Netty官網：https://netty.io/
Netty is an asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients.
Java技術棧方向的朋友應該或多或少都聽說過Netty是對Java中nio ( Non Blocking IO )的封裝，讓我們能快速開發出效能更高、擴充套件性更好的網路應用程式。那麼Netty究竟對nio做了怎樣的封裝呢？本文主要從原始碼角度揭開這層面紗。
原始碼追蹤中，我使用阿里的語雀產品的思維圖記錄主要方法呼叫，上面的圖片是部分截圖，完整原貌見：
https://www.yuque.com/docs/share/02fa3e3d-d485-48e1-9cfe-6722a3ad8915
預備知識
在初探Netty原始碼之前，至少需要理解Reactor Pattern、java.nio基本使用、Netty基本使用，這樣後面才能把Netty的原始碼與java.nio對比著來看。
Reactor Pattern
不識Netty真面目，只緣未讀此真經。Doug Lea (java.util.concurrent包的作者) 在《Scalable IO in Java》中循序漸進地分析瞭如何構建可伸縮的高效能IO服務以及服務模型的演變與進化。文中描述的Reactor Pattern，也被Netty等大多數高效能IO服務框架所借鑑。因此仔細閱讀《Scalable IO in Java》有助於更好地理解Netty框架的架構與設計。詳情見：
http://gee.cs.oswego.edu/dl/cpjslides/nio.pdf
傳統的服務模式
Server端為每一個Client端的連線請求都開啟一個獨立執行緒，也就是所謂的BIO (Blocking IO)，即java.net.ServerSocket包下api的使用。
基於事件驅動模式
Reactor模式
Reactor responds to IO events by dispatching the appropriate handler (Similar to AWT thread)
Handlers perform non-blocking actions (Similar to AWT ActionListeners)
Manage by binding handlers to events (Similar to AWT addActionListener)
(1) 單執行緒版本
(2) 多執行緒版本
(3) 多Reactor版本 (一主多從、多主多從)
Netty正是借鑑了這種多Reactor版本的設計。
快速上手java.nio
- Channels：Connections to files, sockets etc that support non-blocking reads
- Buffers：Array-like objects that can be directly read or written by Channels
- Selectors：Tell which of a set of Channels have IO events
- SelectionKeys：Maintain IO event status and bindings
注意：以下Demo僅專注於主邏輯，沒有處理異常，也沒有關閉資源。
Server端
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.channels.spi.SelectorProvider;
import java.util.Iterator;
public class NIOServer {
private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
public static void main(String[] args) throws IOException {
// ServerSocketChannel.open()
ServerSocketChannel serverSocketChannel = DEFAULT_SELECTOR_PROVIDER.openServerSocketChannel();
serverSocketChannel.configureBlocking(false);
serverSocketChannel.socket().bind(new InetSocketAddress(8080));
// Selector.open()
Selector selector = DEFAULT_SELECTOR_PROVIDER.openSelector();
// register this serverSocketChannel with the selector
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
// selector.select()
while (!Thread.interrupted()) {
selector.select();
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while (it.hasNext()) {
SelectionKey key = it.next();
it.remove();
// handle IO events
handle(key);
}
}
}
private static void handle(SelectionKey key) throws IOException {
if (key.isAcceptable()) {
ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key.channel();
SocketChannel socketChannel = serverSocketChannel.accept();
socketChannel.configureBlocking(false);
socketChannel.register(key.selector(), SelectionKey.OP_READ);
} else if (key.isReadable()) {
SocketChannel socketChannel = (SocketChannel) key.channel();
// read client data
ByteBuffer buffer = ByteBuffer.allocate(1024);
int len = socketChannel.read(buffer);
if (len != -1) {
String msg = String.format("recv client[%s] data：%s", socketChannel.getRemoteAddress(),
new String(buffer.array(), 0, len));
System.out.println(msg);
}
// response client
ByteBuffer data = ByteBuffer.wrap("Hello, NIOClient!".getBytes());
socketChannel.write(data);
key.interestOps(SelectionKey.OP_READ | SelectionKey.OP_WRITE);
} else if (key.isWritable()) {
// ...
