netty4.0.x源码分析—bootstrap

Bootstrap的意思就是引导，辅助的意思，在编写服务端或客户端程序时，我们都需要先new一个bootstrap，然后基于这个bootstrap调用函数，添加eventloop和handler，可见对bootstrap进行分析还是有必要的。

1、bootstrap结构图

bootstrap的结构比较简单，涉及的类和接口很少，如下图所示，其中Bootstrap则是客户端程序用的引导类，ServerBootstrap是服务端程序用的引导类。

2、serverbootstrap分析

这部分，专门对serverbootstrap进行分析，bootstrap过程大同小异就不作详细的分析了。下面是我们编写服务端代码的一般化过程，整个分析过程将基于下面这段代码中用到的函数进行。

// Configure the bootstrap.
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
             .channel(NioServerSocketChannel.class)
             .childHandler(new HexDumpProxyInitializer(remoteHost, remotePort))
             .childOption(ChannelOption.AUTO_READ, false)
             .bind(localPort).sync().channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }

先看关键代码（注意这里面的部分函数是在AbstractBootstrap中定义的）

    private final Map<ChannelOption<?>, Object> childOptions = new LinkedHashMap<ChannelOption<?>, Object>();
    private final Map<AttributeKey<?>, Object> childAttrs = new LinkedHashMap<AttributeKey<?>, Object>();
    private volatile EventLoopGroup childGroup;
    private volatile ChannelHandler childHandler;

    /**
     * Set the {@link EventLoopGroup} for the parent (acceptor) and the child (client). These
     * {@link EventLoopGroup}'s are used to handle all the events and IO for {@link SocketChannel} and
     * {@link Channel}'s.
     */
    public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
        super.group(parentGroup);
        if (childGroup == null) {
            throw new NullPointerException("childGroup");
        }
        if (this.childGroup != null) {
            throw new IllegalStateException("childGroup set already");
        }
        this.childGroup = childGroup;
        return this;
    }

属性值ChildGroup，ChildHandler，是用来处理accpt的Channel的。group函数其实就是将parentGroup和ChildGroup进行赋值，其中parentGroup用于处理accept事件，ChildGroup用于处理accpt的Channel的IO事件。

    //channel函数的实现定义在抽象父类中，其实就是通过newInstance函数生成一个具体的channel对象。
<pre name="code" class="java">    /**
     * The {@link Class} which is used to create {@link Channel} instances from.
     * You either use this or {@link #channelFactory(ChannelFactory)} if your
     * {@link Channel} implementation has no no-args constructor.
     */
    public B channel(Class<? extends C> channelClass) {
        if (channelClass == null) {
            throw new NullPointerException("channelClass");
        }
        return channelFactory(new BootstrapChannelFactory<C>(channelClass));
    }

    /**
     * {@link ChannelFactory} which is used to create {@link Channel} instances from
     * when calling {@link #bind()}. This method is usually only used if {@link #channel(Class)}
     * is not working for you because of some more complex needs. If your {@link Channel} implementation
     * has a no-args constructor, its highly recommend to just use {@link #channel(Class)} for
     * simplify your code.
     */
    @SuppressWarnings("unchecked")
    public B channelFactory(ChannelFactory<? extends C> channelFactory) {
        if (channelFactory == null) {
            throw new NullPointerException("channelFactory");
        }
        if (this.channelFactory != null) {
            throw new IllegalStateException("channelFactory set already");
        }

        this.channelFactory = channelFactory;
        return (B) this;
    }<pre name="code" class="java">    private static final class BootstrapChannelFactory<T extends Channel> implements ChannelFactory<T> {
        private final Class<? extends T> clazz;

        BootstrapChannelFactory(Class<? extends T> clazz) {
            this.clazz = clazz;
        }

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

        @Override
        public String toString() {
            return clazz.getSimpleName() + ".class";
        }
    }

Channel函数比较简单，其实就是通过newInstance函数，生成一个具体的Channel对象，例如服务端的NioServerSocketChannel。

    /**
     * Set the {@link ChannelHandler} which is used to serve the request for the {@link Channel}'s.
     */
    public ServerBootstrap childHandler(ChannelHandler childHandler) {
        if (childHandler == null) {
            throw new NullPointerException("childHandler");
        }
        this.childHandler = childHandler;
        return this;
    }

