netty源碼分析之服務端啟動

ServerBootstrap與Bootstrap分別是netty中服務端與客戶端的引導類，主要負責服務端與客戶端初始化、配置及啟動引導等工作，接下來我們就通過netty源碼中的示例對ServerBootstrap與Bootstrap的源碼進行一個簡單的分析。首先我們知道這兩個類都繼承自AbstractBootstrap類

接下來我們就通過netty源碼中ServerBootstrap的實例入手對其進行一個簡單的分析。

// Configure the server.
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
final EchoServerHandler serverHandler = new EchoServerHandler();
try {
//初始化一個服務端引導類
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup) //設置線程組
.channel(NioServerSocketChannel.class)//設置ServerSocketChannel的IO模型 分為epoll與Nio
.option(ChannelOption.SO_BACKLOG, 100)//設置option參數，保存成一個LinkedHashMap<ChannelOption<?>, Object>()
.handler(new LoggingHandler(LogLevel.INFO))//這個hanlder 只專屬于 ServerSocketChannel 而不是 SocketChannel。
.childHandler(new ChannelInitializer<SocketChannel>() { //這個handler 將會在每個客戶端連接的時候調用。供 SocketChannel 使用。@Override
br/>@Override
ChannelPipeline p = ch.pipeline();
if (sslCtx != null) {
p.addLast(sslCtx.newHandler(ch.alloc()));
}
//p.addLast(new LoggingHandler(LogLevel.INFO));
p.addLast(serverHandler);
}
});

        // Start the server. 啟動服務
        ChannelFuture f = b.bind(PORT).sync();

        // Wait until the server socket is closed.
        f.channel().closeFuture().sync();
    } finally {
        // Shut down all event loops to terminate all threads.
        bossGroup.shutdownGracefully();
        workerGroup.shutdownGracefully();
    }

            接下來我們主要從服務端的socket在哪里初始化與哪里accept連接這兩個問題入手對netty服務端啟動的流程進行分析；

我們首先要知道，netty服務的啟動其實可以分為以下四步：

創建服務端Channel
初始化服務端Channel
注冊Selector
端口綁定
一、創建服務端Channel

1、服務端Channel的創建，主要為以下流程

我們通過跟蹤代碼能夠看到

final ChannelFuture regFuture = initAndRegister();// 初始化并創建 NioServerSocketChannel

我們在initAndRegister()中可以看到channel的初始化。

channel = channelFactory.newChannel(); // 通過 反射工廠創建一個 NioServerSocketChannel

我進一步看newChannel()中的源碼，在ReflectiveChannelFactory這個反射工廠中，通過clazz這個類的反射創建了一個服務端的channel。

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

    既然通過反射，我們就要知道clazz類是什么，那么我我們來看下channelFactory這個工廠類是在哪里初始化的，初始化的時候我們傳入了哪個channel。

這里我們需要看下demo實例中初始化ServerBootstrap時.channel(NioServerSocketChannel.class)這里的具體實現，我們看下源碼

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

    通過上面的代碼我可以直觀的看出正是在這里我們通過NioServerSocketChannel這個類構造了一個反射工廠。

那么到這里就很清楚了，我們創建的Channel就是一個NioServerSocketChannel，那么具體的創建我們就需要看下這個類的構造函數。首先我們看下一個NioServerSocketChannel創建的具體流程

首先是newsocket(),我們先看下具體的代碼，在NioServerSocketChannel的構造函數中我們創建了一個jdk原生的ServerSocketChannel

/**

• Create a new instance
  */
  public NioServerSocketChannel() {
  this(newSocket(DEFAULT_SELECTOR_PROVIDER));//傳入默認的SelectorProvider
  }

private static ServerSocketChannel newSocket(SelectorProvider provider) {
    try {
        /**
         *  Use the {@link SelectorProvider} to open {@link SocketChannel} and so remove condition in
         *  {@link SelectorProvider#provider()} which is called by each ServerSocketChannel.open() otherwise.
         *
         *  See <a >#2308</a>.
         */
        return provider.openServerSocketChannel();//可以看到創建的是jdk底層的ServerSocketChannel 
    } catch (IOException e) {
        throw new ChannelException(
                "Failed to open a server socket.", e);
    }
}

    第二步是通過NioServerSocketChannelConfig配置服務端Channel的構造函數，在代碼中我們可以看到我們把NioServerSocketChannel這個類傳入到了NioServerSocketChannelConfig的構造函數中進行配置

/**

• Create a new instance using the given {@link ServerSocketChannel}.
  */
  public NioServerSocketChannel(ServerSocketChannel channel) {
  super(null, channel, SelectionKey.OP_ACCEPT);//調用父類構造函數,傳入創建的channel
  config = new NioServerSocketChannelConfig(this, javaChannel().socket());
  }

