原生的NIO类图使用有诸多不便，Netty向用户屏蔽了细节，在与用户交界处做了封装。
一、服务端创建时序图
步骤一：创建ServerBootstrap实例
ServerBootstrap是Netty服务端的启动辅助类，它提供了一些列的方法用于设置参数，由于参数太多，使用builder模式。
步骤二：设置并且绑定好Reactor线程池
Netty中的Reactor线程池是EventLoopGroup，它实际上就是EventLoop数组。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。值得说明的是，EventLoop的职责不仅仅是处理网络I/O事件，用户自定义的Task和定时任务Task也统一由EventLoop负责处理，这样线程模型就实现了统一。从调度层面看，也不存在从EvenetLoop线程中再启动其他类型的线程用于异步执行另外的任务，这样就避免了多线程的并发操作和锁竞争，提升了I/O线程的处理和调度性能。
步骤三：设置并绑定服务端Channel
服务端需要创建ServerSocketChannel，对原生NIO类库进行了封装，对应是NioServerSocketChannel
对用户而言，不需要关心服务端Channel的底层实现细节和工作原理，只需要指定具体是哪种服务端
Netty的ServerBootstrap提供了channel方法用于指定服务端Channel的类型。Netty通过工厂反射创建NioServerSocketChannel对象。
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}
步骤四：链路建立的时候创建并且初始化ChannelPipeline。
它本质上是一个负载处理网络事件的职责链，负载管理和执行ChannelHanler。网络事件以事件流的形式在ChannelPipeline中流转，由ChannelPipeline根据ChannelHandler的执行策略调度ChannelHandler的执行。典型的网络事件如下：
（1）链路注册；
（2）链路激活；
（3）链路断开；
（4）接收到请求消息；
（5）请求消息接收并处理完毕；
（6）发送应答消息；
（7）链路发生异常；
（8）发生用户自定义事件。
步骤五：初始化ChannelPipeline完成之后，添加并设置ChannelHandler。
ChannelHandler是Netty提供给用户定制和扩展的关键接口。利用ChannelHandler用户可以完成大多数的功能定制，例如消息编解码、心跳、安全认证、TSL/SSL认证、流量控制和流量整形等。Netty同时也提供了大量的系统ChannelHandler供用户使用，比较实用的系统ChannelHandler总结如下：
（1）系统编解码框架——ByteToMessageCodec；
（2）通用基于长度的半包解码器——LengthFieldBasedFrameDecoder;
（3）码流日志打印Handler——LoggingHandler；
（4）SSL安全认证Handler——SslHandler；
（5）链路空闲检测Handler——IdleStateHandler；
（6）流量整形Handler——ChannelTrafficShapingHandler;
（7）Base64编解码——Base64Decoder和Base64Encoder。
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch)
throws Exception {
ch.pipeline().addLast(
new EchoServerHandler());
}
});
步骤6：绑定并启动监听端口。
在绑定监听端口之前系统会做一系列的初始化和检测工作，完成之后，会启动监听端口，并将ServerSocketChannel注册到Selector上监听客户端连接，相关代码如下。
public ChannelFuture bind(SocketAddress localAddress) {
validate();
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
return doBind(localAddress);
}
步骤7：Selector轮询。
由Reactor线程NioEventLoop负责调度和执行Selector轮询操作，选择准备就绪的Channel集合，相关代码如下
private void select(boolean oldWakenUp) throws IOException {
Selector selector = this.selector;
try {
//此处代码省略...
int selectedKeys = selector.select(timeoutMillis);
selectCnt ++;
//此处代码省略...
