ......
}

@Override
public Channel parent() {
    return parent;
}

　pipeline

pipeline的初始化在构造函数中，目前看时直接固定为使用DefaultChannelPipeline的实现：

private final DefaultChannelPipeline pipeline;

protected AbstractChannel(Channel parent, ChannelId id) {
    ......
    pipeline = new DefaultChannelPipeline(this);
}

public ChannelPipeline pipeline() {
    return pipeline;
}

比较奇怪的是这里pipeline的类型定义是"DefaultChannelPipeline"，　而不是"ChannelPipeline"．这个方式按说不正确，呵呵，验证过将类型修改为ChannelPipeline代码编译和测试都OK．

已经提交了一个pull request 给netty，看看他们能否接受 更新: netty接受了这个commit, 现在pipeline的类型已经修改为ChannelPipeline了.

在AbstractChannel的实现中, 有很多和IO相关的方法是直接delagate给pipeline的, 例如bind()方法:

public ChannelFuture bind(SocketAddress localAddress) {
    return pipeline.bind(localAddress);
}

类似delagate给pipeline的方法包括:

bind()
connect()
disconnect()
close()
deregister()
flush()
read()
write()
writeAndFlush()

unsafe

unsafe的初始化也是在构造函数中，通过定义模板方法newUnsafe()让子类来给出具体的Unsafe的实现：

private final Unsafe unsafe;

protected AbstractChannel(Channel parent, ChannelId id) {
    ......
    unsafe = newUnsafe();
}

protected abstract AbstractUnsafe newUnsafe();

public Unsafe unsafe() {
    return unsafe;
}

在AbstractChannel的实现中, 有部分方法是直接delagate给unsafe的, 例如isWritable()方法:

public boolean isWritable() {
    ChannelOutboundBuffer buf = unsafe.outboundBuffer();
    return buf != null && buf.isWritable();
}

类似delagate给unsafe的方法包括:

isWritable()
bytesBeforeUnwritable()
bytesBeforeWritable()
localAddress()
remoteAddress()

类AbstractChannel