    第三步在父類AbstractNioChannel的構造函數中把創建服務端的Channel設置為非阻塞模式

  /**

• Create a new instance
• @param parent the parent {@link Channel} by which this instance was created. May be {@code null}
• @param ch the underlying {@link SelectableChannel} on which it operates
• @param readInterestOp the ops to set to receive data from the {@link SelectableChannel}
  */
  protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
  super(parent);
  this.ch = ch;//這個ch就是傳入的通過jdk創建的Channel
  this.readInterestOp = readInterestOp;
  try {
  ch.configureBlocking(false);//設置為非阻塞
  } 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);
    }
}

    第四步調用AbstractChannel這個抽象類的構造函數設置Channel的id（每個Channel都有一個id，唯一標識）,unsafe（tcp相關底層操作），pipeline（邏輯鏈）等，而不管是服務的Channel還是客戶端的Channel都繼承自這個抽象類，他們也都會有上述相應的屬性。我們看下AbstractChannel的構造函數

  /**

• Creates a new instance.
• @param parent
• the parent of this channel. {@code null} if there's no parent.
  */
  protected AbstractChannel(Channel parent) {
  this.parent = parent;
  id = newId();//創建Channel唯一標識
  unsafe = newUnsafe();//netty封裝的TCP 相關操作類
  pipeline = newChannelPipeline();//邏輯鏈
  }

     2、初始化服務端創建的Channel

   init(channel);// 初始化這個 NioServerSocketChannel

我們首先列舉下init(channel)中具體都做了哪了些功能：

設置ChannelOptions、ChannelAttrs ,配置服務端Channel的相關屬性；
設置ChildOptions、ChildAttrs，配置每個新連接的Channel的相關屬性；
Config handler，配置服務端pipeline;
add ServerBootstrapAcceptor,添加連接器，對accpet接受到的新連接進行處理，添加一個nio線程；
那么接下來我們通過代碼，對每一步設置進行一下分析：

首先是在SeverBootstrap的init()方法中對ChannelOptions、ChannelAttrs 的配置的關鍵代碼

final Map<ChannelOption<?>, Object> options = options0();//拿到你設置的option
synchronized (options) {
setChannelOptions(channel, options, logger);//設置NioServerSocketChannel相應的TCP參數，其實這一步就是把options設置到channel的config中
}

    final Map<AttributeKey<?>, Object> attrs = attrs0();
    synchronized (attrs) {
        for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
            @SuppressWarnings("unchecked")
            AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
            channel.attr(key).set(e.getValue());
        }
    }

            然后是對ChildOptions、ChildAttrs配置的關鍵代碼

          //可以看到兩個都是局部變量，會在下面設置pipeline時用到

final Entry<ChannelOption<?>, Object>[] currentChildOptions;
final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
synchronized (childOptions) {
currentChildOptions = childOptions.entrySet().toArray(newOptionArray(0));
}
synchronized (childAttrs) {
currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(0));
}

            第三步對服務端Channel的handler進行配置

p.addLast(new ChannelInitializer<Channel>() {@Override
br/>@Override
final ChannelPipeline pipeline = ch.pipeline();
ChannelHandler handler = config.handler();//拿到我們自定義的hanler
if (handler != null) {
pipeline.addLast(handler);
}

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

            第四步添加ServerBootstrapAcceptor連接器，這個是netty向服務端Channel自定義添加的一個handler，用來處理新連接的添加與屬性配置，我們來看下關鍵代碼

          ch.eventLoop().execute(new Runnable() {

@Override
public void run() {
//在這里會把我們自定義的ChildGroup、ChildHandler、ChildOptions、ChildAttrs相關配置傳入到ServerBootstrapAcceptor構造函數中，并綁定到新的連接上
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});

三、注冊Selector

一個服務端的Channel創建完畢后，下一步就是要把它注冊到一個事件輪詢器Selector上，在initAndRegister()中我們把上面初始化的Channel進行注冊

ChannelFuture regFuture = config().group().register(channel);//注冊我們已經初始化過的Channel
而這個register具體實現是在AbstractChannel中的AbstractUnsafe抽象類中的
/**

• 1、先是一系列的判斷。
• 2、判斷當前線程是否是給定的 eventLoop 線程。注意：這點很重要，Netty 線程模型的高性能取決于對于當前執行的Thread 的身份的確定。如果不在當前線程，那么就需要很多同步措施（比如加鎖），上下文切換等耗費性能的操作。
• 3、異步（因為我們這里直到現在還是 main 線程在執行，不屬于當前線程）的執行 register0 方法。*/
  @Override
  br/>*/
  @Override
  if (eventLoop == null) {
  throw new NullPointerException("eventLoop");
  }
  if (isRegistered()) {
  promise.setFailure(new IllegalStateException("registered to an event loop already"));
  return;
  }
  if (!isCompatible(eventLoop)) {
  promise.setFailure(
  new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
  return;
  }