步骤8：当轮询到准备就绪的Channel之后，就由Reactor线程NioEventLoop执行ChannelPipeline的相应方法，最终调度并执行ChannelHandler
步骤9：执行Netty系统ChannelHandler和用户添加定制的ChannelHandler。
ChannelPipeline根据网络事件的类型，调度并执行ChannelHandler，相关代码如下。
public ChannelHandlerContext fireChannelRead(final Object msg) {
invokeChannelRead(findContextInbound(), msg);
return this;
}
二、Netty服务端创建源码分析
1. 创建线程组：
通常会创建两个EventLoopGroup，也可以只创建一个并共享。
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
NioEventLoopGroup实际就是Reactor线程池，负责调度和执行具体的任务：
client接入
网络读写事件处理
用户自定义任务
定时任务
通过ServerBootstrap的group方法将2个EventLoopGroup实例传入：
public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
//1. 调用父类的group方法传入parentGroup
super.group(parentGroup);
//2. 设置childGroup
if (childGroup == null) {
throw new NullPointerException("childGroup");
}
if (this.childGroup != null) {
throw new IllegalStateException("childGroup set already");
}
this.childGroup = childGroup;
return this;
}
如果只传一个参数，则2个线程池会被重用。
public ServerBootstrap group(EventLoopGroup group) {
return group(group, group);
}
2. 设置服务端Channel用于端口监听和客户端链路接入:
根据传入的channel class创建对应的服务端Channel，调用的是ReflectiveChannelFactory
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();
} catch (Throwable t) {
throw new ChannelException("Unable to create Channel from class " + clazz, t);
}
}
@Override
public String toString() {
return StringUtil.simpleClassName(clazz) + ".class";
}
}
3. 下面要设定服务端TCP参数：
Netty使用一个LinkedHashMap来保存
private final Map<ChannelOption<?>, Object> options = new LinkedHashMap<ChannelOption<?>, Object>();
主要参数是TCP的backlog参数，底层C对应的接口为：
int listen(int fd, int backlog);
backlog指定了内核为此套接字接口排队的最大连接个数，内核要为套接字维护2个队列：未连接队列和已连接队列，根据TCP三路握手的三个分节分隔这2个队列。
服务器处于listen状态时，收到客户端syn分节(connect)时在未完成队列中创建一个新的条目，然后用三路握手的第二个分节即服务器的syn响应客户端，此条目在第三个分节到达前(客户端对服务器syn的ack)一直保留在未完成队列中，如果三路握手完成，该条目将从未完成连接队列搬到已完成队列尾部。
当进程调用accept时，从已完成队列中的头部取出一个条目给进程，当已完成队列为空时进程将进入睡眠，直到有条目在已完成队列中才唤醒。backlog则用来规定2个队列总和的最大值，大多数实现值为5，但是在高并发场景中显然不够，比如Lighttpd中此值达到128*8。需要设置此值更大原因是未完成连接队列可能因为客户端syn的到达以及等待握手第三个分节的到达延时而增大。Netty默认是100，用户可以调整。
4. 下面可以为启动辅助类和其父类分别指定Handler。
本质区别就是：
- ServerBootstrap中的Handler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是个工厂类，它为每个新接入的客户端都创建一个新的Handler；
5. 服务端启动的最后一步，就是绑定本地端口，启动服务：
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.registered();
doBind0(regFuture, channel, localAddress, promise);
}
}
});
return promise;
}
}
看红色标注的initAndRegister方法:
调用 channelFactory.newChannel()常见NioServerSocketChannel
然后调用init(channel)方法，由具体子类实现，这里实现的子类是ServerBootrap.
