本系列：
本節主要分析server的啟動過程。
Netty是基於Nio實現的，所以也離不開selector、serverSocketChannel、socketChannel和selectKey等，只不過Netty把這些實現都封裝在了底層。
從示例可以看出，一切從ServerBootstrap開始。
ServerBootstrap例項中需要兩個NioEventLoopGroup例項，分別為boss和work，有不同的分工：
1、 boss負責請求的accept操作。
2、 work負責請求的read、write和處理操作。
NioEventLoopGroup
NioEventLoopGroup主要負責管理eventLoop的生命週期，eventLoop數量預設為處理器個數的兩倍。
NioEventLoopGroup
繼承關係如下：
NioEventLoopGroup
NioEventLoopGroup構造方法：
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public NioEventLoopGroup() { this(0); } public NioEventLoopGroup(int nThreads) { this(nThreads, null); } public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) { this(nThreads, threadFactory, SelectorProvider.provider()); } public NioEventLoopGroup( int nThreads, ThreadFactory threadFactory, final SelectorProvider selectorProvider) { super(nThreads, threadFactory, selectorProvider); } |
MultithreadEventLoopGroup構造方法：
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protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) { super(nThreads == 0? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args); } |
其中 DEFAULT_EVENT_LOOP_THREADS 為處理器數量的兩倍。
MultithreadEventExecutorGroup是核心，管理eventLoop的生命週期，先看看其中幾個變數。
1、children：EventExecutor陣列，儲存eventLoop。
2、chooser：從children中選取一個eventLoop的策略。
構造方法：
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protected MultithreadEventExecutorGroup(int nThreads, ThreadFactory threadFactory, Object... args) { if (nThreads <= 0) { throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads)); } if (threadFactory == null) { threadFactory = newDefaultThreadFactory(); } children = new SingleThreadEventExecutor[nThreads]; if (isPowerOfTwo(children.length)) { chooser = new PowerOfTwoEventExecutorChooser(); } else { chooser = new GenericEventExecutorChooser(); } for (int i = 0; i < nThreads; i ++) { boolean success = false; try { children[i] = newChild(threadFactory, args); success = true; } catch (Exception e) { // TODO: Think about if this is a good exception type throw new IllegalStateException("failed to create a child event loop", e); } finally { if (!success) { for (int j = 0; j < i; j ++) { children[j].shutdownGracefully(); } for (int j = 0; j < i; j ++) { EventExecutor e = children[j]; try { while (!e.isTerminated()) { e.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS); } } catch (InterruptedException interrupted) { Thread.currentThread().interrupt(); break; } } } } } final FutureListener<Object> terminationListener = new FutureListener<Object>() { @Override public void operationComplete(Future<Object> future) throws Exception { if (terminatedChildren.incrementAndGet() == children.length) { terminationFuture.setSuccess(null); } } }; for (EventExecutor e: children) { e.terminationFuture().addListener(terminationListener); } } protected EventExecutor newChild( ThreadFactory threadFactory, Object... args) throws Exception { return new NioEventLoop(this, threadFactory, (SelectorProvider) args[0]); } |
1、 根據陣列的大小，採用不同策略初始化chooser。
如果大小為2的冪次方，則採用PowerOfTwoEventExecutorChooser；否則使用GenericEventExecutorChooser。
判斷一個數是否是2的冪次方的方法，覺得很贊。
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private static boolean isPowerOfTwo(int val) { return (val & -val) == val; } |
2、newChild方法過載，初始化EventExecutor時，實際執行的是NioEventLoopGroup中的newChild方法，所以，children元素的實際型別為NioEventLoop。
接下去看看NioEventLoop類。
NioEventLoop
每個eventLoop會維護一個selector和taskQueue，負責處理客戶端請求和內部任務，如ServerSocketChannel註冊和ServerSocket繫結等。
NioEventLoop
繼承關係如下：
NioEventLoop
構造方法：
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NioEventLoop(NioEventLoopGroup parent, ThreadFactory threadFactory, SelectorProvider selectorProvider) { super(parent, threadFactory, false); if (selectorProvider == null) { throw new NullPointerException("selectorProvider"); } provider = selectorProvider; selector = openSelector(); } |
當看到 selector = openSelector() 時，有沒有覺得親切了許多，這裡先不管 selector，看看SingleThreadEventLoop類。
SingleThreadEventLoop 構造方法：
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protected SingleThreadEventLoop(EventLoopGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) { super(parent, threadFactory, addTaskWakesUp); } |
啥事都沒做…
SingleThreadEventExecutor
從命名上可以看出，這是一個只有一個執行緒的執行緒池， 先看看其中的幾個變數：