}
}
}
Client端
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.nio.channels.spi.SelectorProvider;
import java.util.Iterator;
public class NIOClient {
private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
public static void main(String[] args) throws IOException {
// SocketChannel.open()
SocketChannel socketChannel = DEFAULT_SELECTOR_PROVIDER.openSocketChannel();
socketChannel.configureBlocking(false);
socketChannel.connect(new InetSocketAddress("127.0.0.1", 8080));
// Selector.open()
Selector selector = DEFAULT_SELECTOR_PROVIDER.openSelector();
// register this socketChannel with the selector
socketChannel.register(selector, SelectionKey.OP_CONNECT);
// selector.select()
while (!Thread.interrupted()) {
selector.select();
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while (it.hasNext()) {
SelectionKey key = it.next();
it.remove();
// handle IO events
if (key.isConnectable()) {
SocketChannel channel = (SocketChannel) key.channel();
if (channel.isConnectionPending()) {
channel.finishConnect();
}
channel.configureBlocking(false);
// request server
ByteBuffer buffer = ByteBuffer.wrap("Hello, NIOServer!".getBytes());
channel.write(buffer);
channel.register(selector, SelectionKey.OP_READ);
} else if (key.isReadable()) {
SocketChannel channel = (SocketChannel) key.channel();
// read server data
ByteBuffer buffer = ByteBuffer.allocate(1024);
int len = channel.read(buffer);
if (len != -1) {
String msg = String.format("recv server[%s] data：%s", channel.getRemoteAddress(),
new String(buffer.array(), 0, len));
System.out.println(msg);
}
}
}
}
}
}
快速上手Netty
更多官方example，請參考：
https://github.com/netty/netty/tree/4.1/example/
Server端
import io.netty.bootstrap.ServerBootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.util.CharsetUtil;
public class NettyServer {
public static void main(String[] args) throws Exception {
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap bootstrap = new ServerBootstrap()
.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 1024)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) {
ch.pipeline().addLast(new NettyServerHandler());
}
});
ChannelFuture cf = bootstrap.bind(8080).sync();
cf.channel().closeFuture().sync();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
static class NettyServerHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
String message = String.format("recv client[%s] data：%s", ctx.channel().remoteAddress(),
((ByteBuf) msg).toString(CharsetUtil.UTF_8));
System.out.println(message);
}
@Override
public void channelReadComplete(ChannelHandlerContext ctx) {
ByteBuf buf = Unpooled.copiedBuffer("Hello, NettyClient!".getBytes(CharsetUtil.UTF_8));
ctx.writeAndFlush(buf);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
cause.printStackTrace();
ctx.close();
}
}
}
Client端
import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.util.CharsetUtil;
public class NettyClient {
public static void main(String[] args) throws Exception {
EventLoopGroup group = new NioEventLoopGroup(1);
try {
Bootstrap bootstrap = new Bootstrap()
.group(group)
.channel(NioSocketChannel.class)
.handler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) {
ch.pipeline().addLast(new NettyClientHandler());
}
});
ChannelFuture cf = bootstrap.connect("127.0.0.1", 8080).sync();
cf.channel().closeFuture().sync();
} finally {
group.shutdownGracefully();
}
}
static class NettyClientHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelActive(ChannelHandlerContext ctx) {
ByteBuf buf = Unpooled.copiedBuffer("Hello, NettyServer!".getBytes(CharsetUtil.UTF_8));
ctx.writeAndFlush(buf);
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
String message = String.format("recv server[%s] data：%s", ctx.channel().remoteAddress(),
((ByteBuf) msg).toString(CharsetUtil.UTF_8));
System.out.println(message);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
cause.printStackTrace();
ctx.close();
}
}
}
原始碼追蹤
建議跟著我畫的原始碼走向圖，跟下面的內容，最好也開著debug模式，不理解的地方除錯幾遍。這裡再次貼一下連結：
https://www.yuque.com/docs/share/02fa3e3d-d485-48e1-9cfe-6722a3ad8915
注意：追蹤的是當前最新release的 4.1.58.Final 版本的原始碼。
<dependency>
<groupId>io.netty</groupId>
<artifactId>netty-all</artifactId>
<version>4.1.58.Final</version>
</dependency>
本文出自 行無際的部落格 ：
https://www.cnblogs.com/itwild/
關鍵類
下面先重點看幾個關鍵類的大致情況，方便我們讀程式碼 。因為面向抽象程式設計，如果對常見類的繼承層次一點不瞭解，讀程式碼的過程會讓人崩潰。你懂的！！！
NioEventLoopGroup
類定義：
io.netty.channel.nio.NioEventLoopGroup
/**
* {@link MultithreadEventLoopGroup} implementations which is used for NIO {@link Selector} based {@link Channel}s.
*/
public class NioEventLoopGroup extends MultithreadEventLoopGroup
類圖：
NioEventLoop
類定義：
io.netty.channel.nio.NioEventLoop
/**
* {@link SingleThreadEventLoop} implementation which register the {@link Channel}'s to a
* {@link Selector} and so does the multi-plexing of these in the event loop.
*
*/
public final class NioEventLoop extends SingleThreadEventLoop
類圖：
NioServerSocketChannel
類定義：
io.netty.channel.socket.nio.NioServerSocketChannel
/**
* A {@link io.netty.channel.socket.ServerSocketChannel} implementation which uses
* NIO selector based implementation to accept new connections.
*/
public class NioServerSocketChannel extends AbstractNioMessageChannel
implements io.netty.channel.socket.ServerSocketChannel
類圖：
NioSocketChannel
類定義：
io.netty.channel.socket.nio.NioSocketChannel
/**
* {@link io.netty.channel.socket.SocketChannel} which uses NIO selector based implementation.
*/
public class NioSocketChannel extends AbstractNioByteChannel
implements io.netty.channel.socket.SocketChannel
類圖：
ChannelInitializer
類定義：
io.netty.channel.ChannelInitializer
/**
* A special {@link ChannelInboundHandler} which offers an easy way to initialize a {@link Channel} once it was
* registered to its {@link EventLoop}.
*
* Implementations are most often used in the context of {@link Bootstrap#handler(ChannelHandler)} ,
* {@link ServerBootstrap#handler(ChannelHandler)} and {@link ServerBootstrap#childHandler(ChannelHandler)} to
* setup the {@link ChannelPipeline} of a {@link Channel}.
