一、netty服务启动分析
EventLoopGroup boss = new NioEventLoopGroup();//类图，继承线程池ScheduledExecutorService
EventLoopGroup worker = new NioEventLoopGroup();
ServerBootstrap bootstrap = new ServerBootstrap();
bootstrap.group(boss, worker);
bootstrap.channel(NioServerSocketChannel.class);//利用反射构造NioServerSocketChannel实例
//backlog指定了内核为此套接口排队的最大连接个数，对于给定的监听套接口，内核要维护两个队列：未链接队列和已连接队列，根据TCP三路握手过程中三个分节来分隔这两个队列
bootstrap.option(ChannelOption.SO_BACKLOG, 2048);
bootstrap.option(ChannelOption.SO_KEEPALIVE, true);
bootstrap.option(ChannelOption.TCP_NODELAY, true);
bootstrap.handler(new LoggingServerHandler());//handler与childHandler不同
bootstrap.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new MyChannelHandler1());
ch.pipeline().addLast(new MyChannelHandler2());
ch.pipeline().addLast(new MyChannelHandler3());
}
});
ChannelFuture f = bootstrap.bind(port).sync();//bind方法实现
f.addListener((ChannelFutureListener) future -> {
if (future.isSuccess()) {
//启动成功
}
});
f.channel().closeFuture().sync();
图片.png
创建 ServerBootstrap 实例
设置并绑定 Reactor 线程池
设置并绑定服务端 Channel
创建并初始化 ChannelPipeline
添加并设置 ChannelHandler
绑定并启动监听端口
二、netty服务启动代码分析
1、创建两个EventLoopGroup
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
bossGroup 为 BOSS 线程组，用于服务端接受客户端的连接, workerGroup 为 worker 线程组，用于进行 SocketChannel 的网络读写。当然也可以创建一个线程组，共享使用。
2、创建ServerBootstrap实例
ServerBootStrap为Netty服务端的启动引导类，用于帮助用户快速配置、启动服务端服务。提供的方法如下：
图片.png
ServerBootStrap底层采用装饰者模式。
3、设置并绑定Reactor线程池
b.group(bossGroup, workerGroup)
EventLoopGroup 为 Netty 线程池，它实际上就是 EventLoop 的数组容器。EventLoop 的职责是处理所有注册到本线程多路复用器 Selector 上的 Channel，Selector 的轮询操作由绑定的 EventLoop 线程 run 方法驱动，在一个循环体内循环执行。通俗点讲就是一个死循环，不断的检测 I/O 事件、处理 I/O 事件。
这里设置了两个group，这个其实有点儿像我们工作一样。需要两类型的工人，一个老板（bossGroup）,一个工人（workerGroup），老板负责从外面接活，工人则负责死命干活。所以这里 bossGroup 的作用就是不断地接收新的连接，接收之后就丢给 workerGroup 来处理，workerGroup 负责干活就行（负责客户端连接的 IO 操作）。
4、设置并绑定服务端Channel
.channel(NioServerSocketChannel.class)
调用 ServerBootstrap.channel 方法用于设置服务端使用的 Channel，传递一个 NioServerSocketChannel Class对象，Netty通过工厂类，利用反射创建NioServerSocketChannel 对象，如下：
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}
//最终调用构造函数
public NioServerSocketChannel(ServerSocketChannel channel) {
super(null, channel, SelectionKey.OP_ACCEPT);
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
5、添加并设置ChannelHandler
.handler(new LoggingServerHandler())
.childHandler(new ChannelInitializer(){
//省略代码
})
图片.png
ServerBootstrap 中的 Handler（childHandler()） 是 NioServerSocketChannel 使用的，所有连接该监听端口的客户端都会执行它，父类 AbstractBootstrap 中的 Handler 是一个工厂类，它为每一个新接入的客户端都创建一个新的 Handler。
handler在server初始化它时就会执行，而childHandler会在客户端成功connect后才执行，这是两者的区别。
6、绑定端口，启动服务
ChannelFuture future = b.bind(port).sync();
主要步骤：
负责创建服务端的NioServerSocketChannel实例；
为NioServerSocketChannel的pipeline添加handler；
注册NioServerSocketChannel到selector；
二、源码详解
AbstractBootstrap类doBind方法，绑定端口入口
private ChannelFuture doBind(final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister(); //初始化与注册
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;
}
}
private static void doBind0(
final ChannelFuture regFuture, final Channel channel,
final SocketAddress localAddress, final ChannelPromise promise) {
// This method is invoked before channelRegistered() is triggered. Give user handlers a chance to set up
// the pipeline in its channelRegistered() implementation.