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
channel = channelFactory.newChannel();
init(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);
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;
}
下面关注init方法，主要完成了以下功能：
(1). 设置Socket参数和NioServerSocketChannel的附加属性，代码如下:
final Map<ChannelOption<?>, Object> options = options0();
synchronized (options) {
channel.config().setOptions(options);
} 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());
}
}
(2).将AbstractBootstrap的Handler添加到NioServerSocketChannel的ChannelPipeline中，将用于服务端注册的Handler ServerBootstrapAcceptor添加到ChannelPipeline中，代码如下：
p.addLast(new ChannelInitializer<Channel>() {
@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));
}
});
}
});
到此处，Netty服务端监听的相关资源已经初始化完毕，就剩下最后一步-
6. 注册NioServerSocketChannel到Reactor线程的多路复用器上，然后轮询客户端连接事件。
在分析注册代码之前，我们先通过下图看看目前NioServerSocketChannel的ChannelPipeline的组成：
通过Debug最终发现在AbstractChannel类中的register方法上：
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;
if (eventLoop.inEventLoop()) {
register0(promise);
} 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);
}
}
}
首先判断是否是NioEventLoop自身发起的操作，如果是，则不存在并发操作，直接执行Channel注册；如果由其它线程发起，则封装成一个Task放入消息队列中异步执行。此处，由于是由ServerBootstrap所在线程执行的注册操作，所以会将其封装成Task投递到NioEventLoop中执行：
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();
neverRegistered = false;
registered = true;
// 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);
}
}
将NioServerSocketChannel注册到NioEventLoop的Selector上，代码如下：
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
selectionKey = javaChannel().register(eventLoop().selector, 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;
}
}
}
}
大伙儿可能会很诧异，应该注册OP_ACCEPT（16）到多路复用器上，怎么注册0呢？0表示只注册，不监听任何网络操作。这样做的原因如下：
- 注册方法是多态的，它既可以被NioServerSocketChannel用来监听客户端的连接接入，也可以用来注册SocketChannel，用来监听网络读或者写操作；
- 通过SelectionKey的interestOps(int ops)方法可以方便的修改监听操作位。所以，此处注册需要获取SelectionKey并给AbstractNioChannel的成员变量selectionKey赋值。
注册成功之后，触发ChannelRegistered事件，方法如下：
doRegister();
neverRegistered = false;
registered = true; // 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();
Netty的HeadHandler不需要处理ChannelRegistered事件，所以，直接调用下一个Handler，当ChannelRegistered事件传递到TailHandler后结束，TailHandler也不关心ChannelRegistered事件。
ChannelRegistered事件传递完成后，判断ServerSocketChannel监听是否成功，如果成功，需要出发NioServerSocketChannel的ChannelActive事件，判断方法即isActive().
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();
}
}
其中isActive()是一个多态方法，如果是服务端则，判断监听是否启动；如果是客户端，判断TCP连接是否完成。ChannelActive事件在ChannelPipeline()传递，完成之后根据配置决定是否自动触发Channel的读操作。
读方法最终到：
public final void beginRead() {
assertEventLoop();
if (!isActive()) {
return;
}
try {
doBeginRead();
} catch (final Exception e) {
invokeLater(new Runnable() {
@Override
public void run() {
pipeline.fireExceptionCaught(e);
}
});
close(voidPromise());
}
}
由于不同类型的Channel对读的准备工作不同，因此doBeginRead也是个多态方法。
对于NIO通信，无论是客户端，还是服务端，都要设置网络监听操作位为自己感兴趣的，对于NioServerSocketChannel感兴趣的是OP_ACCEPT(16)，于是修改操作位：
@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);
}
}
某些情况下，当前监听的操作类型和Channel关心的网络事件是一致的，不需要重复注册，所以增加了&的判断。JDK SelectionKey有4种操作类型，分别为：
(1) OP_READ = 1 <<0;
(2) OP_WRTE = 1 <<2;
(3) OP_CONNECT = 1 <<3;
(4) OP_ACCEPT = 1 <<4;
此时，服务器监听启动部分源码已经分析结束。
三、客户端接入源码分析
负责处理网络读写、连接和客户端请求介入的Reactor线程就是NioEventLoop，下面分析如何处理新的客户端接入。
当多路复用器检测到新的Channel时候，默认执行processSelectedKeysOptimized方法.
private void processSelectedKeys() {
if (selectedKeys != null) {
processSelectedKeysOptimized(selectedKeys.flip());
} else {
processSelectedKeysPlain(selector.selectedKeys());
}
}
由于Channel的Attachment是NioServerSocketChannel，所以执行processSelectedKey方法。
if (a instanceof AbstractNioChannel) {
processSelectedKey(k, (AbstractNioChannel) a);
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
processSelectedKey(k, task);
}
继续点入该方法debug，由于监听的是连接操作，会执行unsafe.read()方法。由于不同的Channel执行不同的操作，所以NioUnsafe被设计为接口。
debug发现使用的是:
其read()方法如下所示：
@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;
}
//代码省略...