*
* <pre>
*
* public class MyChannelInitializer extends {@link ChannelInitializer} {
* public void initChannel({@link Channel} channel) {
* channel.pipeline().addLast("myHandler", new MyHandler());
* }
* }
*
* {@link ServerBootstrap} bootstrap = ...;
* ...
* bootstrap.childHandler(new MyChannelInitializer());
* ...
* </pre>
* Be aware that this class is marked as {@link Sharable} and so the implementation must be safe to be re-used.
*
* @param <C> A sub-type of {@link Channel}
*/
@Sharable
public abstract class ChannelInitializer<C extends Channel> extends ChannelInboundHandlerAdapter
類圖：
ChannelInboundHandlerAdapter
類定義：
io.netty.channel.ChannelInboundHandlerAdapter
/**
* Abstract base class for {@link ChannelInboundHandler} implementations which provide
* implementations of all of their methods.
*
* <p>
* This implementation just forward the operation to the next {@link ChannelHandler} in the
* {@link ChannelPipeline}. Sub-classes may override a method implementation to change this.
* </p>
* <p>
* Be aware that messages are not released after the {@link #channelRead(ChannelHandlerContext, Object)}
* method returns automatically. If you are looking for a {@link ChannelInboundHandler} implementation that
* releases the received messages automatically, please see {@link SimpleChannelInboundHandler}.
* </p>
*/
public class ChannelInboundHandlerAdapter extends ChannelHandlerAdapter implements ChannelInboundHandler
類圖：
ServerBootstrap
類定義：
io.netty.bootstrap.ServerBootstrap
/**
* {@link Bootstrap} sub-class which allows easy bootstrap of {@link ServerChannel}
*
*/
public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel>
類圖：
Bootstrap
類定義：
io.netty.bootstrap.Bootstrap
/**
* A {@link Bootstrap} that makes it easy to bootstrap a {@link Channel} to use
* for clients.
*
* <p>The {@link #bind()} methods are useful in combination with connectionless transports such as datagram (UDP).
* For regular TCP connections, please use the provided {@link #connect()} methods.</p>
*/
public class Bootstrap extends AbstractBootstrap<Bootstrap, Channel>
類圖：
Server端啟動過程
下面就正式開始追原始碼。
建立Selector
Selector的建立起於這行程式碼EventLoopGroup bossGroup = new NioEventLoopGroup(1)
io.netty.channel.nio.NioEventLoopGroup
/**
* Create a new instance using the specified number of threads, {@link ThreadFactory} and the
* {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
*/
public NioEventLoopGroup(int nThreads) {
this(nThreads, (Executor) null);
}
public NioEventLoopGroup(int nThreads, Executor executor) {
this(nThreads, executor, SelectorProvider.provider());
}
這裡我們看到了熟悉的SelectorProvider.provider()，如果覺得陌生，建議回到上面快速上手java.nio的程式碼。
往裡面追幾層，就到了NioEventLoopGroup的父類 MultithreadEventExecutorGroup 。
io.netty.util.concurrent.MultithreadEventExecutorGroup
protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
EventExecutorChooserFactory chooserFactory, Object... args) {
if (executor == null) {
executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
}
children = new EventExecutor[nThreads];
for (int i = 0; i < nThreads; i ++) {
children[i] = newChild(executor, args);
}
}
注意： 建立NioEventLoopGroup(int nThreads)時的引數nThreads就傳到了上面程式碼中的children = new EventExecutor[nThreads]。看newChild(executor, args)做了什麼。
io.netty.channel.nio.NioEventLoopGroup
@Override
protected EventLoop newChild(Executor executor, Object... args) throws Exception {
EventLoopTaskQueueFactory queueFactory = args.length == 4 ? (EventLoopTaskQueueFactory) args[3] : null;
return new NioEventLoop(this, executor, (SelectorProvider) args[0],
((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2], queueFactory);
}
io.netty.channel.nio.NioEventLoop
NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler,
EventLoopTaskQueueFactory queueFactory) {
super(parent, executor, false, newTaskQueue(queueFactory), newTaskQueue(queueFactory),
rejectedExecutionHandler);
this.provider = ObjectUtil.checkNotNull(selectorProvider, "selectorProvider");
this.selectStrategy = ObjectUtil.checkNotNull(strategy, "selectStrategy");
final SelectorTuple selectorTuple = openSelector();
this.selector = selectorTuple.selector;
this.unwrappedSelector = selectorTuple.unwrappedSelector;
}
Selector的建立就發生在這行程式碼final SelectorTuple selectorTuple = openSelector();進去看看。
io.netty.channel.nio.NioEventLoop
private SelectorTuple openSelector() {
final Selector unwrappedSelector;
try {
unwrappedSelector = provider.openSelector();
} catch (IOException e) {
throw new ChannelException("failed to open a new selector", e);
}
if (DISABLE_KEY_SET_OPTIMIZATION) {
return new SelectorTuple(unwrappedSelector);
}
// 省略其他程式碼...
