服务端的创建
示例代码
netty源码中有一个netty-example项目，不妨以经典的EchoServer作为楔子。
// 步骤1
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
final EchoServerHandler serverHandler = new EchoServerHandler();
try {
// 步骤2
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 100)
.attr(AttributeKey.newInstance("attr"), "attr")
.handler(new LoggingHandler(LogLevel.INFO))
.childHandler(new channelInitializer<SocketChannel> {
@Override
public void initChannel(SocketChannel ch)throws Exception {
ChannelPipeline p = ch.pipeline();
if (sslCtx != null) {
p.addLast(sslCtx.newHandler(ch.alloc()));
}
p.addLast(serverHandler);
}
})
.childOption(ChannelOption.CONNECT_TIMEOUT_MILLIS, 3000)
.childAttr(AttributeKey.newInstance("childAttr"), "childAttr");
// 步骤3
ChannelFuture f = b.bind(PORT).sync();
// 步骤4
f.channel().closeFuture().sync();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
整个流程可以分为几个步骤
- 创建workerGroup和bossGroup
- 创建ServerBootstrap，并设置参数
- 通过Serverbootstrap引导启动，并绑定端口。这里又可以分为初始化、注册、绑定端口、注册感兴趣事件4个小步骤
- 主线程阻塞
- 设置优雅关闭
步骤1留待下一节，先看步骤2
步骤2
ServerBootstrap首先设置bossGroup为parentGroup，workerGroup为childGroup，然后在channel方法中创建了一个工厂，该生产通过设置进来的泛型,利用反射来生产对象。此处生产对象为ServerSocketChannel。
除此之外还设置了handler、childHandler、option、childOption、attr、childAttr6个属性，根据group的命名规则，可以猜测不带child的属性是给bossGroup内的nioEventLoop使用，而以child开头的属性是给workGroupo内的nioEventLoop使用的。
public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
super.group(parentGroup);
this.childGroup = childGroup;
return this;
}
public B channel(Class<? extends C> channelClass) {
return channelFactory(new ReflectiveChannelFactory<C>
(channelClass)
));
}
public ReflectiveChannelFactory(Class<? extends T> clazz) {
this.constructor = clazz.getConstructor();
}
步骤3
serverBootstrap配置完后,开始服务端真正的启动工作，进入b.bind()方法，一路跳转到AbstractBootstrap.doBind(SocketAddress localAddress)方法。
private ChannelFuture doBind(final SocketAddress localAddress) {
// 初始化并注册
final ChannelFuture regFuture = initAndRegister();
final Channel channel = regFuture.channel();
//若已经注册完成,则开始绑定，否则添加一个监听器,待注册完成后绑定
if (regFuture.isDone()) {
ChannelPromise promise = channel.newPromise();
doBind0(regFuture, channel, localAddress, promise);
return promise;
} else {
final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
regFuture.addListener((ChannelFutureListener) future ->
doBind0(regFuture, channel, localAddress, promise));
return promise;
}
}
步骤3.1
先看看initAndRegister()方法。首先是创建NioServerSocketChannel，通过工厂模式实例化一个channel。在channel的构造函数里，传入OP_ACCEPT感兴趣事件，并设置NioServerSocketChannel为非阻塞模式。在顶级抽象父类里，创建好id、unsafe、pipeline。unsafe对象是关联socket或其他进行IO操作组件的一个类，与jdk的unsafe对象类似，一般不需要用户关注
final ChannelFuture initAndRegister() {
Channel channel = channelFactory.newChannel();
init(channel);
ChannelFuture regFuture = config().group().register(channel);
return regFuture;
}
public T newChannel() {
return constructor.newInstance();
}
public NioServerSocketChannel(ServerSocketChannel channel) {
super(null, channel, SelectionKey.OP_ACCEPT);
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
protected AbstractNioChannel(Channel parent, SelectableChannel ch, inteadInterestOp) {
super(parent);
this.ch = ch;
this.readInterestOp = readInterestOp;
ch.configureBlocking(false);
}
}
AbstractChannel(Channel parent) {
this.parent = parent;
id = newId();
unsafe = newUnsafe();
pipeline = newChannelPipeline();
}
之后开始执行初始化,将bootstrapServer中的option和attr赋予serverSocketChannel,通过pipeline添加一个channelInitializer，进而通过channelInitializer将childOption、childAttr、workerGroup、childHandler保存在一个叫ServerBootstrapAcceptor的handler中。从名字可以猜测，此handler应该是在客户端连接时来处理相关事件的。而channelInitializer会在完成由子类实现的initChannel方法后将自己从pipeline中移除。
void init(Channel channel) {
setChannelOptions(channel, options0().entrySet().toArray(newOptionArray(0)), logger);
setAttributes(channel, attrs0().entrySet().toArray(newAttrArray(0)));
ChannelPipeline p = channel.pipeline();
final EventLoopGroup currentChildGroup = childGroup;
final ChannelHandler currentChildHandler = childHandler;
final Entry<ChannelOption<?>, Object>[] currentChildOptions = childOptions.entrySet().toArray(newOptionArray(0));
final Entry<AttributeKey<?>, Object>[] currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(0));