1、state：執行緒池當前的狀態
2、taskQueue：存放任務的佇列
3、thread：執行緒池維護的唯一執行緒
4、scheduledTaskQueue：定義在其父類AbstractScheduledEventExecutor中，用以儲存延遲執行的任務。
…
構造方法：
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protected SingleThreadEventExecutor(EventExecutorGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) { if (threadFactory == null) { throw new NullPointerException("threadFactory"); } this.parent = parent; this.addTaskWakesUp = addTaskWakesUp; thread = threadFactory.newThread(new Runnable() { @Override public void run() { boolean success = false; updateLastExecutionTime(); try { SingleThreadEventExecutor.this.run(); success = true; } catch (Throwable t) { logger.warn("Unexpected exception from an event executor: ", t); } finally { for (;;) { int oldState = STATE_UPDATER.get(SingleThreadEventExecutor.this); if (oldState >= ST_SHUTTING_DOWN || STATE_UPDATER.compareAndSet( SingleThreadEventExecutor.this, oldState, ST_SHUTTING_DOWN)) { break; } } // Check if confirmShutdown() was called at the end of the loop. if (success && gracefulShutdownStartTime == 0) { logger.error( "Buggy " + EventExecutor.class.getSimpleName() + " implementation; " + SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must be called " + "before run() implementation terminates."); } try { // Run all remaining tasks and shutdown hooks. for (;;) { if (confirmShutdown()) { break; } } } finally { try { cleanup(); } finally { STATE_UPDATER.set(SingleThreadEventExecutor.this, ST_TERMINATED); threadLock.release(); if (!taskQueue.isEmpty()) { logger.warn( "An event executor terminated with " + "non-empty task queue (" + taskQueue.size() + ')'); } terminationFuture.setSuccess(null); } } } } }); threadProperties = new DefaultThreadProperties(thread); taskQueue = newTaskQueue(); } |
程式碼很長，內容很簡單：
1、初始化一個執行緒，並線上程內部執行NioEventLoop類的run方法，當然這個執行緒不會立刻執行。
2、使用LinkedBlockingQueue類初始化taskQueue。
ServerBootstrap
通過serverBootstrap.bind(port)啟動服務，過程如下：
bind過程
doBind實現如下
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private ChannelFuture doBind(final SocketAddress localAddress) { final ChannelFuture regFuture = initAndRegister(); final Channel channel = regFuture.channel(); if (regFuture.cause() != null) { return regFuture; } if (regFuture.isDone()) { // At this point we know that the registration was complete and successful. ChannelPromise promise = channel.newPromise(); doBind0(regFuture, channel, localAddress, promise); return promise; } else { // Registration future is almost always fulfilled already, but just in case it's not. final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel); regFuture.addListener(new ChannelFutureListener() { @Override public void operationComplete(ChannelFuture future) throws Exception { Throwable cause = future.cause(); if (cause != null) { // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an // IllegalStateException once we try to access the EventLoop of the Channel. promise.setFailure(cause); } else { // Registration was successful, so set the correct executor to use. // See https://github.com/netty/netty/issues/2586 promise.executor = channel.eventLoop(); } doBind0(regFuture, channel, localAddress, promise); } }); return promise; } } |
1、方法initAndRegister返回一個ChannelFuture例項regFuture，通過regFuture可以判斷initAndRegister執行結果。
2、如果regFuture.isDone()為true，說明initAndRegister已經執行完，則直接執行doBind0進行socket繫結。
3、否則regFuture新增一個ChannelFutureListener監聽，當initAndRegister執行完成時，呼叫operationComplete方法並執行doBind0進行socket繫結。
所以只有當initAndRegister操作結束之後才能進行bind操作。
initAndRegister
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final ChannelFuture initAndRegister() { final Channel channel = channelFactory().newChannel(); try { init(channel); } catch (Throwable t) { channel.unsafe().closeForcibly(); // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t); } ChannelFuture regFuture = group().register(channel); if (regFuture.cause() != null) { if (channel.isRegistered()) { channel.close(); } else { channel.unsafe().closeForcibly(); } } return regFuture; } |
1、主要負責建立服務端channel，在本例子中，建立了NioServerSocketChannel。
2、為NioServerSocketChannel的pipeline新增handler。
3、註冊NioServerSocketChannel到selector。
NioServerSocketChannel
對Nio的ServerSocketChannel和SelectionKey進行了封裝。
繼承關係：
NioServerSocketChannel繼承關係