return new SelectorTuple(unwrappedSelector,
new SelectedSelectionKeySetSelector(unwrappedSelector, selectedKeySet));
}
這裡我們看到了provider.openSelector()，到這裡，建立出來的Selector就與 EventLoop 關聯在一起了。
同時在建立NioEventLoop時，看看super(parent, executor, false, newTaskQueue(queueFactory), ...)在父類SingleThreadEventLoop幹了什麼。
io.netty.channel.SingleThreadEventLoop
protected SingleThreadEventLoop(EventLoopGroup parent, Executor executor,
boolean addTaskWakesUp, Queue<Runnable> taskQueue, Queue<Runnable> tailTaskQueue,
RejectedExecutionHandler rejectedExecutionHandler) {
super(parent, executor, addTaskWakesUp, taskQueue, rejectedExecutionHandler);
tailTasks = ObjectUtil.checkNotNull(tailTaskQueue, "tailTaskQueue");
}
再往下;
io.netty.util.concurrent.SingleThreadEventExecutor
private final Queue<Runnable> taskQueue;
protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
boolean addTaskWakesUp, Queue<Runnable> taskQueue,
RejectedExecutionHandler rejectedHandler) {
super(parent);
this.addTaskWakesUp = addTaskWakesUp;
this.maxPendingTasks = DEFAULT_MAX_PENDING_EXECUTOR_TASKS;
this.executor = ThreadExecutorMap.apply(executor, this);
this.taskQueue = ObjectUtil.checkNotNull(taskQueue, "taskQueue");
this.rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
}
這裡我們看到了對Queue<Runnable> taskQueue的賦值。
建立ServerSocketChannel
AbstractBootstrap中的initAndRegister()方法是ServerSocketChannel的建立入口。
io.netty.bootstrap.AbstractBootstrap
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
// 1.建立ServerSocketChannel
channel = channelFactory.newChannel();
// 2.初始化ServerSocketChannel
init(channel);
} catch (Throwable t) {
}
// 3.將ServerSocketChannel註冊到Selector上
ChannelFuture regFuture = config().group().register(channel);
return regFuture;
}
Server端的啟動最核心的也就是上面加註釋的三步。按照順序先從ServerSocketChannel的建立講起。
ServerSocketChannel的建立用了工廠模式+反射機制。具體見ReflectiveChannelFactory
io.netty.channel.ReflectiveChannelFactory
/**
* A {@link ChannelFactory} that instantiates a new {@link Channel} by invoking its default constructor reflectively.
*/
public class ReflectiveChannelFactory<T extends Channel> implements ChannelFactory<T> {
private final Constructor<? extends T> constructor;
public ReflectiveChannelFactory(Class<? extends T> clazz) {
this.constructor = clazz.getConstructor();
}
@Override
public T newChannel() {
return constructor.newInstance();
}
}
還記得在前面的bootstrap.channel(NioServerSocketChannel.class)這行程式碼嗎？傳入的Class就是用於反射生成Channel例項的。這裡是Server端，顯然需要進NioServerSocketChannel看如何建立的。
io.netty.channel.socket.nio.NioServerSocketChannel
private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
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 href="https://github.com/netty/netty/issues/2308">#2308</a>.
*/
return provider.openServerSocketChannel();
} catch (IOException e) {
throw new ChannelException(
"Failed to open a server socket.", e);
}
}
public NioServerSocketChannel() {
this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}
public NioServerSocketChannel(ServerSocketChannel channel) {
super(null, channel, SelectionKey.OP_ACCEPT);
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
provider.openServerSocketChannel()這行程式碼也就建立出來了ServerSocketChannel。再往父類裡面追，看做了些什麼。super(null, channel, SelectionKey.OP_ACCEPT);
io.netty.channel.nio.AbstractNioChannel
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
super(parent);
this.ch = ch;
this.readInterestOp = readInterestOp;
ch.configureBlocking(false);
}
this.readInterestOp = readInterestOp把感興趣的操作賦值給readInterestOp，上面傳過來的是SelectionKey.OP_ACCEPT。
ch.configureBlocking(false)把剛才建立出來的channel設定為非阻塞。繼續往父類追。
io.netty.channel.AbstractChannel
protected AbstractChannel(Channel parent) {
this.parent = parent;
id = newId();
unsafe = newUnsafe();
pipeline = newChannelPipeline();
}
protected DefaultChannelPipeline newChannelPipeline() {
return new DefaultChannelPipeline(this);
}
這裡看到建立了ChannelPipeline，並關聯到Channel上。再往下走一步。
io.netty.channel.DefaultChannelPipeline
protected DefaultChannelPipeline(Channel channel) {
this.channel = ObjectUtil.checkNotNull(channel, "channel");
succeededFuture = new SucceededChannelFuture(channel, null);
voidPromise = new VoidChannelPromise(channel, true);
tail = new TailContext(this);
head = new HeadContext(this);
head.next = tail;
tail.prev = head;
}
此時ChannelPipeline大致如下：
head --> tail
初始化ServerSocketChannel
回到上面提到的重要的第2步： init(channel); 注意，實現類為ServerBootstrap，因為是Server端嘛。
io.netty.bootstrap.ServerBootstrap
@Override
void init(Channel channel) {
ChannelPipeline p = channel.pipeline();
final EventLoopGroup currentChildGroup = childGroup;
final ChannelHandler currentChildHandler = childHandler;
p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(final Channel ch) {
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));
}
});
}
});
}
在ChannelPipeline加了一個ChannelHandler。此時ChannelPipeline大致如下：
head --> ChannelInitializer --> tail
一旦serverSocketChannel註冊到EventLoop(或者說Selector)上，便會觸發這裡initChannel的呼叫。避免繞暈了，這裡暫時不去探究具體的呼叫邏輯。後面呼叫到這裡的時候，再回過頭來仔細探究。
ServerSocketChannel註冊到Selector上
回到上面提到的重要的第3步：config().group().register(channel);
通過分析類的繼承層次(或者debug也行)可以跟蹤呼叫到SingleThreadEventLoop的register方法。
io.netty.channel.SingleThreadEventLoop
@Override
public ChannelFuture register(Channel channel) {
return register(new DefaultChannelPromise(channel, this));
}
@Override
public ChannelFuture register(final ChannelPromise promise) {
ObjectUtil.checkNotNull(promise, "promise");
promise.channel().unsafe().register(this, promise);
return promise;
}
再往下跟，最終呼叫的是AbstractChannel的register方法，如下：
io.netty.channel.AbstractChannel
@Override
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
AbstractChannel.this.eventLoop = eventLoop;
eventLoop.execute(new Runnable() {
@Override
public void run() {
register0(promise);
}
});
}
往下跟eventLoop.execute()
io.netty.util.concurrent.SingleThreadEventExecutor
private void execute(Runnable task, boolean immediate) {
addTask(task);
startThread();
}
addTask(task)把上面的Runnable放入到上面提到的Queue<Runnable> taskQueue，過程見如下程式碼：
io.netty.util.concurrent.SingleThreadEventExecutor
/**
* Add a task to the task queue, or throws a {@link RejectedExecutionException} if this instance was shutdown
* before.
