netty4源码分析-socket

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netty4源码分析-socket

服务端启动的第一步必须先创建一个监听套接字ServerSocketChannel，该过程是由ChannelFuture f = b.bind(port)中的bind触发。下面详细分析其过程：

Bind源码如下，代码位于ServerBootstrap的父类AbstractBootstrap

//AbstractBootstrap
public ChannelFuture bind(int inetPort) {
        return bind(new InetSocketAddress(inetPort));
}

public ChannelFuture bind(SocketAddress localAddress) {
        validate();
        if (localAddress == null) {
            throw new NullPointerException("localAddress");
        }
        return doBind(localAddress);
    }

validate()方法的作用为：校验：bossGroup、BootstrapChannelFactory、childHandler非空。如果childGroup为空，则复用bossGroup，将bossGroup赋值给childGroup。

接着来看doBind的逻辑：

//AbstractBootstrap
private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regPromise = initAndRegister();
        final Channel channel = regPromise.channel();
        final ChannelPromise promise = channel.newPromise();
        if (regPromise.isDone()) {
            doBind0(regPromise, channel, localAddress, promise);
        } else {
            regPromise.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    doBind0(future, channel, localAddress, promise);
                }
            });
        }
        return promise;
    }

重点分析里面的initAndRegister()方法

//AbstractBootstrap
final ChannelFuture initAndRegister() {
        final Channel channel = channelFactory().newChannel();
        try {
            init(channel);
        } catch (Throwable t) {
            channel.unsafe().closeForcibly();
            return channel.newFailedFuture(t);
        }

        ChannelPromise regPromise = channel.newPromise();
        group().register(channel, regPromise);
        if (regPromise.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }
        return regPromise;
    }

a）首先分析以下代码：

final Channel channel = channelFactory().newChannel()

channelFactory()方法返回之前创建的BootstrapChannelFactory，里面的newChannel()方法会根据反射创建一个ServerSocketChannel

//BootstrapChannelFactory       
public T newChannel() {
            try {
                return clazz.newInstance();
            } catch (Throwable t) {
                throw new ChannelException("Unable to create Channel from class " + clazz, t);
            }
        }

注：clazz是在服务端启动的这段代码（b.group(bossGroup,workerGroup).channel(NioServerSocketChannel.class)中设置的。

clazz.newInstance()会调用NioServerSocketChannel的默认构造函数

// NioServerSocketChannel
public NioServerSocketChannel() {
        super(null, newSocket(), SelectionKey.OP_ACCEPT);
        config = new DefaultServerSocketChannelConfig(this, javaChannel().socket());
}
private static ServerSocketChannel newSocket() {
        try {
            return ServerSocketChannel.open();
        } catch (IOException e) {
            throw new ChannelException(
                    "Failed to open a server socket.", e);
        }
    }

注意newSocket中的这行代码：

return ServerSocketChannel.open();

此处就是服务端监听套接字ServerSocketChannel创建的地方。

既然是使用NIO，那么设置创建的ServerSocketChannel为非阻塞是在哪个地方发生的呢？看下这行代码

super(null, newSocket(), SelectionKey.OP_ACCEPT);

它会对NioServerSocketChannel的父类进行初始化：NioServerSocketChannel的父类是AbstractNioMessageChannel，其构造方法仅仅初始化其父类AbstractNioChannel，父类构造方法如下：

//AbstractNioChannel
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) {
                if (logger.isWarnEnabled()) {
                    logger.warn(
                            "Failed to close a partially initialized socket.", e2);
                }
            }
            throw new ChannelException("Failed to enter non-blocking mode.", e);
        }
    }

ch.configureBlocking(false)此处就将之前创建的ServerSocketChannel设置为非阻塞模式。

该方法里还有三点需要注意：

1、super(parent)会调用AbstractNioChannel的父类AbstractChannel的构造方法

// AbstractChannel.java
protected AbstractChannel(Channel parent) {
        this.parent = parent;
        unsafe = newUnsafe();
        pipeline = new DefaultChannelPipeline(this);
    }

newUnsafe()是由子类AbstractNioMessageChannel实现的，里面实例化了一个内部类NioMessageUnsafe（注：该类很重要，里面定义了read方法，会触发accept的调用，后面对其重点分析）。

2、this.readInterestOp = readInterestOp：设置channel的ops为SelectionKey.OP_ACCEPT（值为16）

3、pipeline = new DefaultChannelPipeline(this)，创建作用于ServerSocketChannel的管道Pipeline

// DefaultChannelPipeline
public DefaultChannelPipeline(Channel channel) {
        if (channel == null) {
            throw new NullPointerException("channel");
        }
        this.channel = channel;

        TailHandler tailHandler = new TailHandler();
        tail = new DefaultChannelHandlerContext(this, null, generateName(tailHandler), tailHandler);

        HeadHandler headHandler = new HeadHandler(channel.unsafe());
        head = new DefaultChannelHandlerContext(this, null, generateName(headHandler), headHandler);

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

DefaultChannelPipeline维护了一个以DefaultChannelHandlerContext为元素的双向链表结构，Head是一个Outbound处理器，而tail是一个Inbound处理器。经过此步骤后，管道中的处理器链表为：Head->tail。

b）再来分析以下代码

init(channel)

该方法由子类ServerBootstrap实现

// ServerBootstrap.java
void init(Channel channel) throws Exception {
        final Map<ChannelOption<?>, Object> options = options();
        synchronized (options) {
            channel.config().setOptions(options);
        }

        final Map<AttributeKey<?>, Object> attrs = attrs();
        synchronized (attrs) {
            for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
                @SuppressWarnings("unchecked")
                AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
                channel.attr(key).set(e.getValue());
            }
        }

        ChannelPipeline p = channel.pipeline();
        if (handler() != null) {
            p.addLast(handler());
        }

        final EventLoopGroup currentChildGroup = childGroup;
        final ChannelHandler currentChildHandler = childHandler;
        final Entry<ChannelOption<?>, Object>[] currentChildOptions;
        final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
        synchronized (childOptions) {
            currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
        }
        synchronized (childAttrs) {
            currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
        }

        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(Channel ch) throws Exception {
                ch.pipeline().addLast(new ServerBootstrapAcceptor(
                        currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
            }
        });
    }

该方法主要做了两件事：

1、设置NioServerSocketChannel的options和attrs，并存储之后用于SocketChannel的options和attrs。

2、为NioServerSocketChannel对应的管道增加一个Inbound处理器ChannelInitializer。经过此步骤后，管道中的处理器链表为：head(outbound)->ChannelInitializer(inbound)->tail(inbound)。注意ChannelInitializer的实现方法initChannel，里面会当channelRegistered事件发生时将ServerBootstrapAcceptor加入到管道中。 c）最后分析以下代码：

group().register(channel, regPromise);

netty4源码分析-socket

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