ServerBootstrap與Bootstrap分別是netty中服務(wù)端與客戶端的引導(dǎo)類里烦，主要負(fù)責(zé)服務(wù)端與客戶端初始化束昵、配置及啟動引導(dǎo)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(jìn)行一個簡單的分析捌袜。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(jìn)行一個簡單的分析琅轧。

// 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(wù)端引導(dǎo)類 ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) //設(shè)置線程組 .channel(NioServerSocketChannel.class)//設(shè)置ServerSocketChannel的IO模型 分為epoll與Nio .option(ChannelOption.SO_BACKLOG, 100)//設(shè)置option參數(shù)，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搭伤。 .childHandler(new ChannelInitializer<SocketChannel>() { //這個handler 將會在每個客戶端連接的時候調(diào)用只怎。供 SocketChannel 使用。 @Override public void initChannel(SocketChannel ch) throws Exception { 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(wù) 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(wù)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(wù)端啟動的流程進(jìn)行分析闷畸；

?我們首先要知道尝盼，netty服務(wù)的啟動其實可以分為以下四步：

創(chuàng)建服務(wù)端Channel

初始化服務(wù)端Channel

注冊Selector

端口綁定

一、創(chuàng)建服務(wù)端Channel

1佑菩、服務(wù)端Channel的創(chuàng)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(chuàng)建 NioServerSocketChannel

我們在initAndRegister()中可以看到channel的初始化裁赠。

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

?我進(jìn)一步看newChannel()中的源碼殿漠，在ReflectiveChannelFactory這個反射工廠中，通過clazz這個類的反射創(chuàng)建了一個服務(wù)端的channel佩捞。

@Override

? ? public T newChannel() {

? ? ? ? try {

? ? ? ? ? ? return clazz.getConstructor().newInstance();//反射創(chuàng)建

? ? ? ? } catch (Throwable t) {

? ? ? ? ? ? throw new ChannelException("Unable to create Channel from class " + clazz, t);

? ? ? ? }

? ? }

既然通過反射绞幌，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一忱，初始化的時候我們傳入了哪個channel莲蜘。

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(xiàn)，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(gòu)造了一個反射工廠帘营。

那么到這里就很清楚了票渠，我們創(chuàng)建的Channel就是一個NioServerSocketChannel，那么具體的創(chuàng)建我們就需要看下這個類的構(gòu)造函數(shù)芬迄。首先我們看下一個NioServerSocketChannel創(chuàng)建的具體流程

首先是newsocket(),我們先看下具體的代碼问顷，在NioServerSocketChannel的構(gòu)造函數(shù)中我們創(chuàng)建了一個jdk原生的ServerSocketChannel

/**

? ? * Create a new instance

? ? */

? ? public NioServerSocketChannel() {

? ? ? ? this(newSocket(DEFAULT_SELECTOR_PROVIDER));//傳入默認(rèn)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(chuàng)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(wù)端Channel的構(gòu)造函數(shù)，在代碼中我們可以看到我們把NioServerSocketChannel這個類傳入到了NioServerSocketChannelConfig的構(gòu)造函數(shù)中進(jìn)行配置

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

第三步在父類AbstractNioChannel的構(gòu)造函數(shù)中把創(chuàng)建服務(wù)端的Channel設(shè)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(chuàng)建的Channel this.readInterestOp = readInterestOp; try { ch.configureBlocking(false);//設(shè)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(diào)用AbstractChannel這個抽象類的構(gòu)造函數(shù)設(shè)置Channel的id（每個Channel都有一個id，唯一標(biāo)識）,unsafe（tcp相關(guān)底層操作）杜窄，pipeline（邏輯鏈）等肠骆，而不管是服務(wù)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(yīng)的屬性塞耕。我們看下AbstractChannel的構(gòu)造函數(shù)

/** * 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(chuàng)建Channel唯一標(biāo)識 unsafe = newUnsafe();//netty封裝的TCP 相關(guān)操作類 pipeline = newChannelPipeline();//邏輯鏈 }