*/
protected void addTask(Runnable task) {
ObjectUtil.checkNotNull(task, "task");
if (!offerTask(task)) {
reject(task);
}
}
final boolean offerTask(Runnable task) {
if (isShutdown()) {
reject();
}
return taskQueue.offer(task);
}
把task放入taskQueue後，就到startThread()這行程式碼了，進去瞧瞧。
io.netty.util.concurrent.SingleThreadEventExecutor
private void startThread() {
doStartThread();
}
private void doStartThread() {
executor.execute(new Runnable() {
@Override
public void run() {
SingleThreadEventExecutor.this.run();
}
});
}
繼續追executor.execute，到這裡才真正建立新的執行緒執行SingleThreadEventExecutor.this.run(), thread名稱大致為nioEventLoopGroup-2-1，見如下程式碼：
io.netty.util.concurrent.ThreadPerTaskExecutor
@Override
public void execute(Runnable command) {
threadFactory.newThread(command).start();
}
SingleThreadEventExecutor.this.run()實際執行的程式碼如下：
io.netty.channel.nio.NioEventLoop
@Override
protected void run() {
int selectCnt = 0;
for (;;) {
try {
int strategy;
try {
strategy = selectStrategy.calculateStrategy(selectNowSupplier, hasTasks());
switch (strategy) {
case SelectStrategy.CONTINUE:
continue;
case SelectStrategy.BUSY_WAIT:
// fall-through to SELECT since the busy-wait is not supported with NIO
case SelectStrategy.SELECT:
long curDeadlineNanos = nextScheduledTaskDeadlineNanos();
if (curDeadlineNanos == -1L) {
curDeadlineNanos = NONE; // nothing on the calendar
}
nextWakeupNanos.set(curDeadlineNanos);
try {
if (!hasTasks()) {
strategy = select(curDeadlineNanos);
}
} finally {
// This update is just to help block unnecessary selector wakeups
// so use of lazySet is ok (no race condition)
nextWakeupNanos.lazySet(AWAKE);
}
// fall through
default:
}
} catch (IOException e) {
// If we receive an IOException here its because the Selector is messed up. Let's rebuild
// the selector and retry. https://github.com/netty/netty/issues/8566
// ...
continue;
}
selectCnt++;
cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
boolean ranTasks;
if (ioRatio == 100) {
try {
if (strategy > 0) {
processSelectedKeys();
}
} finally {
// Ensure we always run tasks.
ranTasks = runAllTasks();
}
} else if (strategy > 0) {
final long ioStartTime = System.nanoTime();
try {
processSelectedKeys();
} finally {
// Ensure we always run tasks.
final long ioTime = System.nanoTime() - ioStartTime;
ranTasks = runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}
} else {
ranTasks = runAllTasks(0); // This will run the minimum number of tasks
}
} finally {
// Always handle shutdown even if the loop processing threw an exception.
}
}
}
先簡單解釋一下上面的程式碼，部分細節後面再扣。run()方法裡面是個死迴圈，大致是這樣的，這裡的描述並不完全準確，是這麼個意思，taskQueue裡面如果有task，就不斷poll執行佇列裡的task，具體見runAllTasks()；否則，就selector.select()，若有IO事件，則通過processSelectedKeys()來處理。
講到這裡，正好剛才不是往taskQueue裡放了個Runnable嗎，再貼一下上面那個Runnable的程式碼
new Runnable() {
@Override
public void run() {
register0(promise);
}
};
於是就要執行Runnable裡面register0(promise)了。
io.netty.channel.AbstractChannel
private void register0(ChannelPromise promise) {
//(1)把ServerSocketChannel註冊到了Selector上
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.
