本文为原创，转载请注明出处
netty4源码分析-socket
服务端启动的第一步必须先创建一个监听套接字ServerSocketChannel，该过程是由ChannelFuture f = b.bind(port)中的bind触发。下面详细分析其过程：
Bind源码如下，代码位于ServerBootstrap的父类AbstractBootstrap
//AbstractBootstrap
public ChannelFuture bind(int inetPort) {
return bind(new InetSocketAddress(inetPort));
}
public ChannelFuture bind(SocketAddress localAddress) {
validate();
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
return doBind(localAddress);
}
validate()方法的作用为：校验：bossGroup、BootstrapChannelFactory、childHandler非空。如果childGroup为空，则复用bossGroup，将bossGroup赋值给childGroup。
接着来看doBind的逻辑：
//AbstractBootstrap
private ChannelFuture doBind(final SocketAddress localAddress) {
final ChannelFuture regPromise = initAndRegister();
final Channel channel = regPromise.channel();
final ChannelPromise promise = channel.newPromise();
if (regPromise.isDone()) {
doBind0(regPromise, channel, localAddress, promise);
} else {
regPromise.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
doBind0(future, channel, localAddress, promise);
}
});
}
return promise;
}
重点分析里面的initAndRegister()方法
//AbstractBootstrap
final ChannelFuture initAndRegister() {
final Channel channel = channelFactory().newChannel();
try {
init(channel);
} catch (Throwable t) {
channel.unsafe().closeForcibly();
return channel.newFailedFuture(t);
}
ChannelPromise regPromise = channel.newPromise();
group().register(channel, regPromise);
if (regPromise.cause() != null) {
if (channel.isRegistered()) {
channel.close();
} else {
channel.unsafe().closeForcibly();
}
}
return regPromise;
}
a）首先分析以下代码：
final Channel channel = channelFactory().newChannel()
channelFactory()方法返回之前创建的BootstrapChannelFactory，里面的newChannel()方法会根据反射创建一个ServerSocketChannel
//BootstrapChannelFactory
public T newChannel() {
try {
return clazz.newInstance();
} catch (Throwable t) {
throw new ChannelException("Unable to create Channel from class " + clazz, t);
}
}
注：clazz是在服务端启动的这段代码（b.group(bossGroup,workerGroup).channel(NioServerSocketChannel.class)中设置的。
clazz.newInstance()会调用NioServerSocketChannel的默认构造函数
// NioServerSocketChannel
public NioServerSocketChannel() {
super(null, newSocket(), SelectionKey.OP_ACCEPT);
config = new DefaultServerSocketChannelConfig(this, javaChannel().socket());
}
private static ServerSocketChannel newSocket() {
try {
return ServerSocketChannel.open();
} catch (IOException e) {
throw new ChannelException(
"Failed to open a server socket.", e);
}
}
注意newSocket中的这行代码：
return ServerSocketChannel.open();
此处就是服务端监听套接字ServerSocketChannel创建的地方。
既然是使用NIO，那么设置创建的ServerSocketChannel为非阻塞是在哪个地方发生的呢？看下这行代码
super(null, newSocket(), SelectionKey.OP_ACCEPT);
它会对NioServerSocketChannel的父类进行初始化：NioServerSocketChannel的父类是AbstractNioMessageChannel，其构造方法仅仅初始化其父类AbstractNioChannel，父类构造方法如下：
//AbstractNioChannel
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
super(parent);
this.ch = ch;
this.readInterestOp = readInterestOp;
try {
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);
}
}
ch.configureBlocking(false)此处就将之前创建的ServerSocketChannel设置为非阻塞模式。
该方法里还有三点需要注意：
1、super(parent)会调用AbstractNioChannel的父类AbstractChannel的构造方法
// AbstractChannel.java
protected AbstractChannel(Channel parent) {
this.parent = parent;
unsafe = newUnsafe();
pipeline = new DefaultChannelPipeline(this);
}
newUnsafe()是由子类AbstractNioMessageChannel实现的，里面实例化了一个内部类NioMessageUnsafe（注：该类很重要，里面定义了read方法，会触发accept的调用，后面对其重点分析）。
2、this.readInterestOp = readInterestOp：设置channel的ops为SelectionKey.OP_ACCEPT（值为16）
3、pipeline = new DefaultChannelPipeline(this)，创建作用于ServerSocketChannel的管道Pipeline
// DefaultChannelPipeline
public DefaultChannelPipeline(Channel channel) {
if (channel == null) {
throw new NullPointerException("channel");
}
this.channel = channel;
TailHandler tailHandler = new TailHandler();
tail = new DefaultChannelHandlerContext(this, null, generateName(tailHandler), tailHandler);
HeadHandler headHandler = new HeadHandler(channel.unsafe());
head = new DefaultChannelHandlerContext(this, null, generateName(headHandler), headHandler);
head.next = tail;
tail.prev = head;
}
DefaultChannelPipeline维护了一个以DefaultChannelHandlerContext为元素的双向链表结构，Head是一个Outbound处理器，而tail是一个Inbound处理器。经过此步骤后，管道中的处理器链表为：Head->tail。
b）再来分析以下代码
init(channel)
该方法由子类ServerBootstrap实现
// ServerBootstrap.java
void init(Channel channel) throws Exception {
final Map<ChannelOption<?>, Object> options = options();
synchronized (options) {
channel.config().setOptions(options);
}
final Map<AttributeKey<?>, Object> attrs = attrs();
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();
if (handler() != null) {
p.addLast(handler());
}
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 {
ch.pipeline().addLast(new ServerBootstrapAcceptor(
currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
该方法主要做了两件事：
1、设置NioServerSocketChannel的options和attrs，并存储之后用于SocketChannel的options和attrs。
2、为NioServerSocketChannel对应的管道增加一个Inbound处理器ChannelInitializer。经过此步骤后，管道中的处理器链表为：head(outbound)->ChannelInitializer(inbound)->tail(inbound)。注意ChannelInitializer的实现方法initChannel，里面会当channelRegistered事件发生时将ServerBootstrapAcceptor加入到管道中。