}
对doReadMessages方法进行分析，发现它实际就是接受新的客户端连接并且创建NioSocketChannel:
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = javaChannel().accept();
try {
if (ch != null) {
buf.add(new NioSocketChannel(this, ch));
return 1;
}
} catch (Throwable t) {
logger.warn("Failed to create a new channel from an accepted socket.", t);
try {
ch.close();
} catch (Throwable t2) {
logger.warn("Failed to close a socket.", t2);
}
}
return 0;
}
接收到新的连接之后，触发ChannelPipeLine的ChannelRead方法，代码如下：
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
pipeline.fireChannelRead(readBuf.get(i));
}
于是触发pipeLine调用链，事件在ChannelPipeline中传递，执行ServerBootstrapAcceptor中的channelRead方法，代码如下：
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel) msg;
child.pipeline().addLast(childHandler); //(1)
for (Entry<ChannelOption<?>, Object> e: childOptions) {
try {
if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {
logger.warn("Unknown channel option: " + e);
}
} catch (Throwable t) {
logger.warn("Failed to set a channel option: " + child, t);
}
}
//(2)
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
//(3)
try {
childGroup.register(child).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (!future.isSuccess()) {
forceClose(child, future.cause());
}
}
});
} catch (Throwable t) {
forceClose(child, t);
}
}
上面方法主要分为3个步骤：
(1) 加入childHandler到客户端SocketChannel的ChannelPipeline中
(2) 设置SocketChannel的TCP参数
(3) 注册SocketChannel到多路复用器
注意这里register注册也是注册操作位为0.
执行完注册后，紧接着会触发ChannelReadComplete事件。
Netty的Header和Tailer本身不关于这个事件，因此ChannelReadComplete是直接透传， 执行完ChannelReadComplete后，接着执行PipeLine的read()方法，最终到HeadHandler的read()方法。read()方法在前面说过，会修改操作位，此时这里debug发现把操作位修改OP_READ。
此时，客户端连接处理完成，可以进行网络读写等I/O操作。
四、Netty客户端创建流程
1、用户线程创建Bootstrap
Bootstrap b = new Bootstrap();
Bootstrap是Socket客户端创建工具类，通过API设置创建客户端相关的参数，异步发起客户端连接。
2、创建处理客户端连接、IO读写的Reactor线程组NioEventLoopGroup
EventLoopGroup group = new NioEventLoopGroup();
3、通过Bootstrap的ChannelFactory和用户指定的Channel类型创建用于客户端连接的NioSocketChannel
b.group(group).channel(NioSocketChannel.class).option(ChannelOption.TCP_NODELAY, true)
此处的NioSocketChannel类似于Java NIO提供的SocketChannel。
4、创建默认的channel Handler pipeline
b.group(group).channel(NioSocketChannel.class).option(ChannelOption.TCP_NODELAY, true)
.handler(new ChannelInitializer<SocketChannel>()
{
@Override
public void initChannel(SocketChannel ch) throws Exception
{
ch.pipeline().addLast(new HelloClientHandler());
}
});
用于调度和执行网络事件。
5、异步发起TCP连接
// 发起异步连接操作
ChannelFuture f = b.connect(host, port).sync();
SocketChannel执行connect()操作后有以下三种结果：
- 连接成功，然会true;
- 暂时没有连接上，服务器端没有返回ACK应答，连接结果不确定，返回false。此种结果下，需要将NioSocketChannel中的selectionKey设置为OP_CONNECT，监听连接结果；
- 接连失败，直接抛出I/O异常
6、注册对应的网络监听状态位到多路复用器