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(() -> pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)));
}
});
}
步骤3.2
初始化完毕后便是注册,首先判断是否注册以及传进来的eventLoop参数是否属于NioEventLoop类，然后判断当前线程是否是NioEventLoop线程，若是，则直接注册，否则添加到eventLoop的线程池中,通过创建nioEventLoop线程来执行注册。在register0——doRegister方法链中，我们看到netty最终调用了jdk底层的channel绑定了selector，由于此时还未绑定端口，所以ops即感兴趣事件是0。同时，把this，即NioServerSocketChannel作为attachment添加到selectionKey上，这是为了之后在select出事件时，可以获取channel进行操作。当注册完毕后，调用pipeline进行注册事件传播。如果设置了自动读取，还会立即开始一次读取。
// 通过unsafe对象进行注册
public ChannelFuture register(final ChannelPromise promise) {
promise.channel().unsafe().register(this, promise);
return promise;
}
// 如果当前线程是之前nioEventLoop绑定的线程则直接注册，否则添加到eventLoop的线程池中
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
AbstractChannel.this.eventLoop = eventLoop;
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
eventLoop.execute(() -> register0(promise));
}
}
// 注册后调用pipeline进行一些事件传播
private void register0(ChannelPromise promise) {
boolean firstRegistration = neverRegistered;
doRegister();
neverRegistered = false;
registered = true;
pipeline.invokeHandlerAddedIfNeeded();
pipeline.fireChannelRegistered();
if (isActive()) {
if (firstRegistration) {
pipeline.fireChannelActive();
} else if (config().isAutoRead()) {
beginRead();
}
}
}
// 最终调用jdk底层channel进行注册
protected void doRegister() throws Exception {
for (;;) {
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
}
}
步骤3.3
回到AbstractBootStrap, 开始执行doBind0方法。这也是需要由nioEventLoop执行的，所以也丢到了线程池里。
private static void doBind0(final ChannelFuture regFuture, final Channel channel,
final SocketAddress localAddress, final ChannelPromise promise) {
channel.eventLoop().execute(new Runnable() {
public void run() {
if (regFuture.isSuccess()) {
channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
} else {
promise.setFailure(regFuture.cause());
}
}
});
}
// 调用pipeline-tail进行绑定
public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
return pipeline.bind(localAddress, promise);
}
public final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
return tail.bind(localAddress, promise);
}
@Override
public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
// 调用下一个OutBoundHandlerContext执行,默认传递到headContext
final AbstractChannelHandlerContext next = findContextOutbound(MASK_BIND);
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
next.invokeBind(localAddress, promise);
} else {
safeExecute(executor, new Runnable() {
@Override
public void run() {
next.invokeBind(localAddress, promise);
}
}, promise, null);
}
return promise;
}
// 传到headContext，调用unsafe来执行。
@Override
public void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) {
unsafe.bind(localAddress, promise);
}
// AbstractUnsafe的bind
public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
doBind(localAddress);
if (!wasActive && isActive()) {
invokeLater(new Runnable() {
@Override
public void run() {
pipeline.fireChannelActive();
}
});
}
// unsafe最终调用jdk的channel完成绑定操作
protected void doBind(SocketAddress localAddress) throws Exception {
javaChannel().bind(localAddress, config.getBacklog());
}
nioEventLoop通过pipeline的方式绑定,pipeline调用tailContext的bind方法，tailContext又会不断寻找下一个OutBoundHandler来执行bind方法，默认会传到headContext，headContext再调用底层的unsafe来执行bind。unsafe完成bind后，会通知pipeline调用fireChannelActive方法。这里绑定端口便完成了
步骤3.4
绑定端口后还需要注册感兴趣事件，这是通过fireChannelActive触发的。active事件首先会传递到headContext，
public void channelActive(ChannelHandlerContext ctx) {
ctx.fireChannelActive();
readIfIsAutoRead();
}
headContext继续将active事件传播，然后调用readIfIsAutoRead方法。此方法会调用channel的read方法，继而调用pipeline的read事件进行传播，又回到headContext，headContext又调用unsafe的beginRead如下:
protected void doBeginRead() throws Exception {
final SelectionKey selectionKey = this.selectionKey;
readPending = true;
final int interestOps = selectionKey.interestOps();
if ((interestOps & readInterestOp) == 0) {
selectionKey.interestOps(interestOps | readInterestOp);
}
}
在beginRead方法中,将channel注册感兴趣事件为创建NioServerSocketChannel时传入的OP_ACCEPT事件。
至此，初始化、注册selector、绑定、注册感兴趣事件4个小步骤都完成了
步骤4
主线程调用DefaultChannelPromise的sync方法，sync方法调用父类的await方法,阻塞在此处, 从此任由netty自由发挥，知道netty关闭了线程
步骤5
当netty报错或关闭后，主线程结束阻塞，开始执行netty优雅关闭机制。待续