Netty系列-2 NioServerSocketChannel和NioSocketChannel介绍

背景

本文介绍Netty的通道组件NioServerSocketChannel和NioSocketChannel，从源码的角度介绍其实现原理。

1.NioServerSocketChannel

Netty本质是对NIO的封装和增强，因此Netty框架中必然包含了对于ServerSocketChannel的构建、配置以及向选择器注册，如下所示:

java 复制代码

// 创建ServerSocketChannel对象
ServerSocketChannel serverSocketChannel = SelectorProvider.provider().openServerSocketChannel();

// ServerSocketChannel通道设置为非阻塞
serverSocketChannel.configureBlocking(false);

// 将ServerSocketChannel通道注册至选择器
serverSocketChannel.register(Selector, opts, attachment);

// 接收客户端连接得到SocketChannel通道
SocketChannel socketChannel = serverSocketChannel.accept();

其中的构建和配置过程发生在NioServerSocketChannel的实例化过程。

1.1 NioServerSocketChannel构造函数

NioServerSocketChannel实例化过程包含了对serverSocketChannel的创建以及配置

Netty启动时，通过反射调用NioServerSocketChannel的无参构造函数创建NioServerSocketChannel对象.

java 复制代码

private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();

public NioServerSocketChannel() {
    this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}

public NioServerSocketChannel(ServerSocketChannel channel) {
    super(null, channel, SelectionKey.OP_ACCEPT);
    config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}

DEFAULT_SELECTOR_PROVIDER是Provider对象，用于创建通道和选择器，newSocket方法返回一个ServerSocketChannel对象，如下所示:

java 复制代码

private static ServerSocketChannel newSocket(SelectorProvider provider) {
    try {
        return provider.openServerSocketChannel();
    } catch (IOException e) {
        throw new ChannelException("Failed to open a server socket.", e);
    }
}

NioServerSocketChannel中还维护了一个config对象用于储存该通道相关的配置，后续通过通道对象的config()方法获取该config对象。

继续调用父类的构造方法:

java 复制代码

protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    super(parent);
    this.ch = ch;
    this.readInterestOp = readInterestOp;
    try {
        ch.configureBlocking(false);
    } catch (IOException e) {
        try {
            ch.close();
        } catch (IOException e2) {
            logger.warn("Failed to close a partially initialized socket.", e2);
        }

        throw new ChannelException("Failed to enter non-blocking mode.", e);
    }
}

// super(parent)内容如下:
protected AbstractChannel(Channel parent) {
    this.parent = parent;
    id = newId();
    unsafe = newUnsafe();
    pipeline = newChannelPipeline();
}

因此NioServerSocketChannel中包含如下属性: