Netty源码分析——Channel初始化
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前面的章节相对来说比较容易理解，从本节开始，真正的挑战开始了，加油！
本节从这段代码开始。
ServerBootstrap serverBootstrap = new ServerBootstrap()
.group(boss, worker)
.channel(NioServerSocketChannel.class)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new ServerHandler());
}
});
ChannelFuture channelFuture = serverBootstrap.bind(6666).sync(); //绑定端口
ServerBootstrap是netty服务端应用程序的引导器，Bootstrap是socket客户端程序的的引导器，相对来说比服务端简单。
这部分采用链式编程，每次方法的调用返回本身ServerBootstrap的引用。
进入ServerBootstrap类中的group方法，在ServerBootstrap类中保存了boss和worker事件循环组（EventLoopGroup）的引用，为了后续使用。
再看channel方法，传入NioServerSocketChannel.class作为参数。执行channelFactory(new ReflectiveChannelFactory<C>(channelClass));
这里会实例化一个Channel工厂ReflectiveChannelFactory，当下面初始化channel时就会通过调用newChannel()来反射出对应channel的实例
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}
ReflectiveChannelFactory类的其他方法省略
@Override public T newChannel() { try { return clazz.newInstance(); } catch (Throwable t) { throw new ChannelException("Unable to create Channel from class " + clazz, t); } }
再继续执行进入这个方法，会在当前对象中保存channel工厂的实例，为后续调用。
@Deprecated
@SuppressWarnings("unchecked")
public B channelFactory(ChannelFactory<? extends C> channelFactory) {
if (channelFactory == null) {
throw new NullPointerException("channelFactory");
}
if (this.channelFactory != null) {
throw new IllegalStateException("channelFactory set already");
}
this.channelFactory = channelFactory;
return (B) this;
}
接下来执行ChannelFuture channelFuture = serverBootstrap.bind(6666).sync();这里完成的工作很多，一步一步往下看。
经过几步的追踪回进入以下方法
private ChannelFuture doBind(final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister(); //初始化和注册通道，这里完成两个主要的操作，本节的初始化就在这里开始。
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.registered();
doBind0(regFuture, channel, localAddress, promise);
}
}
});
return promise;
}
}
进入initAndRegister方法
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
channel = channelFactory.newChannel(); //这里就时上文提到的调用channel工厂的newChannel方法发生出一个NioServerSocketChannel，这里有一个细节需要提醒一下，实例化NioServerSocketChannel时，会实例化一个ChannelPipline，
ChannelPipline是一个很重要的概念，这里提一下，记住每一个Channel都对应一个ChannelPipline，就是在实例化Channel的时候同时实例化的。
init(channel); //这里就是初始化这个通道
} catch (Throwable t) {
if (channel != null) {
// channel can be null if newChannel crashed (eg SocketException("too many open files"))
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 = config().group().register(channel); //这里时下节要说的注册这个通道的方法入口，预告一下
if (regFuture.cause() != null) {
if (channel.isRegistered()) {
channel.close();
} else {
channel.unsafe().closeForcibly();
}
}
// If we are here and the promise is not failed, it's one of the following cases:
// 1) If we attempted registration from the event loop, the registration has been completed at this point.
// i.e. It's safe to attempt bind() or connect() now because the channel has been registered.
// 2) If we attempted registration from the other thread, the registration request has been successfully
// added to the event loop's task queue for later execution.
// i.e. It's safe to attempt bind() or connect() now:
// because bind() or connect() will be executed *after* the scheduled registration task is executed
// because register(), bind(), and connect() are all bound to the same thread.
return regFuture;
}
才进入主题init(channel)方法，因为抽象方法，程序会进入到ServerBootstrap类中重写的方法
@Override
void init(Channel channel) throws Exception {
final Map<ChannelOption<?>, Object> options = options0();
synchronized (options) {
channel.config().setOptions(options); //将配置的选项设置到通道，本例中没有设置通道选项
}
final Map<AttributeKey<?>, Object> attrs = attrs0();
synchronized (attrs) { //将属性设置到通道
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(); //获取pipline
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>() { //在pipline中加入一个ChannelInitializer处理器。
@Override
public void initChannel(Channel ch) throws Exception {
final ChannelPipeline pipeline = ch.pipeline();
ChannelHandler handler = config.handler();
if (handler != null) {
pipeline.addLast(handler);
}
// We add this handler via the EventLoop as the user may have used a ChannelInitializer as handler.
// In this case the initChannel(...) method will only be called after this method returns. Because
// of this we need to ensure we add our handler in a delayed fashion so all the users handler are
// placed in front of the ServerBootstrapAcceptor.
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
pipeline.addLast(new ServerBootstrapAcceptor(
currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
}
要理解上面代码的意图，需要后面channelpipline的知识，这里先简单介绍一下。简单来说一个channel就管理一个channelpipline，两者相互关联，都保存着对方的引用。
要解释channelpipline就要提到channelhandler，channelhandler就是真正处理socket事件的处理器，而channelpipline中保存着一个channelhandler的链表（实际上是channelhandlercontext链表，这里简单理解，后续详细说明），这里的p.addLast(new ChannelInitializer<Channel>()方法就是相当于在channelhandler链表中中加入了一个节点，用于处理下一节要说的注册的。这里的ChannelInitializer实际上就是一个入站处理器，这里重写的方法还会再下一节再次说明。我还会再回来的！
至此通道的初始化完成，主要初始化通道的一些基本信息，未后续的处理做好准备。
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