Bootstrap的意思就是引导，辅助的意思，在编写服务端或客户端程序时，我们都需要先new一个bootstrap，然后基于这个bootstrap调用函数，添加eventloop和handler，可见对bootstrap进行分析还是有必要的。
1、bootstrap结构图
bootstrap的结构比较简单，涉及的类和接口很少，如下图所示，其中Bootstrap则是客户端程序用的引导类，ServerBootstrap是服务端程序用的引导类。
2、serverbootstrap分析
这部分，专门对serverbootstrap进行分析，bootstrap过程大同小异就不作详细的分析了。下面是我们编写服务端代码的一般化过程，整个分析过程将基于下面这段代码中用到的函数进行。
// Configure the bootstrap.
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.childHandler(new HexDumpProxyInitializer(remoteHost, remotePort))
.childOption(ChannelOption.AUTO_READ, false)
.bind(localPort).sync().channel().closeFuture().sync();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
先看关键代码（注意这里面的部分函数是在AbstractBootstrap中定义的）
private final Map<ChannelOption<?>, Object> childOptions = new LinkedHashMap<ChannelOption<?>, Object>();
private final Map<AttributeKey<?>, Object> childAttrs = new LinkedHashMap<AttributeKey<?>, Object>();
private volatile EventLoopGroup childGroup;
private volatile ChannelHandler childHandler;
/**
* Set the {@link EventLoopGroup} for the parent (acceptor) and the child (client). These
* {@link EventLoopGroup}'s are used to handle all the events and IO for {@link SocketChannel} and
* {@link Channel}'s.
*/
public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
super.group(parentGroup);
if (childGroup == null) {
throw new NullPointerException("childGroup");
}
if (this.childGroup != null) {
throw new IllegalStateException("childGroup set already");
}
this.childGroup = childGroup;
return this;
}
属性值ChildGroup，ChildHandler，是用来处理accpt的Channel的。group函数其实就是将parentGroup和ChildGroup进行赋值，其中parentGroup用于处理accept事件，ChildGroup用于处理accpt的Channel的IO事件。
//channel函数的实现定义在抽象父类中，其实就是通过newInstance函数生成一个具体的channel对象。
<pre name="code" class="java"> /**
* The {@link Class} which is used to create {@link Channel} instances from.
* You either use this or {@link #channelFactory(ChannelFactory)} if your
* {@link Channel} implementation has no no-args constructor.
*/
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new BootstrapChannelFactory<C>(channelClass));
}
/**
* {@link ChannelFactory} which is used to create {@link Channel} instances from
* when calling {@link #bind()}. This method is usually only used if {@link #channel(Class)}
* is not working for you because of some more complex needs. If your {@link Channel} implementation
* has a no-args constructor, its highly recommend to just use {@link #channel(Class)} for
* simplify your code.
*/
@SuppressWarnings("unchecked")
public B channelFactory(ChannelFactory<? extends C> channelFactory) {
if (channelFactory == null) {
throw new NullPointerException("channelFactory");
}
if (this.channelFactory != null) {
throw new IllegalStateException("channelFactory set already");
}
this.channelFactory = channelFactory;
return (B) this;
}<pre name="code" class="java"> private static final class BootstrapChannelFactory<T extends Channel> implements ChannelFactory<T> {
private final Class<? extends T> clazz;
BootstrapChannelFactory(Class<? extends T> 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 clazz.getSimpleName() + ".class";
}
}
Channel函数比较简单，其实就是通过newInstance函数，生成一个具体的Channel对象，例如服务端的NioServerSocketChannel。
/**
* Set the {@link ChannelHandler} which is used to serve the request for the {@link Channel}'s.
*/
public ServerBootstrap childHandler(ChannelHandler childHandler) {
if (childHandler == null) {
throw new NullPointerException("childHandler");
}
this.childHandler = childHandler;
return this;
}
上面的函数即给serverbootstrap的childHandler赋值。
/**
* Allow to specify a {@link ChannelOption} which is used for the {@link Channel} instances once they get created
* (after the acceptor accepted the {@link Channel}). Use a value of {@code null} to remove a previous set
* {@link ChannelOption}.