上面的函数即给serverbootstrap的childHandler赋值。

    /**
     * Allow to specify a {@link ChannelOption} which is used for the {@link Channel} instances once they get created
     * (after the acceptor accepted the {@link Channel}). Use a value of {@code null} to remove a previous set
     * {@link ChannelOption}.
     */
    public <T> ServerBootstrap childOption(ChannelOption<T> childOption, T value) {
        if (childOption == null) {
            throw new NullPointerException("childOption");
        }
        if (value == null) {
            synchronized (childOptions) {
                childOptions.remove(childOption);
            }
        } else {
            synchronized (childOptions) {
                childOptions.put(childOption, value);
            }
        }
        return this;
    }

上面的函数是指定accpt的channel的属性，channel有很多属性，比如SO_TIMEOUT时间，Buf长度等等。

    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind() {
        validate();
        SocketAddress localAddress = this.localAddress;
        if (localAddress == null) {
            throw new IllegalStateException("localAddress not set");
        }
        return doBind(localAddress);
    }

     /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(int inetPort) {
        return bind(new InetSocketAddress(inetPort));
    }

    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(String inetHost, int inetPort) {
        return bind(new InetSocketAddress(inetHost, inetPort));
    }

<pre name="code" class="java">    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(SocketAddress localAddress) {
        validate();
        if (localAddress == null) {
            throw new NullPointerException("localAddress");
        }
        return doBind(localAddress);
    }

    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;
    }<pre name="code" class="java">    private static void doBind0(
            final ChannelFuture regFuture, final Channel channel,
            final SocketAddress localAddress, final ChannelPromise promise) {

        // This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up
        // the pipeline in its channelRegistered() implementation.

        channel.eventLoop().execute(new Runnable() {
            @Override
            public void run() {
                if (regFuture.isSuccess()) {
                    channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
                } else {
                    promise.setFailure(regFuture.cause());
                }
            }
        });
    }

Bind函数层层调用过来之后，最后就调用Channel的bind函数了，下面再看channel的bind函数是如何处理的。定义在AbstractChannel中：

    @Override
    public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
        return pipeline.bind(localAddress, promise);
    }

channel的bind函数，最终就是调用pipeline的bind，而pipeline的bind实际上就是调用contexthandler的bind，之个之前分析write和flush的时候说过了。所以这里直接看contexthandler的bind函数。下面是定义：

    @Override
    public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
        if (localAddress == null) {
            throw new NullPointerException("localAddress");
        }
        validatePromise(promise, false);

        final DefaultChannelHandlerContext next = findContextOutbound();
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            next.invokeBind(localAddress, promise);
        } else {
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    next.invokeBind(localAddress, promise);
                }
            });
        }

        return promise;
    }<pre name="code" class="java">    private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
        try {
            ((ChannelOutboundHandler) handler).bind(this, localAddress, promise);
        } catch (Throwable t) {
            notifyOutboundHandlerException(t, promise);
        }
    }

最终调用Handler的bind函数，还记得之前说的outbound类型的事件吗，这类事件提供了默认的实现方法，HeadHandler的bind函数，下面是它的定义：

        @Override
        public void bind(
                ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
                throws Exception {
            unsafe.bind(localAddress, promise);
        }

我们又看到了unsafe这个苦力了，最终的操作还是得由它来完成啊，赶紧去看看这个bind函数吧，

        @Override
        public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
            if (!ensureOpen(promise)) {
                return;
            }

            // See: https://github.com/netty/netty/issues/576
            if (!PlatformDependent.isWindows() && !PlatformDependent.isRoot() &&
                Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
                localAddress instanceof InetSocketAddress &&
                !((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress()) {
                // Warn a user about the fact that a non-root user can't receive a
                // broadcast packet on *nix if the socket is bound on non-wildcard address.
                logger.warn(
                        "A non-root user can't receive a broadcast packet if the socket " +
                        "is not bound to a wildcard address; binding to a non-wildcard " +
                        "address (" + localAddress + ") anyway as requested.");
            }

            boolean wasActive = isActive();
            try {
                doBind(localAddress);
            } catch (Throwable t) {
                closeIfClosed();
                promise.setFailure(t);
                return;
            }
            if (!wasActive && isActive()) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireChannelActive();
                    }
                });
            }
            promise.setSuccess();
        }

上面的代码最终调用了Channel的doBind函数，这里我们的Channel是NioServerSocketChannel，所以最终就是调用它的bind函数了，代码如下

    @Override
    protected void doBind(SocketAddress localAddress) throws Exception {
        javaChannel().socket().bind(localAddress, config.getBacklog());
    }