構造方法：
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public NioServerSocketChannel() { this(newSocket(DEFAULT_SELECTOR_PROVIDER)); } private static ServerSocketChannel newSocket(SelectorProvider provider) { try { return provider.openServerSocketChannel(); } catch (IOException e) { throw new ChannelException( "Failed to open a server socket.", e); } } public NioServerSocketChannel(ServerSocketChannel channel) { super(null, channel, SelectionKey.OP_ACCEPT); config = new NioServerSocketChannelConfig(this, javaChannel().socket()); } |
1、方法newSocket利用 provider.openServerSocketChannel() 生成Nio中的ServerSocketChannel物件。
2、設定SelectionKey.OP_ACCEPT事件。
AbstractNioMessageChannel構造方法
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protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) { super(parent, ch, readInterestOp); } |
啥也沒做…
AbstractNioChannel構造方法
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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); } } |
設定當前ServerSocketChannel為非阻塞通道。
AbstractChannel構造方法
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protected AbstractChannel(Channel parent) { this.parent = parent; unsafe = newUnsafe(); pipeline = new DefaultChannelPipeline(this); } |
1、初始化unsafe。
這裡的Unsafe並非是jdk中底層Unsafe類，用來負責底層的connect、register、read和write等操作。
2、初始化pipeline。
每個Channel都有自己的pipeline，當有請求事件發生時，pipeline負責呼叫相應的hander進行處理。
unsafe和pipeline的具體實現原理會在後續進行分析。
回到ServerBootstrap的init(Channel channel)方法，新增handler到channel的pipeline中。
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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(); 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 { ChannelPipeline pipeline = ch.pipeline(); ChannelHandler handler = handler(); if (handler != null) { pipeline.addLast(handler); } pipeline.addLast(new ServerBootstrapAcceptor( currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)); } }); } |
1、設定channel的options和attrs。
2、在pipeline中新增一個ChannelInitializer物件，每個client請求進來都會執行initChannel方法。
init執行完，需要把當前channel註冊到EventLoopGroup。
其實最終目的是為了實現Nio中把ServerSocket註冊到selector上，這樣就可以實現client請求的監聽了。
看看Netty中是如何實現的：
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public ChannelFuture register(Channel channel, ChannelPromise promise) { return next().register(channel, promise); } public EventLoop next() { return (EventLoop) super.next(); } public EventExecutor next() { return children[Math.abs(childIndex.getAndIncrement() % children.length)]; } |
因為EventLoopGroup中維護了多個eventLoop，next方法會呼叫chooser策略找到下一個eventLoop，並執行eventLoop的register方法進行註冊。
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public ChannelFuture register(final Channel channel, final ChannelPromise promise) { ... channel.unsafe().register(this, promise); return promise; } |
channel.unsafe()是什麼？
NioServerSocketChannel初始化時，會建立一個NioMessageUnsafe例項，用於實現底層的register、read、write等操作。
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eventLoop.execute(new Runnable() { @Override public void run() { register0(promise); } }); private void register0(ChannelPromise promise) { try { if (!ensureOpen(promise)) { return; } Runnable postRegisterTask = doRegister(); registered = true; promise.setSuccess(); pipeline.fireChannelRegistered(); if (postRegisterTask != null) { postRegisterTask.run(); } if (isActive()) { pipeline.fireChannelActive(); } } catch (Throwable t) { // Close the channel directly to avoid FD leak. closeForcibly(); if (!promise.tryFailure(t)) { } closeFuture.setClosed(); } } public void execute(Runnable task) { if (task == null) { throw new NullPointerException("task"); } boolean inEventLoop = inEventLoop(); if (inEventLoop) { addTask(task); } else { startThread(); addTask(task); if (isShutdown() && removeTask(task)) { reject(); } } if (!addTaskWakesUp) { wakeup(inEventLoop); } } |
1、register0方法提交到eventLoop執行緒池中執行，這個時候會啟動eventLoop中的執行緒。
2、方法doRegister()才是最終Nio中的註冊方法，方法javaChannel()獲取ServerSocketChannel。
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protected Runnable doRegister() throws Exception { boolean selected = false; for (;;) { try { selectionKey = javaChannel().register(eventLoop().selector, 0, this); return null; } 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; } } } } |