//(2)觸發pipeline中的ChannelHandler的handlerAdded()方法呼叫
pipeline.invokeHandlerAddedIfNeeded();
safeSetSuccess(promise);
//(3)觸發pipeline中的ChannelInboundHandler的channelRegistered()方法呼叫
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();
}
}
}
上面我按照自己的理解，在程式碼中加了少許註釋，下面按照我註釋的順序依次解釋一下。
(1) doRegister()
io.netty.channel.nio.AbstractNioChannel
@Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
}
}
這裡顯然是把ServerSocketChannel註冊到了Selector上。
(2) pipeline.invokeHandlerAddedIfNeeded()
作用：觸發pipeline中的ChannelHandler的handlerAdded()方法呼叫
io.netty.channel.DefaultChannelPipeline
final void invokeHandlerAddedIfNeeded() {
if (firstRegistration) {
firstRegistration = false;
// We are now registered to the EventLoop. It's time to call the callbacks for the ChannelHandlers,
// that were added before the registration was done.
callHandlerAddedForAllHandlers();
}
}
上面的註釋清晰地告訴我們，現在ServerSocketChannel已經註冊到EventLoop上，是時候該呼叫Pipeline中的ChannelHandlers。到這裡，就能與上面初始化ServerSocketChannel對接起來了，猜測應該會觸發上面的ChannelInitializer的呼叫。
io.netty.channel.DefaultChannelPipeline
private void callHandlerAddedForAllHandlers() {
final PendingHandlerCallback pendingHandlerCallbackHead;
synchronized (this) {
pendingHandlerCallbackHead = this.pendingHandlerCallbackHead;
// Null out so it can be GC'ed.
this.pendingHandlerCallbackHead = null;
}
// This must happen outside of the synchronized(...) block as otherwise handlerAdded(...) may be called while
// holding the lock and so produce a deadlock if handlerAdded(...) will try to add another handler from outside
// the EventLoop.
PendingHandlerCallback task = pendingHandlerCallbackHead;
while (task != null) {
task.execute();
task = task.next;
}
}
這裡需要先解釋一下為什麼又突然冒出來PendingHandlerCallback。是這樣的，在addLast(ChannelHandler... handlers)時，實際上調了下面的方法。
io.netty.channel.DefaultChannelPipeline
public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
final AbstractChannelHandlerContext newCtx;
synchronized (this) {
newCtx = newContext(group, filterName(name, handler), handler);
addLast0(newCtx);
// If the registered is false it means that the channel was not registered on an eventLoop yet.
// In this case we add the context to the pipeline and add a task that will call
// ChannelHandler.handlerAdded(...) once the channel is registered.
if (!registered) {
newCtx.setAddPending();
callHandlerCallbackLater(newCtx, true);
return this;
}
EventExecutor executor = newCtx.executor();
if (!executor.inEventLoop()) {
callHandlerAddedInEventLoop(newCtx, executor);
return this;
}
}
callHandlerAdded0(newCtx);
return this;
}
看到上面的3行註釋沒有，就解釋了上面的PendingHandlerCallback從哪裡來的。翻譯一下就是，在往Pipeline中新增ChannelHandler時，如果Channel還沒有註冊到EventLoop上，就將當前的AbstractChannelHandlerContext封裝到PendingHandlerCallback裡去，等著後面觸發呼叫。
回到正題，PendingHandlerCallback.execute()幾經周折，會呼叫ChannelHandler的handlerAdded(),如下所示：
io.netty.channel.AbstractChannelHandlerContext
final void callHandlerAdded() throws Exception {
// We must call setAddComplete before calling handlerAdded. Otherwise if the handlerAdded method generates
// any pipeline events ctx.handler() will miss them because the state will not allow it.
if (setAddComplete()) {
handler().handlerAdded(this);
}
}
那麼再回頭看看ChannelInitializer
io.netty.channel.ChannelInitializer
/**
* {@inheritDoc} If override this method ensure you call super!
*/
@Override
public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
if (ctx.channel().isRegistered()) {
if (initChannel(ctx)) {
removeState(ctx);
}
}
}
private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
if (initMap.add(ctx)) { // Guard against re-entrance.
initChannel((C) ctx.channel());
return true;
}
return false;
}
/**
* This method will be called once the {@link Channel} was registered. After the method returns this instance
* will be removed from the {@link ChannelPipeline} of the {@link Channel}.
*
* @param ch the {@link Channel} which was registered.
* @throws Exception is thrown if an error occurs. In that case it will be handled by
* {@link #exceptionCaught(ChannelHandlerContext, Throwable)} which will by default close
* the {@link Channel}.