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());
}
}
});
}
initAndRegister方法,创建和初始化channel
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
channel = channelFactory.newChannel(); //创建服务端Channel
init(channel);//初始化Channel
} 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); //注册selector
if (regFuture.cause() != null) {
if (channel.isRegistered()) {
channel.close();
} else {
channel.unsafe().closeForcibly();
}
}
#channelFactory.newChannel()通过反射创建实例
public class ReflectiveChannelFactory<T extends Channel> implements ChannelFactory<T> {
private final Class<? extends T> clazz;
public ReflectiveChannelFactory(Class<? extends T> clazz) {
if (clazz == null) {
throw new NullPointerException("clazz");
}
this.clazz = clazz;
}
@Override
public T newChannel() {
try {
return clazz.newInstance();
//clazz由AbstractBootstrap.channel方法传入，bootstrap.channel(NioServerSocketChannel.class);
} catch (Throwable t) {
throw new ChannelException("Unable to create Channel from class " + clazz, t);
}
}
@Override
public String toString() {
return StringUtil.simpleClassName(clazz) + ".class";
}
}
#AbstractBootstrap
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}
#NioServerSocketChannel构造函数
public NioServerSocketChannel(ServerSocketChannel channel) {
super(null, channel, SelectionKey.OP_ACCEPT);//调用AbstractNioChannel构造函数
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
#AbstractNioChannel构造函数
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
super(parent);
this.ch = ch;
this.readInterestOp = readInterestOp;
try {
ch.configureBlocking(false);//非阻塞方式
} 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);
}
}
#init方法，初始化channel参数，添加handler
void init(Channel channel) throws Exception {
final Map<ChannelOption<?>, Object> options = options0();
synchronized (options) {
channel.config().setOptions(options);
}
final Map<AttributeKey<?>, Object> attrs = attrs0();
synchronized (attrs) {//服务端Channel属性
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 {
final ChannelPipeline pipeline = ch.pipeline();
ChannelHandler handler = config.handler();//传入hander
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.
//默认添加的ServerBootstrapAcceptor的hander，连接接入器处理新链接接入时，初始化Options和Attrs
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
pipeline.addLast(new ServerBootstrapAcceptor(
currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
}
register方法，注册selector
#unsafe类register方法
//注册到Reactor线程的多路复用器上监听新客户端的接入
public final void register(final ChannelPromise promise) {
if (eventLoop.inEventLoop()) {//是否在当前eventLoop中
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {//不在当前eventLoop中，异步执行
@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();
promise.setFailure(t);
}
}
}
#unsafe类register0方法
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 (!ensureOpen(promise)) {
return;
}
doRegister();
registered = true;
promise.setSuccess();
pipeline.fireChannelRegistered();
if (isActive()) {
pipeline.fireChannelActive();
}
} catch (Throwable t) {
// Close the channel directly to avoid FD leak.
closeForcibly();
closeFuture.setClosed();
if (!promise.tryFailure(t)) {
logger.warn(
"Tried to fail the registration promise, but it is complete already. " +
"Swallowing the cause of the registration failure:", t);
}
}
}
#AbstractNioChannel类doRegister方法
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
selectionKey = javaChannel().register(eventLoop().selector, 0, this);
//获取selectionKey ，通过SelectionKey的interestOps(int ops)方法可以修改监听操作位，注册OP_ACCEPT（16）到多路复用器上
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;
}
}
}
}
附录：
ChannelOption参数说明
1、ChannelOption.SO_BACKLOG
ChannelOption.SO_BACKLOG对应的是tcp/ip协议listen函数中的backlog参数，函数listen(int socketfd,int backlog)用来初始化服务端可连接队列，
服务端处理客户端连接请求是顺序处理的，所以同一时间只能处理一个客户端连接，多个客户端来的时候，服务端将不能处理的客户端连接请求放在队列中等待处理，backlog参数指定了队列的大小
2、ChannelOption.SO_REUSEADDR
ChanneOption.SO_REUSEADDR对应于套接字选项中的SO_REUSEADDR，这个参数表示允许重复使用本地地址和端口，
比如，某个服务器进程占用了TCP的80端口进行监听，此时再次监听该端口就会返回错误，使用该参数就可以解决问题，该参数允许共用该端口，这个在服务器程序中比较常使用，
比如某个进程非正常退出，该程序占用的端口可能要被占用一段时间才能允许其他进程使用，而且程序死掉以后，内核一需要一定的时间才能够释放此端口，不设置SO_REUSEADDR
就无法正常使用该端口。
3、ChannelOption.SO_KEEPALIVE
Channeloption.SO_KEEPALIVE参数对应于套接字选项中的SO_KEEPALIVE，该参数用于设置TCP连接，当设置该选项以后，连接会测试链接的状态，这个选项用于可能长时间没有数据交流的
连接。当设置该选项以后，如果在两小时内没有数据的通信时，TCP会自动发送一个活动探测数据报文。
4、ChannelOption.SO_SNDBUF和ChannelOption.SO_RCVBUF
ChannelOption.SO_SNDBUF参数对应于套接字选项中的SO_SNDBUF，ChannelOption.SO_RCVBUF参数对应于套接字选项中的SO_RCVBUF这两个参数用于操作接收缓冲区和发送缓冲区
的大小，接收缓冲区用于保存网络协议站内收到的数据，直到应用程序读取成功，发送缓冲区用于保存发送数据，直到发送成功。
5、ChannelOption.SO_LINGER
ChannelOption.SO_LINGER参数对应于套接字选项中的SO_LINGER,Linux内核默认的处理方式是当用户调用close（）方法的时候，函数返回，在可能的情况下，尽量发送数据，不一定保证
会发生剩余的数据，造成了数据的不确定性，使用SO_LINGER可以阻塞close()的调用时间，直到数据完全发送
6、ChannelOption.TCP_NODELAY
ChannelOption.TCP_NODELAY参数对应于套接字选项中的TCP_NODELAY,该参数的使用与Nagle算法有关
Nagle算法是将小的数据包组装为更大的帧然后进行发送，而不是输入一次发送一次,因此在数据包不足的时候会等待其他数据的到了，组装成大的数据包进行发送，虽然该方式有效提高网络的有效
负载，但是却造成了延时，而该参数的作用就是禁止使用Nagle算法，使用于小数据即时传输，于TCP_NODELAY相对应的是TCP_CORK，该选项是需要等到发送的数据量最大的时候，一次性发送