7、由多路复用器在I/O中轮询个Channel，处理连接结果
8、如果连接成功，设置Future结果，发送连接成功事件，触发ChannelPipeline执行
9、由ChannelPipeline调度执行系统和用户的ChannelHandler，执行业务逻辑
五、客户端创建源码分析
Netty客户端创建流程非常繁琐，这里只针对关键步骤进行分析。
5.1 客户端连接辅助类Bootstrap
1. 设置I/O线程组：只需要一个线程组EventLoopGroup
2. TCP参数设置：
public <T> B option(ChannelOption<T> option, T value) {
if (option == null) {
throw new NullPointerException("option");
}
if (value == null) {
synchronized (options) {
options.remove(option);
}
} else {
synchronized (options) {
options.put(option, value);
}
}
return (B) this;
}
主要TCP参数如下：
- (1) SO_TIMEOUT: 控制读取操作将阻塞多少毫秒，如果返回值为0，计时器就被禁止了，该线程将被无限期阻塞。
- (2) SO_SNDBUF: 套接字使用的发送缓冲区大小
- (3) SO_RCVBUF: 套接字使用的接收缓冲区大小
- (4) SO_REUSEADDR : 是否允许重用端口
- (5) CONNECT_TIMEOUT_MILLIS: 客户端连接超时时间，原生NIO不提供该功能，Netty使用的是自定义连接超时定时器检测和超时控制
- (6) TCP_NODELAY : 是否使用Nagle算法
3. channel接口
同样使用反射创建NioSocketChannel
4. 设置Handler接口
Bootstrap为了简化Handler的编排，提供了ChannelInitializer，当TCP链路注册成功后，调用initChannel接口:
public final void channelRegistered(ChannelHandlerContext ctx) throws Exception {
// Normally this method will never be called as handlerAdded(...) should call initChannel(...) and remove
// the handler.
if (initChannel(ctx)) {
// we called initChannel(...) so we need to call now pipeline.fireChannelRegistered() to ensure we not
// miss an event.
ctx.pipeline().fireChannelRegistered();
} else {
// Called initChannel(...) before which is the expected behavior, so just forward the event.
ctx.fireChannelRegistered();
}
}
其中InitChannel为抽象接口，即下面红色标注的代码，用户便是在这个方法中设置ChannelHandler:
private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
if (initMap.putIfAbsent(ctx, Boolean.TRUE) == null) { // Guard against re-entrance.
try {
initChannel((C) ctx.channel());
} catch (Throwable cause) {
// Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
// We do so to prevent multiple calls to initChannel(...).
exceptionCaught(ctx, cause);
} finally {
remove(ctx);
}
return true;
}
return false;
}
5.2 客户端连接操作
1. 首先要创建和初始化NioSocketChannel
private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();
final Channel channel = regFuture.channel();
//.....
创建之后，初始化，然后在注册：
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
channel = channelFactory.newChannel();
init(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);
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;
}
2. 创建完成后，连接操作会异步执行，最终调用到HeadContext的connect方法.
public void connect(
ChannelHandlerContext ctx,
SocketAddress remoteAddress, SocketAddress localAddress,
ChannelPromise promise) throws Exception {
unsafe.connect(remoteAddress, localAddress, promise);
}
的connect操作如下：
if (doConnect(remoteAddress, localAddress)) {
fulfillConnectPromise(promise, wasActive);
} else {
//...
