一、Channel概述

Channel即Socket封裝，提供了I/O的基本操作。從以下子接口中可以看出Netty對不同的底層協議提供了對應的channel來處理，例如：TCP/IP、UDP/IP、SCTP/IP、HTTP2等。

Channel類圖

Channel子接口

二、實例化流程

從客戶端引導類示例中查看Channel初始化過程。示例中使用NIOSocketChannel作為通信通道，在JAVA中通信中會建立socket連接。

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通過ChannelFactory創建，下面看一下ChannelFactory類圖。

ChannelFactory類圖

ReflectiveChannelFactory提供了newChannel()方法通過反射實例化。示例中通過b.channel(NioSocketChannel.class)將NioSocketChannel.class賦值給ReflectiveChannelFactory的成員變量Constructor<? extends T> constructor，Channel在connect的時候實例化，下面為實例化調用鏈路。

Channel實例化鏈路

三、實例化過程

1.客戶端實例化過程

了解了Channel初始化調用鏈，再來看下以NioSocketChannel為例初始化做了哪些事情。下面是NioSocketChannel的四個構造重載方法。

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
   }
}

代碼解讀
@1 默認使用SelectorProvider.provider()
@2 使用Provider創建SocketChannel。provider.openSocketChannel()->new SocketChannelImpl(this)。
@3 設置NioChannel非阻塞模式

小結：客戶端NioSocketChannel實例化過程中已經回到所熟悉的java nio。創建了通道SocketChannel，并設置為非阻塞。

2.服務端實例化過程

Channel服務端的實例化流程與客戶端是相同的，下面以NioServerSocketChannel為例走查實例化過程。服務端引導初始化示例代碼如下。

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的構造方法。

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      
 }

代碼解讀

@1 使用默認Provider類SelectorProvider
@2 開啟服務端通道ServerSocketChannel。provider.openServerSocketChannel()->new ServerSocketChannelImpl(this)
@3 將ServerSocketChanne設置為非阻塞

小結：服務端NioServerSocketChannel的實例化過程同樣回到熟悉的Java NIO，創建非阻塞ServerSocketChanne通道。

3.實例化其他事項

在實例化的過程中，會調父類的構造方法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的唯一標識，下面看下DefaultChannelId的生成規則。

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);
}

小結：默認的ChannelId由machineId、processId、sequence、timestamp、random構成。machineId：可以由參數io.netty.machineId自定義，默認為8位隨機byte構成
processId：可以由參數io.netty.processId自定義，默認為4位進程ID
sequence：原子自增序號AtomicInteger，每創建一個Chanenl會進行自增
timestamp：8位的timestamp
random：4位的隨機整數

@2 unsafe初始化

unsafe即I/O的核心操作，byte的讀寫都靠它來處理。服務端NioServerSocketChannel初始化使用NioMessageUnsafe。客戶端NioSocketChannel初始化使用NioSocketChannelUnsafe。以NIO為例看下Unsafe的類圖結構。

unsafe類圖結構

@3 ChannelPipeline初始化

默認使用DefaultChannelPipeline，從構造方法可以看出為鏈表結構，詳細分析另文分析。

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;
}