Netty系列-netty的初體驗

一、前言

   最近看了netty原始碼，打算寫個博客記下來，方便后面再復習，同時希望也能方便看到的人，在研究netty的時候，多少能方便點，

二、環境搭建

   git clone netty的代碼下來，或者可以fork到自己的git 倉庫，然后git clone下來，

  后面的版本統一用

<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-all</artifactId>
    <version>4.1.6.Final</version>
</dependency>

三、例子研究

如下是服務端標準的代碼案例，bossgroup主要是用來接收連接請求的，workergroup主要是用來處理讀寫請求的

 1    EventLoopGroup bossGroup = new NioEventLoopGroup(1);
 2         EventLoopGroup workerGroup = new NioEventLoopGroup();
 3         final EchoServerHandler serverHandler = new EchoServerHandler();
 4         try {
 5             ServerBootstrap b = new ServerBootstrap();
 6             b.group(bossGroup, workerGroup)
 7              .channel(NioServerSocketChannel.class)
 8              .option(ChannelOption.SO_BACKLOG, 100)
 9              .handler(new LoggingHandler(LogLevel.INFO))
10              .childHandler(new ChannelInitializer<SocketChannel>() {
11                  @Override
12                  public void initChannel(SocketChannel ch) throws Exception {
13                      ChannelPipeline p = ch.pipeline();
14                      if (sslCtx != null) {
15                          p.addLast(sslCtx.newHandler(ch.alloc()));
16                      }
17                      //p.addLast(new LoggingHandler(LogLevel.INFO));
18                      p.addLast(serverHandler);
19                  }
20              });
21 
22             // Start the server.
23             ChannelFuture f = b.bind(PORT).sync();

前面5-20都是初始化，我們先看23行，bind方法，一路跟下去，分為三部分

 1 private ChannelFuture doBind(final SocketAddress localAddress) {
 2         final ChannelFuture regFuture = initAndRegister();
 3         final Channel channel = regFuture.channel();
 4         if (regFuture.cause() != null) {
 5             return regFuture;
 6         }
 7 
 8         if (regFuture.isDone()) {
 9             // At this point we know that the registration was complete and successful.
10             ChannelPromise promise = channel.newPromise();
11             doBind0(regFuture, channel, localAddress, promise);
12             return promise;
13         } else {
14             // Registration future is almost always fulfilled already, but just in case it's not.
15             final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
16             regFuture.addListener(new ChannelFutureListener() {
17                 @Override
18                 public void operationComplete(ChannelFuture future) throws Exception {
19                     Throwable cause = future.cause();
20                     if (cause != null) {
21                         // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
22                         // IllegalStateException once we try to access the EventLoop of the Channel.
23                         promise.setFailure(cause);
24                     } else {
25                         // Registration was successful, so set the correct executor to use.
26                         // See https://github.com/netty/netty/issues/2586
27                         promise.registered();
28 
29                         doBind0(regFuture, channel, localAddress, promise);
30                     }
31                 }
32             });
33             return promise;
34         }
35     }

第一是 initAndRegister方法

 在這里，channelFactory啥時候初始化的？我們回到標準案例那里

 跟進去看看

public B channel(Class<? extends C> channelClass) {
        if (channelClass == null) {
            throw new NullPointerException("channelClass");
        }
        return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
    }

跟到底會發現下面這段

public B channelFactory(ChannelFactory<? extends C> channelFactory) {
        if (channelFactory == null) {
            throw new NullPointerException("channelFactory");
        }
        if (this.channelFactory != null) {
            throw new IllegalStateException("channelFactory set already");
        }
        //初始化cannelFactory
        this.channelFactory = channelFactory;
        return self();
    }

所以很明顯，cannelFactory是ReflectiveChannelFactory，我們繼續看ReflectiveChannelFactory的newChannel方法

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

這就可以看出，channel的創建是通過工廠模式，反射創建無參建構式的，實體就是我們初始化傳進去的 NioServerSocketChannel，我們把channel的創建看完，繼續跟它的建構式

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

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

DEFAULT_SELECTOR_PROVIDER 是根據作業系統選擇的provider，而newSocket其實就是根據provider到jdk底層去獲取對應的serversockerchannel，我們繼續this，

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

呼叫父類的構造方法，注意引數 SelectionKey.OP_ACCEPT，繼續

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);
        }
    }

這個建構式就是把前面生成的channel和accept事件保存起來，并設定該channel為非阻塞模式，是不是就是nio的代碼方式，我們繼續看super

protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }

id我們暫時不管，這里會生成一個unsafe 來操作bytebuffer的，還生成了pipeline，這個主要是為了執行我們初始化設定的一些handdler，我們后面分析；到這里把channel的初始化分析完了，回到之前的initAndRegister方法，我們繼續往下看有個init方法，它有兩個實作，一個是客戶端的BootStrap，一個是服務端的ServerBootStrap，做的事情都差不多，我們看下ServerBootStrap的，

