一、Channel概述

Channel即Socket封装，提供了I/O的基本操作。从以下子接口中可以看出Netty对不同的底层协议提供了对应的channel来处理，例如：TCP/IP、UDP/IP、SCTP/IP、HTTP2等。

Channel类图

Channel子接口

二、实例化流程

从客户端引导类示例中查看Channel初始化过程。示例中使用NIOSocketChannel作为通信通道，在JAVA中通信中会建立socket连接。

EventLoopGroup workerGroup = new NioEventLoopGroup();
Http2ClientInitializer initializer = new Http2ClientInitializer(sslCtx, Integer.MAX_VALUE);
Bootstrap b = new Bootstrap();
b.group(workerGroup);
b.channel(NioSocketChannel.class);
b.option(ChannelOption.SO_KEEPALIVE, true);
b.remoteAddress(HOST, PORT);
b.handler(initializer);
Channel channel = b.connect().syncUninterruptibly().channel();

Channel通过ChannelFactory创建，下面看一下ChannelFactory类图。

ChannelFactory类图

ReflectiveChannelFactory提供了newChannel()方法通过反射实例化。示例中通过b.channel(NioSocketChannel.class)将NioSocketChannel.class赋值给ReflectiveChannelFactory的成员变量Constructor<? extends T> constructor，Channel在connect的时候实例化，下面为实例化调用链路。

Channel实例化链路

三、实例化过程

1.客户端实例化过程

了解了Channel初始化调用链，再来看下以NioSocketChannel为例初始化做了哪些事情。下面是NioSocketChannel的四个构造重载方法。

public NioSocketChannel() {
    this(DEFAULT_SELECTOR_PROVIDER); // @1
}
public NioSocketChannel(SelectorProvider provider) {
    this(newSocket(provider)); // @2
}
public NioSocketChannel(SocketChannel socket) {
    this(null, socket);
}
public NioSocketChannel(Channel parent, SocketChannel socket) {
    super(parent, socket);
    config = new NioSocketChannelConfig(this, socket.socket());
}
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    super(parent);
    this.ch = ch;
    this.readInterestOp = readInterestOp;
    ch.configureBlocking(false); // @3
   }
}

代码解读
@1 默认使用SelectorProvider.provider()
@2 使用Provider创建SocketChannel。provider.openSocketChannel()->new SocketChannelImpl(this)。
@3 设置NioChannel非阻塞模式

小结：客户端NioSocketChannel实例化过程中已经回到所熟悉的java nio。创建了通道SocketChannel，并设置为非阻塞。

2.服务端实例化过程

Channel服务端的实例化流程与客户端是相同的，下面以NioServerSocketChannel为例走查实例化过程。服务端引导初始化示例代码如下。

EventLoopGroup group = new NioEventLoopGroup();
ServerBootstrap b = new ServerBootstrap();
b.option(ChannelOption.SO_BACKLOG, 1024);
b.group(group)
 .channel(NioServerSocketChannel.class)
 .handler(new LoggingHandler(LogLevel.INFO))
 .childHandler(new Http2ServerInitializer(sslCtx));
Channel ch = b.bind(PORT).sync().channel();

NioServerSocketChannel的构造方法。

public NioServerSocketChannel() {
    this(newSocket(DEFAULT_SELECTOR_PROVIDER)); // @1
}
public NioServerSocketChannel(SelectorProvider provider) {
    this(newSocket(provider)); // @2
}
 public NioServerSocketChannel(ServerSocketChannel channel) {
    super(null, channel, SelectionKey.OP_ACCEPT);
    config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    super(parent);
    this.ch = ch;
    this.readInterestOp = readInterestOp;
    ch.configureBlocking(false); // @3      
 }

代码解读

@1 使用默认Provider类SelectorProvider
@2 开启服务端通道ServerSocketChannel。provider.openServerSocketChannel()->new ServerSocketChannelImpl(this)
@3 将ServerSocketChanne设置为非阻塞

小结：服务端NioServerSocketChannel的实例化过程同样回到熟悉的Java NIO，创建非阻塞ServerSocketChanne通道。

3.实例化其他事项

在实例化的过程中，会调父类的构造方法super(parent)。

protected AbstractChannel(Channel parent) {
    this.parent = parent;
    id = newId(); // @1
    unsafe = newUnsafe(); // @2
    pipeline = newChannelPipeline(); // @3
}

@1 ChannelId初始化

ChannelId是Channel的唯一标识，下面看下DefaultChannelId的生成规则。

private DefaultChannelId() {
    data = new byte[macHINE_ID.length + PROCESS_ID_LEN + SEQUENCE_LEN + TIMESTAMP_LEN + RANDOM_LEN];
    int i = 0;
    // machineId
    System.arraycopy(MACHINE_ID, 0, data, i, MACHINE_ID.length);
    i += MACHINE_ID.length;
    // processId
    i = writeInt(i, PROCESS_ID);
    // sequence
    i = writeInt(i, nextSequence.getAndIncrement());
    // timestamp (kind of)
    i = writeLong(i, Long.reverse(System.nanoTime()) ^ System.currentTimeMillis());
    // random
    int random = PlatformDependent.threadLocalRandom().nextInt();
    i = writeInt(i, random);
    assert i == data.length;

    hashCode = Arrays.hashCode(data);
}

小结：默认的ChannelId由machineId、processId、sequence、timestamp、random构成。machineId：可以由参数io.netty.machineId自定义，默认为8位随机byte构成
processId：可以由参数io.netty.processId自定义，默认为4位进程ID
sequence：原子自增序号AtomicInteger，每创建一个Chanenl会进行自增
timestamp：8位的timestamp
random：4位的随机整数

@2 unsafe初始化

unsafe即I/O的核心操作，byte的读写都靠它来处理。服务端NioServerSocketChannel初始化使用NioMessageUnsafe。客户端NioSocketChannel初始化使用NioSocketChannelUnsafe。以NIO为例看下Unsafe的类图结构。

unsafe类图结构

@3 ChannelPipeline初始化

默认使用DefaultChannelPipeline，从构造方法可以看出为链表结构，详细分析另文分析。

protected DefaultChannelPipeline(Channel channel) {
    this.channel = ObjectUtil.checkNotNull(channel, "channel");
    succeededFuture = new SucceededChannelFuture(channel, null);
    voidPromise =  new VoidChannelPromise(channel, true);

    tail = new TailContext(this);
    head = new HeadContext(this);

    head.next = tail;
    tail.prev = head;
}