*/
protected abstract void initChannel(C ch) throws Exception;
原來，最終會觸發initChannel呼叫，所以上面初始化ServerSocketChannel時重寫的initChannel會在這時執行。
p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(final Channel ch) {
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));
}
});
}
});
這裡的initChannel執行之後，此時ChannelPipeline大致如下：
head --> tail
值得注意的是，此時ServerBootstrapAcceptor暫時並沒有被放入ChannelPipeline中，而同樣是放到了上面提到的Queue<Runnable> taskQueue佇列中，如下：
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
至於ServerBootstrapAcceptor裡面幹了啥，等到後面再細說。
來，繼續。上面講清楚了doRegister()和pipeline.invokeHandlerAddedIfNeeded()，接下來看pipeline.fireChannelRegistered()。
(3) pipeline.fireChannelRegistered()
作用：觸發pipeline中的ChannelInboundHandler的channelRegistered()方法呼叫
還是往裡面簡單追一下原始碼。
io.netty.channel.AbstractChannelHandlerContext
static void invokeChannelRegistered(final AbstractChannelHandlerContext next) {
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
next.invokeChannelRegistered();
} else {
executor.execute(new Runnable() {
@Override
public void run() {
next.invokeChannelRegistered();
}
});
}
}
private void invokeChannelRegistered() {
if (invokeHandler()) {
try {
// 這裡觸發了channelRegistered()方法呼叫
((ChannelInboundHandler) handler()).channelRegistered(this);
} catch (Throwable t) {
invokeExceptionCaught(t);
}
} else {
fireChannelRegistered();
}
}
到這裡，register0()這個task就執行完了。但是還記得這個task執行過程中，又往taskQueue中新增了一個Runnable嗎？
new Runnable() {
@Override
public void run() {
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
}
此時會poll到新加的task，見如下程式碼：
io.netty.util.concurrent.SingleThreadEventExecutor
protected boolean runAllTasks(long timeoutNanos) {
for (;;) {
safeExecute(task);
task = pollTask();
if (task == null) {
lastExecutionTime = ScheduledFutureTask.nanoTime();
break;
}
}
afterRunningAllTasks();
this.lastExecutionTime = lastExecutionTime;
return true;
}
執行完這個新增的Runnable後，此時ChannelPipeline大致如下：
head --> ServerBootstrapAcceptor --> tail
此時，taskQueue中的task都執行完了，EventLoop執行緒執行selector.select()，等待客戶端的連線。
到這裡，Server端也就成功啟動了。
Client端啟動過程
建立Selector
與Server端完全一致。
建立SocketChannel
入口與Server端一樣，不一樣的地方在於Client端是bootstrap.channel(NioSocketChannel.class)，所以需要看NioSocketChannel的實現。這裡也不必多說。
初始化SocketChannel
Client端的就比較簡單了，如下：
io.netty.bootstrap.Bootstrap
@Override
void init(Channel channel) {
ChannelPipeline p = channel.pipeline();
p.addLast(config.handler());
}
SocketChannel註冊到Selector上
前面的過程與Server端基本一樣，執行完doRegister()，執行pipeline.invokeHandlerAddedIfNeeded()時，沒有Server端複雜(因為Server端初始化SocketChannel，加了個新增ServerBootstrapAcceptor到ChannelPipeline的task)。
前面分析過，這個過程會觸發initChannel呼叫，所以這時會執行使用者編寫的ChannelInitializer，也就是會執行ch.pipeline().addLast(new NettyClientHandler())，將使用者編寫的NettyClientHandler插入到ChannelPipeline中。
連線Server
註冊成功後，會執行連線Server的回撥。
io.netty.bootstrap.Bootstrap
private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();
final Channel channel = regFuture.channel();
if (regFuture.isDone()) {
return doResolveAndConnect0(channel, remoteAddress, localAddress, channel.newPromise());
} 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 {
// Directly obtain the cause and do a null check so we only need one volatile read in case of a
// failure.
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.registered();
doResolveAndConnect0(channel, remoteAddress, localAddress, promise);
}
}
});
return promise;
}
}
需要看doResolveAndConnect0(), 裡面又呼叫的是doConnect()
io.netty.bootstrap.Bootstrap
private static void doConnect(
final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise connectPromise) {
// This method is invoked before channelRegistered() is triggered. Give user handlers a chance to set up
// the pipeline in its channelRegistered() implementation.
final Channel channel = connectPromise.channel();
channel.eventLoop().execute(new Runnable() {
@Override
public void run() {
if (localAddress == null) {
channel.connect(remoteAddress, connectPromise);
} else {
channel.connect(remoteAddress, localAddress, connectPromise);
}
connectPromise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
}
});
}
最終呼叫的是：
io.netty.channel.socket.nio.NioSocketChannel#doConnect()
@Override
protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
boolean success = false;
try {
boolean connected = SocketUtils.connect(javaChannel(), remoteAddress);
if (!connected) {
selectionKey().interestOps(SelectionKey.OP_CONNECT);
}
success = true;
return connected;
} finally {
if (!success) {
doClose();
}
}
}
再看SocketUtils.connect(javaChannel(), remoteAddress)
io.netty.util.internal.SocketUtils
public static boolean connect(final SocketChannel socketChannel, final SocketAddress remoteAddress)
throws IOException {
try {
return AccessController.doPrivileged(new PrivilegedExceptionAction<Boolean>() {
@Override
public Boolean run() throws IOException {
return socketChannel.connect(remoteAddress);
}
});
} catch (PrivilegedActionException e) {
throw (IOException) e.getCause();
}
}
這裡我們看到了熟悉的socketChannel.connect(remoteAddress)。
Server與Client通訊
上面詳細介紹了Server端的啟動過程，Client端的啟動過程，Client也向Server發出了連線請求。這時再回過頭來看Server端。
Server端感知到了IO事件，會在io.netty.channel.nio.NioEventLoop的run()方法裡，呼叫processSelectedKeys()，對於每個IO事件，最終呼叫的是processSelectedKey()來處理。
io.netty.channel.nio.NioEventLoop
private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
try {
int readyOps = k.readyOps();
// We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
// the NIO JDK channel implementation may throw a NotYetConnectedException.