ServerSocketChannel註冊完之後，通知pipeline執行fireChannelRegistered方法，pipeline中維護了handler連結串列，通過遍歷連結串列，執行InBound型別handler的channelRegistered方法，最終執行init中新增的ChannelInitializer handler。
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public final void channelRegistered(ChannelHandlerContext ctx) throws Exception { boolean removed = false; boolean success = false; try { initChannel((C) ctx.channel()); ctx.pipeline().remove(this); removed = true; ctx.fireChannelRegistered(); success = true; } catch (Throwable t) { logger.warn("Failed to initialize a channel. Closing: " + ctx.channel(), t); } finally { if (!removed) { ctx.pipeline().remove(this); } if (!success) { ctx.close(); } } } |
1、initChannel方法最終把ServerBootstrapAcceptor新增到ServerSocketChannel的pipeline，負責accept客戶端請求。
2、在pipeline中刪除對應的handler。
3、觸發fireChannelRegistered方法，可以自定義handler的channelRegistered方法。
到目前為止，ServerSocketChannel完成了初始化並註冊到seletor上，啟動執行緒執行selector.select()方法準備接受客戶端請求。
細心的同學已經發現，ServerSocketChannel的socket還未繫結到指定埠，那麼這一塊Netty是如何實現的？
Netty把註冊操作放到eventLoop中執行。
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private static void doBind0( final ChannelFuture regFuture, final Channel channel, final SocketAddress localAddress, final ChannelPromise promise) { 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()); } } }); } public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) { return pipeline.bind(localAddress, promise); } @Override public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) { return tail.bind(localAddress, promise); } @Override public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) { if (localAddress == null) { throw new NullPointerException("localAddress"); } validatePromise(promise, false); return findContextOutbound().invokeBind(localAddress, promise); } private ChannelFuture invokeBind(final SocketAddress localAddress, final ChannelPromise promise) { EventExecutor executor = executor(); if (executor.inEventLoop()) { invokeBind0(localAddress, promise); } else { executor.execute(new Runnable() { @Override public void run() { invokeBind0(localAddress, promise); } }); } return promise; } private void invokeBind0(SocketAddress localAddress, ChannelPromise promise) { try { ((ChannelOutboundHandler) handler()).bind(this, localAddress, promise); } catch (Throwable t) { notifyOutboundHandlerException(t, promise); } } @Override public void bind( ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception { unsafe.bind(localAddress, promise); } |
最終由unsafe實現埠的bind操作。
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public final void bind(final SocketAddress localAddress, final ChannelPromise promise) { if (!ensureOpen(promise)) { return; } try { boolean wasActive = isActive(); ... doBind(localAddress); promise.setSuccess(); if (!wasActive && isActive()) { pipeline.fireChannelActive(); } } catch (Throwable t) { promise.setFailure(t); closeIfClosed(); } } protected void doBind(SocketAddress localAddress) throws Exception { javaChannel().socket().bind(localAddress, config.getBacklog()); } |
bind完成後，且ServerSocketChannel也已經註冊完成，則觸發pipeline的fireChannelActive方法，所以在這裡可以自定義fireChannelActive方法，預設執行tail的fireChannelActive。
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@Override public ChannelPipeline fireChannelActive() { head.fireChannelActive(); if (channel.config().isAutoRead()) { channel.read(); } return this; } |
channel.read()方法會觸發pipeline的行為：
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@Override public Channel read() { pipeline.read(); return this; } @Override public ChannelPipeline read() { tail.read(); return this; } @Override public ChannelHandlerContext read() { findContextOutbound().invokeRead(); return this; } private void invokeRead() { EventExecutor executor = executor(); if (executor.inEventLoop()) { invokeRead0(); } else { Runnable task = invokeRead0Task; if (task == null) { invokeRead0Task = task = new Runnable() { @Override public void run() { invokeRead0(); } }; } executor.execute(task); } } private void invokeRead0() { try { ((ChannelOutboundHandler) handler()).read(this); } catch (Throwable t) { notifyHandlerException(t); } } |
最終會在pipeline中找到handler執行read方法，預設是head。
至此為止，server已經啟動完成。
END。
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