ServerBootstrap与Bootstrap分别是netty中服务端与客户端的引导类，主要负责服务端与客户端初始化、配置及启动引导等工作，接下来我们就通过netty源码中的示例对ServerBootstrap与Bootstrap的源码进行一个简单的分析。首先我们知道这两个类都继承自AbstractBootstrap类
接下来我们就通过netty源码中ServerBootstrap的实例入手对其进行一个简单的分析。
// Configure the server.
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup(); final EchoServerHandler serverHandler = new EchoServerHandler(); try { //初始化一个服务端引导类
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup) //设置线程组
.channel(NioServerSocketChannel.class)//设置ServerSocketChannel的IO模型 分为epoll与Nio
.option(ChannelOption.SO_BACKLOG, 100)//设置option参数，保存成一个LinkedHashMap<ChannelOption<?>, Object>()
.handler(new LoggingHandler(LogLevel.INFO))//这个hanlder 只专属于 ServerSocketChannel 而不是 SocketChannel。
.childHandler(new ChannelInitializer<SocketChannel>() { //这个handler 将会在每个客户端连接的时候调用。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. 启动服务
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();
}接下来我们主要从服务端的socket在哪里初始化与哪里accept连接这两个问题入手对netty服务端启动的流程进行分析；
我们首先要知道，netty服务的启动其实可以分为以下四步：
创建服务端Channel
初始化服务端Channel
注册Selector
端口绑定
一、创建服务端Channel
1、服务端Channel的创建，主要为以下流程
我们通过跟踪代码能够看到
final ChannelFuture regFuture = initAndRegister();// 初始化并创建 NioServerSocketChannel
我们在initAndRegister()中可以看到channel的初始化。
channel = channelFactory.newChannel(); // 通过 反射工厂创建一个 NioServerSocketChannel
我进一步看newChannel()中的源码，在ReflectiveChannelFactory这个反射工厂中，通过clazz这个类的反射创建了一个服务端的channel。
@Override public T newChannel() { try { return clazz.getConstructor().newInstance();//反射创建
} catch (Throwable t) { throw new ChannelException("Unable to create Channel from class " + clazz, t);
}
}既然通过反射，我们就要知道clazz类是什么，那么我我们来看下channelFactory这个工厂类是在哪里初始化的，初始化的时候我们传入了哪个channel。
这里我们需要看下demo实例中初始化ServerBootstrap时.channel(NioServerSocketChannel.class)这里的具体实现，我们看下源码
public B channel(Class<? extends C> channelClass) { if (channelClass == null) { throw new NullPointerException("channelClass");
} return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}通过上面的代码我可以直观的看出正是在这里我们通过NioServerSocketChannel这个类构造了一个反射工厂。
那么到这里就很清楚了，我们创建的Channel就是一个NioServerSocketChannel，那么具体的创建我们就需要看下这个类的构造函数。首先我们看下一个NioServerSocketChannel创建的具体流程
首先是newsocket(),我们先看下具体的代码，在NioServerSocketChannel的构造函数中我们创建了一个jdk原生的ServerSocketChannel
/**
* Create a new instance */
public NioServerSocketChannel() { this(newSocket(DEFAULT_SELECTOR_PROVIDER));//传入默认的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 href="https://github.com/netty/netty/issues/2308">#2308</a>. */
return provider.openServerSocketChannel();//可以看到创建的是jdk底层的ServerSocketChannel
} catch (IOException e) { throw new ChannelException( "Failed to open a server socket.", e);
}
}第二步是通过NioServerSocketChannelConfig配置服务端Channel的构造函数，在代码中我们可以看到我们把NioServerSocketChannel这个类传入到了NioServerSocketChannelConfig的构造函数中进行配置
/**
* Create a new instance using the given {@link ServerSocketChannel}. */
public NioServerSocketChannel(ServerSocketChannel channel) { super(null, channel, SelectionKey.OP_ACCEPT);//调用父类构造函数,传入创建的channel
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}第三步在父类AbstractNioChannel的构造函数中把创建服务端的Channel设置为非阻塞模式
/**
* 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;//这个ch就是传入的通过jdk创建的Channel
this.readInterestOp = readInterestOp; try {
ch.configureBlocking(false);//设置为非阻塞
} 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);
}
}第四步调用AbstractChannel这个抽象类的构造函数设置Channel的id（每个Channel都有一个id，唯一标识）,unsafe（tcp相关底层操作），pipeline（逻辑链）等，而不管是服务的Channel还是客户端的Channel都继承自这个抽象类，他们也都会有上述相应的属性。我们看下AbstractChannel的构造函数
/**
* 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();//创建Channel唯一标识
unsafe = newUnsafe();//netty封装的TCP 相关操作类
pipeline = newChannelPipeline();//逻辑链
}2、初始化服务端创建的Channel
init(channel);// 初始化这个 NioServerSocketChannel
