在Java中，网络通信是通过Socket来进行的，那么在Netty中，服务端的Socket是在哪里进行初始化的？并且在哪里进行accept连接？ Netty里的Channel是啥，有啥作用呢？带着这三个问题，进入本文的Netty服务端启动分析。
本文分析将分为四大步：
- 创建服务端Channel；
- 初始化服务端Channel；
- 注册selector；
- 端口绑定；
创建服务端Channel又可以分为四步，如下：
- ServerBootstrap#bind() 用户代码入口；
- initAndRegister() 初始化并注册；
- newChannel() 创建服务端channel；
首先看下下图简易版的Netty服务端启动代码。
public final class Server {
public static void main(String[] args) throws Exception {
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.childOption(ChannelOption.TCP_NODELAY, true)
.childAttr(AttributeKey.newInstance("childAttr"), "childAttrValue")
.handler(new ServerHandler())
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch) {
ch.pipeline().addLast(new AuthHandler());
//..
}
});
ChannelFuture f = b.bind(8888).sync();
f.channel().closeFuture().sync();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}服务端构建好ServerBootstrap之后，通过bind()方法进行绑定。进入ServerBootstrap的父类AbstractBootstrap后，一路调用来到AbstractBootstrap#doBind()方法，首先就是初始化并注册Channel。
在initAndRegister处channelFactory是ReflectiveChannelFactory，具体赋值处是在ServerBootstrap#channel()方法中定义的，并且传入的channel是：NioServerSocketChannel，上图中可以见。
查看到ReflectiveChannelFactory#newChannel()方法，实际就是调用的NioServerSocketChannel#newInstance()方法反射构建一个channel对象。
那么，我们看下NioServerSocketChannel底层是如何获取通过反射创建服务端Channel的呢？
以下部分源码均在NioServerSocketChannel类中
public NioServerSocketChannel() {
this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}而newSocket()方法是一个静态方法
private static ServerSocketChannel newSocket(SelectorProvider provider) {
try {
// 通过SelectorProvider来获取一个ServerSocketChannel，SelectorProvider是通过SPI来获取的
// 此处返回一个ServerSocketChannelImpl对象
return provider.openServerSocketChannel();
} catch (IOException e) {
throw new ChannelException(
"Failed to open a server socket.", e);
}
} public NioServerSocketChannel(ServerSocketChannel channel) {
// 调用抽象父类AbstractNioChannel构造方法，注意此处服务端Channel注册的是OP_ACCEPT事件
super(null, channel, SelectionKey.OP_ACCEPT);
// TCP参数配置类
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}AbstractNioChannel类
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
// 调用父类AbstractChannel的构造方法
super(parent);
this.ch = ch;
this.readInterestOp = readInterestOp;
try {
// 关闭blocking，关闭阻塞模式：比较重要
ch.configureBlocking(false);
} 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);
}
}AbstractChannel类
protected AbstractChannel(Channel parent) {
this.parent = parent;
// 每个channel的唯一标识
id = newId();
// 底层io操作工具类
unsafe = newUnsafe();
// channel里的逻辑链pipeline（非常重要）
pipeline = newChannelPipeline();
}总结下服务端创建Channel的三件重要事情：
- 通过反射来创建JDK底层的channel；
- 设置Channel为非阻塞模式ch.configureBlocking(false);
- 创建一个pipeline对象；
初始化服务端Channel可以分为如下的几步：
- set ChannelOptions，ChannelAttrs 设置options和attrs
- set Child Options，ChildAttrs，为服务端创建的子链接创建options和attrs
- config handler，配置服务端pipeline
- add ServerBootstrapAcceptor，添加连接器
ServerBoostrap端初始化过程
void init(Channel channel) throws Exception {
// 获取用户配置的options
final Map<ChannelOption<?>, Object> options = options0();
synchronized (options) {
channel.config().setOptions(options);
}
// 配置attrs
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());
}
}
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();
if (handler != null) {
pipeline.addLast(handler);
}
// 添加ServerBootstrapAccetor
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
pipeline.addLast(new ServerBootstrapAcceptor(
currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
}整个注册selector过程可以分为以下几步：
- AbstractChannel#register(channel) 入口
- this.eventLoop = eventLoop 绑定线程
- register0() 实际注册
- doRegister() 调用jdk底层进行注册
- invokeHandlerAddedIfNeeded()
- fireChannelRegistered() 传播注册成功的事件
AbstractChannel
@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;
}
// 设置AbstractChannel的eventLoop
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);
}
}
}AbstractChannel.AbstractUnsafe#register0()
private void register0(ChannelPromise promise) {
try {
if (!promise.setUncancellable() || !ensureOpen(promise)) {
return;
}
boolean firstRegistration = neverRegistered;
doRegister();
neverRegistered = false;
registered = true;
// 调用handlerAdd事件回调
pipeline.invokeHandlerAddedIfNeeded();
safeSetSuccess(promise);
// 调用register事件回调
pipeline.fireChannelRegistered();
if (isActive()) {
if (firstRegistration) {
pipeline.fireChannelActive();
} else if (config().isAutoRead()) {
beginRead();
}
}
} catch (Throwable t) {
closeForcibly();
closeFuture.setClosed();
safeSetFailure(promise, t);
}
}AbstractNioChannel.java
@Override
protected void doRegister() throws Exception {
boolean selected = false;
// 这里是个小技巧，for(;;)比while(true)效率要高很多
for (;;) {
try {
// 将通道channel注册到selector上
selectionKey = javaChannel().register(eventLoop().selector, 0, this);
return;
} catch (CancelledKeyException e) {
if (!selected) {
eventLoop().selectNow();
selected = true;
} else {
throw e;
}
}
}
}就这样，NioServerSocketChannel就以Accept事件注册到了Selector上了。
端口绑定流程如下：
- AbstractUnsafe#bind() 入口
- dobind()
- javaChannel().bind() jdk底层绑定
- pipeline.fireChanelActive() 传播事件
- HeadContext.readIfIsAutoRead()
- dobind()
AbstractChannel.AbstractUnsafe#bind()
@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.isRoot()) {
// 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.");
}
// 是否active
boolean wasActive = isActive();
try {
// 调用jdk底层代码进行绑定
doBind(localAddress);
} catch (Throwable t) {
safeSetFailure(promise, t);
closeIfClosed();
return;
}
if (!wasActive && isActive()) {
invokeLater(new Runnable() {
@Override
public void run() {
pipeline.fireChannelActive();
}
});
}
safeSetSuccess(promise);
}NioServerSocketChannel.java
@Override
protected void doBind(SocketAddress localAddress) throws Exception {
// 判断jdk版本
if (PlatformDependent.javaVersion() >= 7) {
javaChannel().bind(localAddress, config.getBacklog());
} else {
javaChannel().socket().bind(localAddress, config.getBacklog());
}
}Netty服务端核心启动流程主要是为了创建NioServerSocketChannel，然后将其注册在Selector上，总结下核心步骤如下：
- NioServerSocket#newSocket() 获取服务端channel
- ServerBootstrap#init() 对服务端channel进行初始化
- AbstractChannel.AbstractUnsafe#register() 将服务端Channel注册到Selector上
- AbstractChannel.AbstractUnsafe#doBind() 注册端口号