其实它最终也是调用了JDK的Channel的socket bind函数。

看到这里，你是否会觉得有点怪异，为什么没有注册accpt事件啊，一般的我们的server socket都是要注册accpt事件到selector，用于监听连接。如果你发现了这个问题，说明你是理解socket的编程的，^_^。实际上是前面在分析bind的时候我们漏掉了一个重要的函数，initAndRegister，下面再来看看它的定义：

    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();
            }
        }

        // If we are here and the promise is not failed, it's one of the following cases:
        // 1) If we attempted registration from the event loop, the registration has been completed at this point.
        //    i.e. It's safe to attempt bind() or connect() now beause the channel has been registered.
        // 2) If we attempted registration from the other thread, the registration request has been successfully
        //    added to the event loop's task queue for later execution.
        //    i.e. It's safe to attempt bind() or connect() now:
        //         because bind() or connect() will be executed *after* the scheduled registration task is executed
        //         because register(), bind(), and connect() are all bound to the same thread.

        return regPromise;
    }

在这里，我们看到了我们之前介绍event时说的register函数，它就是用于将Channel注册到eventloop中去的。eventloop经过层层调用，最终调用了SingleThreadEventLoop类中的register函数，下面是它的定义：

    @Override
    public ChannelFuture register(final Channel channel, final ChannelPromise promise) {
        if (channel == null) {
            throw new NullPointerException("channel");
        }
        if (promise == null) {
            throw new NullPointerException("promise");
        }

        channel.unsafe().register(this, promise);
        return promise;
    }

还是逃离不了unsafe对象的调用，前面说了那么多的unsafe，这个函数猜都可以猜测出执行过程了，这里就不细细的列举代码了。

还有一个init函数，这里需要说明一下，代码如下：

    @Override
    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));
            }
        });
    }

它就是用来处理channel 的pipeline，并添加一个ServerBootstrapAcceptor的handler，继续看看这个handler的定义，我们就会明白它的意图。

    private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {

        private final EventLoopGroup childGroup;
        private final ChannelHandler childHandler;
        private final Entry<ChannelOption<?>, Object>[] childOptions;
        private final Entry<AttributeKey<?>, Object>[] childAttrs;

        @SuppressWarnings("unchecked")
        ServerBootstrapAcceptor(
                EventLoopGroup childGroup, ChannelHandler childHandler,
                Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
            this.childGroup = childGroup;
            this.childHandler = childHandler;
            this.childOptions = childOptions;
            this.childAttrs = childAttrs;
        }

        @Override
        @SuppressWarnings("unchecked")
        public void channelRead(ChannelHandlerContext ctx, Object msg) {
            Channel child = (Channel) msg;

            child.pipeline().addLast(childHandler);

            for (Entry<ChannelOption<?>, Object> e: childOptions) {
                try {
                    if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {
                        logger.warn("Unknown channel option: " + e);
                    }
                } catch (Throwable t) {
                    logger.warn("Failed to set a channel option: " + child, t);
                }
            }

            for (Entry<AttributeKey<?>, Object> e: childAttrs) {
                child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
            }

            try {
                childGroup.register(child);
            } catch (Throwable t) {
                child.unsafe().closeForcibly();
                logger.warn("Failed to register an accepted channel: " + child, t);
            }
        }

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            final ChannelConfig config = ctx.channel().config();
            if (config.isAutoRead()) {
                // stop accept new connections for 1 second to allow the channel to recover
                // See https://github.com/netty/netty/issues/1328
                config.setAutoRead(false);
                ctx.channel().eventLoop().schedule(new Runnable() {
                    @Override
                    public void run() {
                       config.setAutoRead(true);
                    }
                }, 1, TimeUnit.SECONDS);
            }
            // still let the exceptionCaught event flow through the pipeline to give the user
            // a chance to do something with it
            ctx.fireExceptionCaught(cause);
        }
    }

上面就是这个handler的全部代码，它重写了ChannelRead函数，它的目的其实是想将server端accept的channel注册到ChildGroup的eventloop中，这样就可以理解，服务端代码workerGroup这个eventloop的作用了，它终于在这里体现出了它的作用了。

3、总结

这篇文章主要是分析了serverbootstrap的全过程，通过对这个的分析，我们清晰的看到了平时编写socket服务端代码时对bind，register事件，以及accept channel等的处理。

http://blog.csdn.net/pingnanlee/article/details/11973769