?2蚀腿、初始化服務(wù)端創(chuàng)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(shè)置ChannelOptions、ChannelAttrs ,配置服務(wù)端Channel的相關(guān)屬性扫外；

設(shè)置ChildOptions莉钙、ChildAttrs，配置每個新連接的Channel的相關(guān)屬性畏浆；

Config handler胆胰，配置服務(wù)端pipeline;

add ServerBootstrapAcceptor,添加連接器，對accpet接受到的新連接進(jìn)行處理刻获，添加一個nio線程蜀涨；

那么接下來我們通過代碼，對每一步設(shè)置進(jìn)行一下分析：

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

final Map<ChannelOption<?>, Object> options = options0();//拿到你設(shè)置的option synchronized (options) { setChannelOptions(channel, options, logger);//設(shè)置NioServerSocketChannel相應(yīng)的TCP參數(shù)厚柳，其實這一步就是把options設(shè)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(guān)鍵代碼

//可以看到兩個都是局部變量沐兵，會在下面設(shè)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(wù)端Channel的handler進(jìn)行配置

p.addLast(new ChannelInitializer<Channel>() { @Override public void initChannel(final Channel ch) throws Exception { 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連接器别垮，這個是netty向服務(wù)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扎谎，我們來看下關(guān)鍵代碼

ch.eventLoop().execute(new Runnable() { @Override public void run() { //在這里會把我們自定義的ChildGroup碳想、ChildHandler、ChildOptions毁靶、ChildAttrs相關(guān)配置傳入到ServerBootstrapAcceptor構(gòu)造函數(shù)中胧奔，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(wù)端的Channel創(chuàng)建完畢后预吆，下一步就是要把它注冊到一個事件輪詢器Selector上龙填，在initAndRegister()中我們把上面初始化的Channel進(jìn)行注冊

ChannelFuture regFuture = config().group().register(channel);//注冊我們已經(jīng)初始化過的Channel

而這個register具體實現(xiàn)是在AbstractChannel中的AbstractUnsafe抽象類中的

/** * 1、先是一系列的判斷拐叉。 * 2岩遗、判斷當(dāng)前線程是否是給定的 eventLoop 線程。注意：這點很重要凤瘦，Netty 線程模型的高性能取決于對于當(dāng)前執(zhí)行的Thread 的身份的確定宿礁。如果不在當(dāng)前線程，那么就需要很多同步措施（比如加鎖）廷粒，上下文切換等耗費性能的操作窘拯。 * 3红且、異步（因為我們這里直到現(xiàn)在還是 main 線程在執(zhí)行，不屬于當(dāng)前線程）的執(zhí)行 register0 方法涤姊。 */ @Override public final void register(EventLoop eventLoop, final ChannelPromise promise) { 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(jìn)入register0()方法看下注冊過程的具體實現(xiàn)

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(fā)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(xiàn)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(wù)端的channel注冊到了指定的selector上暇番，下面就是服務(wù)端口的邦迪

三、端口綁定

首先我們梳理下netty中服務(wù)端口綁定的流程

我們來看下AbstarctUnsafe中bind()方法的具體實現(xiàn)

@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(fā)ChannelActive事件 } }); } safeSetSuccess(promise); }

在doBind(localAddress)中netty實現(xiàn)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(fā)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事件往下傳播思喊，然后調(diào)用readIfIsAutoRead()方法出觸發(fā)channel的read事件壁酬，而它最終調(diào)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上恨课。

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

1、channelFactory.newChannel()剂公，其實就是創(chuàng)建jdk底層channel希俩，并初始化id、piepline等屬性纲辽；

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

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

4怠硼、doBind0(regFuture, channel, localAddress, promise)鬼贱，完成服務(wù)端端口的監(jiān)聽，并把accept事件注冊到selector上香璃；

以上就是對netty服務(wù)端啟動流程進(jìn)行的一個簡單分析吩愧，有很多細(xì)節(jié)沒有關(guān)注與深入，其中如有不足與不正確的地方還望指出與海涵增显。