        AbstractChannel.this.eventLoop = eventLoop;//綁定線程

        if (eventLoop.inEventLoop()) {
            register0(promise);//實際的注冊過程
        } else {
            try {
                eventLoop.execute(new Runnable() {
                    @Override
                    public void run() {
                        register0(promise);
                    }
                });
            } catch (Throwable t) {
                logger.warn(
                        "Force-closing a channel whose registration task was not accepted by an event loop: {}",
                        AbstractChannel.this, t);
                closeForcibly();
                closeFuture.setClosed();
                safeSetFailure(promise, t);
            }
        }
    }

首先我們對整個注冊的流程做一個梳理

接下來我們進入register0()方法看下注冊過程的具體實現

private void register0(ChannelPromise promise) {
try {
// check if the channel is still open as it could be closed in the mean time when the register
// call was outside of the eventLoop
if (!promise.setUncancellable() || !ensureOpen(promise)) {
return;
}
boolean firstRegistration = neverRegistered;
doRegister();//jdk channel的底層注冊
neverRegistered = false;
registered = true;

            // 觸發綁定的handler事件
            // 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.
            if (isActive()) {
                if (firstRegistration) {
                    pipeline.fireChannelActive();
                } else if (config().isAutoRead()) {
                    // This channel was registered before and autoRead() is set. This means we need to begin read
                    // again so that we process inbound data.
                    //
                    // See https://github.com/netty/netty/issues/4805
                    beginRead();
                }
            }
        } catch (Throwable t) {
            // Close the channel directly to avoid FD leak.
            closeForcibly();
            closeFuture.setClosed();
            safeSetFailure(promise, t);
        }
    }
AbstractNioChannel中doRegister()的具體實現就是把jdk底層的channel綁定到eventLoop的selecor上

@Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
//把channel注冊到eventLoop上的selector上
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
} catch (CancelledKeyException e) {
if (!selected) {
// Force the Selector to select now as the "canceled" SelectionKey may still be
// cached and not removed because no Select.select(..) operation was called yet.
eventLoop().selectNow();
selected = true;
} else {
// We forced a select operation on the selector before but the SelectionKey is still cached
// for whatever reason. JDK bug ?
throw e;
}
}
}
}

    到這里netty就把服務端的channel注冊到了指定的selector上，下面就是服務端口的邦迪

三、端口綁定

首先我們梳理下netty中服務端口綁定的流程

我們來看下AbstarctUnsafe中bind()方法的具體實現

@Override
public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
assertEventLoop();

        if (!promise.setUncancellable() || !ensureOpen(promise)) {
            return;
        }

        // See: https://github.com/netty/netty/issues/576
        if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
            localAddress instanceof InetSocketAddress &&
            !((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() &&
            !PlatformDependent.isWindows() && !PlatformDependent.maybeSuperUser()) {
            // 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();//判斷綁定是否完成
        try {
            doBind(localAddress);//底層jdk綁定端口
        } catch (Throwable t) {
            safeSetFailure(promise, t);
            closeIfClosed();
            return;
        }

        if (!wasActive && isActive()) {
            invokeLater(new Runnable() {
                @Override
                public void run() {
                    pipeline.fireChannelActive();//觸發ChannelActive事件
                }
            });
        }

        safeSetSuccess(promise);
    }

            在doBind(localAddress)中netty實現了jdk底層端口的綁定

          @Override

protected void doBind(SocketAddress localAddress) throws Exception {
if (PlatformDependent.javaVersion() >= 7) {
javaChannel().bind(localAddress, config.getBacklog());
} else {
javaChannel().socket().bind(localAddress, config.getBacklog());
}
}

在 pipeline.fireChannelActive()中會觸發pipeline中的channelActive()方法

@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
ctx.fireChannelActive();

        readIfIsAutoRead();
    }

在channelActive中首先會把ChannelActive事件往下傳播，然后調用readIfIsAutoRead()方法出觸發channel的read事件，而它最終調用AbstractNioChannel中的doBeginRead()方法

@Override
protected void doBeginRead() throws Exception {
// Channel.read() or ChannelHandlerContext.read() was called
final SelectionKey selectionKey = this.selectionKey;
if (!selectionKey.isValid()) {
return;
}

    readPending = true;

    final int interestOps = selectionKey.interestOps();
    if ((interestOps & readInterestOp) == 0) {
        selectionKey.interestOps(interestOps | readInterestOp);//readInterestOp為  SelectionKey.OP_ACCEPT
    }
}

在doBeginRead()方法，netty會把accept事件注冊到Selector上。

到此我們對netty服務端的啟動流程有了一個大致的了解，整體可以概括為下面四步：

1、channelFactory.newChannel()，其實就是創建jdk底層channel，并初始化id、piepline等屬性；

2、init(channel)，添加option、attr等屬性，并添加ServerBootstrapAcceptor連接器；

3、config().group().register(channel)，把jdk底層的channel注冊到eventLoop上的selector上；

4、doBind0(regFuture, channel, localAddress, promise)，完成服務端端口的監聽，并把accept事件注冊到selector上；

以上就是對netty服務端啟動流程進行的一個簡單分析，有很多細節沒有關注與深入，其中如有不足與不正確的地方還望指出與海涵。