@Override
    void init(Channel channel) throws Exception {
        // 把代碼啟動的時候設定的引數放到它該有的位置上
        final Map<ChannelOption<?>, Object> options = options0();
        synchronized (options) {
             // options設定到channel上
            setChannelOptions(channel, options, logger);
        }

        final Map<AttributeKey<?>, Object> attrs = attrs0();
        synchronized (attrs) {
            // 遍歷attr事件，設定到channel上
            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();
            ......

        // 把所有handler組裝成pipeline
        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(final Channel ch) throws Exception {
                final ChannelPipeline pipeline = ch.pipeline();
                ChannelHandler handler = config.handler();
                if (handler != null) {
                    pipeline.addLast(handler);
                }

                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });
    }            

省略了些代碼，主要還是把之前一開始初始化保存的物件綁到對應的channel上，然后放到一個 inboundhandler型別-ServerBootstrapAcceptor物件上，并給放到pipeline 鏈上，

我們繼續看initAndRegister的另一行代碼

ChannelFuture regFuture = config().group().register(channel);

 config().group()，在這里是 NioEventLoopGroup，register執行的是它的父類 MultithreadEventLoopGroup

public ChannelFuture register(Channel channel) {
        return next().register(channel);
    }

next方法有兩種實作，在NioEventLoopGroup初始化的時候會呼叫它的父類建構式，如果執行緒數是2的次方就實體化 PowerOfTwoEventExecutorChooser，否則就是GenericEventExecutorChooser，我們看看兩個實作的有啥區別

PowerOfTwoEventExecutorChooser：
public EventExecutor next() {
            return executors[idx.getAndIncrement() & executors.length - 1];
        }


GenericEventExecutorChooser：
public EventExecutor next() {
            return executors[Math.abs(idx.getAndIncrement() % executors.length)];
        }

一個按位與，一個是取模運算，明顯按位與快一點，所以推薦設定2的n次方

講完chooser選擇后，繼續看register，因為我們是 NioEventLoop，它繼承于 SingleThreadEventLoop，所以我們看它的register

public ChannelFuture register(final ChannelPromise promise) {
        ObjectUtil.checkNotNull(promise, "promise");
        promise.channel().unsafe().register(this, promise);
        return promise;
    }

從DefaultChannelPromise 拿到niosocketchannel，拿到對應的unsafe，而 AbstractUnsafe 是 AbstractChannel的內部類，

public final void register(EventLoop eventLoop, final ChannelPromise promise) {
           ......
            AbstractChannel.this.eventLoop = eventLoop;

            if (eventLoop.inEventLoop()) {
                register0(promise);
            } else {
                try {
                    eventLoop.execute(new Runnable() {
                        @Override
                        public void run() {
                            register0(promise);
                        }
                    });
                } 
            }
        }

繼續看 register0

private void register0(ChannelPromise promise) {
               .......
                doRegister();
               .......
                // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
                // user may already fire events through the pipeline in the ChannelFutureListener.
                pipeline.invokeHandlerAddedIfNeeded();

                safeSetSuccess(promise);
                pipeline.fireChannelRegistered();
                // Only fire a channelActive if the channel has never been registered. This prevents firing
                // multiple channel actives if the channel is deregistered and re-registered.
               .......
        }

繼續 doRegister

protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
                return;
            }
        }
    }

呼叫底層的jdk channel來注冊selector，拿到一個selectionKey，initAndRegister方法算是結束了，主要是初始化channel和注冊selector的，接下來看看 doBind0 方法，

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()) {
                    // 通過pipeline 從tail到head節點執行，最終在對應的NioServerSocketChannel執行bind方法
                    channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
                } else {
                    promise.setFailure(regFuture.cause());
                }
            }
        });
    }

我們繼續往下跟會發現，扔給執行緒池的任務異步執行，從AbstractChannel開始做bind操作，通過每個channel對應的 DefaultChannelPipeline 來執行bind，最侄訓通過 pipeline 從tail執行到head節點，最終跑到NioServerSocketChannel 類

protected void doBind(SocketAddress localAddress) throws Exception {
        // 最終執行到jdk底層的 bind方法
        if (PlatformDependent.javaVersion() >= 7) {
            javaChannel().bind(localAddress, config.getBacklog());
        } else {
            javaChannel().socket().bind(localAddress, config.getBacklog());
        }
    }

最終bind 方法執行完畢，

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