3. doConnect三种可能结果
- 连接成功，然会true;
- 暂时没有连接上，服务器端没有返回ACK应答，连接结果不确定，返回false。此种结果下，需要将NioSocketChannel中的selectionKey设置为OP_CONNECT，监听连接结果；
- 接连失败，直接抛出I/O异常
异步返回之后，需要判断连接结果，如果成功，则触发ChannelActive事件。最终会将NioSocketChannel中的selectionKey设置为SelectionKey.OP_READ，用于监听网络读操作。
5.3 异步连接结果通知
NioEventLoop的Selector轮询客户端连接Channel，当服务端返回应答后，进行判断。依旧是NioEventLoop中的processSelectedKey方法：
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();
}
下面分析finishConnect方法：
@Override
public final void finishConnect() {
// Note this method is invoked by the event loop only if the connection attempt was
// neither cancelled nor timed out. assert eventLoop().inEventLoop(); try {
boolean wasActive = isActive();
doFinishConnect(); //判断SocketChannel的连接结果，true表示成功
fulfillConnectPromise(connectPromise, wasActive); //触发链路激活
} catch (Throwable t) {
fulfillConnectPromise(connectPromise, annotateConnectException(t, requestedRemoteAddress));
} finally {
// Check for null as the connectTimeoutFuture is only created if a connectTimeoutMillis > 0 is used
// See https://github.com/netty/netty/issues/1770
if (connectTimeoutFuture != null) {
connectTimeoutFuture.cancel(false);
}
connectPromise = null;
}
}
fulfillConnectPromise方法则触发链路激活事件，并由ChannelPipeline进行传播:
private void fulfillConnectPromise(ChannelPromise promise, boolean wasActive) {
if (promise == null) {
// Closed via cancellation and the promise has been notified already.
return;
}
// Get the state as trySuccess() may trigger an ChannelFutureListener that will close the Channel.
// We still need to ensure we call fireChannelActive() in this case.
boolean active = isActive();
// trySuccess() will return false if a user cancelled the connection attempt.
boolean promiseSet = promise.trySuccess();
// Regardless if the connection attempt was cancelled, channelActive() event should be triggered,
// because what happened is what happened.
if (!wasActive && active) {
pipeline().fireChannelActive();
}
// If a user cancelled the connection attempt, close the channel, which is followed by channelInactive().
if (!promiseSet) {
close(voidPromise());
}
}
跟之前类似，将网络监听修改为读操作。
5.4 客户端连接超时机制
由Netty自己实现的客户端超时机制，在AbstractNioChannel的connect方法中：
public final void connect(
final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
if (!promise.setUncancellable() || !ensureOpen(promise)) {
return;
} try {
if (connectPromise != null) {
// Already a connect in process.
throw new ConnectionPendingException();
} boolean wasActive = isActive();
if (doConnect(remoteAddress, localAddress)) {
fulfillConnectPromise(promise, wasActive);
} else {
connectPromise = promise;
requestedRemoteAddress = remoteAddress; // Schedule connect timeout.
int connectTimeoutMillis = config().getConnectTimeoutMillis();
if (connectTimeoutMillis > 0) {
connectTimeoutFuture = eventLoop().schedule(new Runnable() {
@Override
public void run() {
ChannelPromise connectPromise = AbstractNioChannel.this.connectPromise;
ConnectTimeoutException cause =
new ConnectTimeoutException("connection timed out: " + remoteAddress);
if (connectPromise != null && connectPromise.tryFailure(cause)) {
close(voidPromise());
}
}
}, connectTimeoutMillis, TimeUnit.MILLISECONDS);
} promise.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (future.isCancelled()) {
if (connectTimeoutFuture != null) {
connectTimeoutFuture.cancel(false);
}
connectPromise = null;
close(voidPromise());
}
}
});
}
} catch (Throwable t) {
promise.tryFailure(annotateConnectException(t, remoteAddress));
closeIfClosed();
}
}
一旦超时定时器执行，则说明客户端超时，构造异常，将异常结果设置到connectPromise中，同时关闭客户端句柄。
如果在超时之前获取结果，则直接删除定时器，防止其被触发。