if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
// remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
// See https://github.com/netty/netty/issues/924
int ops = k.interestOps();
ops &= ~SelectionKey.OP_CONNECT;
k.interestOps(ops);
unsafe.finishConnect();
}
// Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
if ((readyOps & SelectionKey.OP_WRITE) != 0) {
// Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
ch.unsafe().forceFlush();
}
// Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
// to a spin loop
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
unsafe.read();
}
} catch (CancelledKeyException ignored) {
unsafe.close(unsafe.voidPromise());
}
}
這裡是SelectionKey.OP_ACCEPT，當然走的是unsafe.read()
io.netty.channel.nio.AbstractNioMessageChannel
private final class NioMessageUnsafe extends AbstractNioUnsafe {
private final List<Object> readBuf = new ArrayList<Object>();
@Override
public void read() {
assert eventLoop().inEventLoop();
final ChannelConfig config = config();
final ChannelPipeline pipeline = pipeline();
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
allocHandle.incMessagesRead(localRead);
} while (allocHandle.continueReading());
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
pipeline.fireChannelRead(readBuf.get(i));
}
readBuf.clear();
allocHandle.readComplete();
pipeline.fireChannelReadComplete();
} finally {
// ...
}
}
}
這裡面有很重要的兩個方法，doReadMessages(readBuf)和pipeline.fireChannelRead()
io.netty.channel.socket.nio.NioServerSocketChannel
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = SocketUtils.accept(javaChannel());
try {
if (ch != null) {
buf.add(new NioSocketChannel(this, ch));
return 1;
}
} catch (Throwable t) {
// ...
}
return 0;
}
io.netty.util.internal.SocketUtils
public static SocketChannel accept(final ServerSocketChannel serverSocketChannel) throws IOException {
try {
return AccessController.doPrivileged(new PrivilegedExceptionAction<SocketChannel>() {
@Override
public SocketChannel run() throws IOException {
return serverSocketChannel.accept();
}
});
} catch (PrivilegedActionException e) {
throw (IOException) e.getCause();
}
}
serverSocketChannel接受了Client端的連線後，將該socketChannel放到了List中。
而後遍歷該List，將每個socketChannel傳入pipeline.fireChannelRead()中。
還記得當前serverSocketChannel的ChannelPipeline有哪些ChannelHandler嗎？
head --> ServerBootstrapAcceptor --> tail
接下來就需要重點看下ServerBootstrapAcceptor的邏輯
io.netty.bootstrap.ServerBootstrap#ServerBootstrapAcceptor
private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
private final EventLoopGroup childGroup;
private final ChannelHandler childHandler;
private final Runnable enableAutoReadTask;
ServerBootstrapAcceptor(
final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
this.childGroup = childGroup;
this.childHandler = childHandler;
enableAutoReadTask = new Runnable() {
@Override
public void run() {
channel.config().setAutoRead(true);
}
};
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel) msg;
child.pipeline().addLast(childHandler);
setChannelOptions(child, childOptions, logger);
setAttributes(child, childAttrs);
childGroup.register(child).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (!future.isSuccess()) {
forceClose(child, future.cause());
}
}
});
}
}
ServerBootstrapAcceptor將建立好連線的socketChannel註冊到workerGroup中的某個EventLoop(或者說是Selector)上，而且將使用者編寫的childHandler加到了每個socketChannel的ChannelPipeline中。ServerBootstrapAcceptor相當於起了轉發的作用，建立好連線後Channel實際的讀寫IO事件是由workerGroup中的EventLoop來處理。
再回過頭來，看Reactor模式的多Reactor版本(一主多從)，不知道你是否能get到其中的含義？
注意：上面程式碼裡的childGroup就是來自我們在寫Server端NettyServer程式碼時定義的workerGroup
EventLoopGroup workerGroup = new NioEventLoopGroup();
我覺得能堅持看到這個地方的朋友應該能明白，只是這裡又囉嗦了一下。
講到這裡，我覺得其實後面Client端的情況都不用講了，已經很清晰了。不過為了文章的完整性，還是寫下去比較好。
Server端accept連線請求後，Client端此時同樣也有了IO事件。同樣還是走processSelectedKey()那個方法，不過執行的分支不一樣。
int readyOps = k.readyOps();
// We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
// the NIO JDK channel implementation may throw a NotYetConnectedException.
if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
// remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
// See https://github.com/netty/netty/issues/924
int ops = k.interestOps();
ops &= ~SelectionKey.OP_CONNECT;
k.interestOps(ops);
unsafe.finishConnect();
}
最終呼叫doFinishConnect()，如下：
io.netty.channel.socket.nio.NioSocketChannel
@Override
protected void doFinishConnect() throws Exception {
if (!javaChannel().finishConnect()) {
throw new Error();
}
}
之後，Client端與Server端就可以通過Channel讀寫資料，通過ChannelPipeline中的ChannelHandler對資料decode、compute、encode。
寫在後面
至此，本篇就大致講清楚了Netty的Server端和Client端的整個啟動並通訊的過程以及如何對nio進行封裝的。這裡再貼一張在網路上流傳較廣的Netty工作原理圖，相信此時再看這張圖應該無比親切吧。
整個過程確實比較繞。但回過頭再看，有一個清晰的思路，然後時刻記著與nio的程式碼做對比，多點耐心也還能堅持下去，另外遇到搞不明白的地方再配合debug，會輕鬆許多。最後，由於本人能力有限，文中如有錯誤的理解、不恰當的描述，歡迎指出！