我们首先列举下init(channel)中具体都做了哪了些功能：
设置ChannelOptions、ChannelAttrs ,配置服务端Channel的相关属性；
设置ChildOptions、ChildAttrs，配置每个新连接的Channel的相关属性；
Config handler，配置服务端pipeline;
add ServerBootstrapAcceptor,添加连接器，对accpet接受到的新连接进行处理，添加一个nio线程；
那么接下来我们通过代码，对每一步设置进行一下分析：
首先是在SeverBootstrap的init()方法中对ChannelOptions、ChannelAttrs 的配置的关键代码
final Map<ChannelOption<?>, Object> options = options0();//拿到你设置的option
synchronized (options) {
setChannelOptions(channel, options, logger);//设置NioServerSocketChannel相应的TCP参数，其实这一步就是把options设置到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());
}
}然后是对ChildOptions、ChildAttrs配置的关键代码
//可以看到两个都是局部变量，会在下面设置pipeline时用到
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));
}第三步对服务端Channel的handler进行配置
p.addLast(new ChannelInitializer<Channel>() {
@Override public void initChannel(final Channel ch) throws Exception { final ChannelPipeline pipeline = ch.pipeline();
ChannelHandler handler = config.handler();//拿到我们自定义的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连接器，这个是netty向服务端Channel自定义添加的一个handler，用来处理新连接的添加与属性配置，我们来看下关键代码
ch.eventLoop().execute(new Runnable() {
@Override public void run() { //在这里会把我们自定义的ChildGroup、ChildHandler、ChildOptions、ChildAttrs相关配置传入到ServerBootstrapAcceptor构造函数中，并绑定到新的连接上
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});三、注册Selector
一个服务端的Channel创建完毕后，下一步就是要把它注册到一个事件轮询器Selector上，在initAndRegister()中我们把上面初始化的Channel进行注册
ChannelFuture regFuture = config().group().register(channel);//注册我们已经初始化过的Channel
而这个register具体实现是在AbstractChannel中的AbstractUnsafe抽象类中的
/**
* 1、先是一系列的判断。
* 2、判断当前线程是否是给定的 eventLoop 线程。注意：这点很重要，Netty 线程模型的高性能取决于对于当前执行的Thread 的身份的确定。如果不在当前线程，那么就需要很多同步措施（比如加锁），上下文切换等耗费性能的操作。
* 3、异步（因为我们这里直到现在还是 main 线程在执行，不属于当前线程）的执行 register0 方法。 */
@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;//绑定线程
if (eventLoop.inEventLoop()) {
register0(promise);//实际的注册过程
} 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);
}
}
}
首先我们对整个注册的流程做一个梳理
接下来我们进入register0()方法看下注册过程的具体实现
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的底层注册
neverRegistered = false;
registered = true; // 触发绑定的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()的具体实现就是把jdk底层的channel绑定到eventLoop的selecor上
@Override protected void doRegister() throws Exception { boolean selected = false; for (;;) { try { //把channel注册到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;
}
}
}
}到这里netty就把服务端的channel注册到了指定的selector上，下面就是服务端口的邦迪
三、端口绑定
首先我们梳理下netty中服务端口绑定的流程
我们来看下AbstarctUnsafe中bind()方法的具体实现
@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();//判断绑定是否完成
try {
doBind(localAddress);//底层jdk绑定端口
} catch (Throwable t) {
safeSetFailure(promise, t);
closeIfClosed(); return;
} if (!wasActive && isActive()) {
invokeLater(new Runnable() {
@Override public void run() {
pipeline.fireChannelActive();//触发ChannelActive事件 }
});
}
safeSetSuccess(promise);
}在doBind(localAddress)中netty实现了jdk底层端口的绑定
@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()中会触发pipeline中的channelActive()方法
@Override public void channelActive(ChannelHandlerContext ctx) throws Exception {
ctx.fireChannelActive();
readIfIsAutoRead();
}在channelActive中首先会把ChannelActive事件往下传播，然后调用readIfIsAutoRead()方法出触发channel的read事件，而它最终调用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为 SelectionKey.OP_ACCEPT }
}在doBeginRead()方法，netty会把accept事件注册到Selector上。
到此我们对netty服务端的启动流程有了一个大致的了解，整体可以概括为下面四步：
1、channelFactory.newChannel()，其实就是创建jdk底层channel，并初始化id、piepline等属性；
2、init(channel)，添加option、attr等属性，并添加ServerBootstrapAcceptor连接器；
3、config().group().register(channel)，把jdk底层的channel注册到eventLoop上的selector上；
4、doBind0(regFuture, channel, localAddress, promise)，完成服务端端口的监听，并把accept事件注册到selector上；
以上就是对netty服务端启动流程进行的一个简单分析，有很多细节没有关注与深入，其中如有不足与不正确的地方还望指出与海涵。