*/
public <T> ServerBootstrap childOption(ChannelOption<T> childOption, T value) {
if (childOption == null) {
throw new NullPointerException("childOption");
}
if (value == null) {
synchronized (childOptions) {
childOptions.remove(childOption);
}
} else {
synchronized (childOptions) {
childOptions.put(childOption, value);
}
}
return this;
}
上面的函数是指定accpt的channel的属性，channel有很多属性，比如SO_TIMEOUT时间，Buf长度等等。
/**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind() {
validate();
SocketAddress localAddress = this.localAddress;
if (localAddress == null) {
throw new IllegalStateException("localAddress not set");
}
return doBind(localAddress);
}
/**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind(int inetPort) {
return bind(new InetSocketAddress(inetPort));
}
/**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind(String inetHost, int inetPort) {
return bind(new InetSocketAddress(inetHost, inetPort));
}
<pre name="code" class="java"> /**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind(SocketAddress localAddress) {
validate();
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
return doBind(localAddress);
}
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;
}<pre name="code" class="java"> 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());
}
}
});
}
Bind函数层层调用过来之后，最后就调用Channel的bind函数了，下面再看channel的bind函数是如何处理的。定义在AbstractChannel中：
@Override
public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
return pipeline.bind(localAddress, promise);
}
channel的bind函数，最终就是调用pipeline的bind，而pipeline的bind实际上就是调用contexthandler的bind，之个之前分析write和flush的时候说过了。所以这里直接看contexthandler的bind函数。下面是定义：
@Override
public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
validatePromise(promise, false);
final DefaultChannelHandlerContext next = findContextOutbound();
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
next.invokeBind(localAddress, promise);
} else {
executor.execute(new Runnable() {
@Override
public void run() {
next.invokeBind(localAddress, promise);
}
});
}
return promise;
}<pre name="code" class="java"> private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
try {
((ChannelOutboundHandler) handler).bind(this, localAddress, promise);
} catch (Throwable t) {
notifyOutboundHandlerException(t, promise);
}
}
最终调用Handler的bind函数，还记得之前说的outbound类型的事件吗，这类事件提供了默认的实现方法，HeadHandler的bind函数，下面是它的定义：
@Override
public void bind(
ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
throws Exception {
unsafe.bind(localAddress, promise);
}
我们又看到了unsafe这个苦力了，最终的操作还是得由它来完成啊，赶紧去看看这个bind函数吧，
@Override
public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
if (!ensureOpen(promise)) {
return;
}
// See: https://github.com/netty/netty/issues/576
if (!PlatformDependent.isWindows() && !PlatformDependent.isRoot() &&
Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
localAddress instanceof InetSocketAddress &&
!((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress()) {
// 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.");
}
boolean wasActive = isActive();
try {
doBind(localAddress);
} catch (Throwable t) {
closeIfClosed();
promise.setFailure(t);
return;
}
if (!wasActive && isActive()) {
invokeLater(new Runnable() {
@Override
public void run() {
pipeline.fireChannelActive();
}
});
}
promise.setSuccess();
}
上面的代码最终调用了Channel的doBind函数，这里我们的Channel是NioServerSocketChannel，所以最终就是调用它的bind函数了，代码如下
@Override
protected void doBind(SocketAddress localAddress) throws Exception {
javaChannel().socket().bind(localAddress, config.getBacklog());
}
其实它最终也是调用了JDK的Channel的socket bind函数。
看到这里，你是否会觉得有点怪异，为什么没有注册accpt事件啊，一般的我们的server socket都是要注册accpt事件到selector，用于监听连接。如果你发现了这个问题，说明你是理解socket的编程的，^_^。实际上是前面在分析bind的时候我们漏掉了一个重要的函数，initAndRegister，下面再来看看它的定义：
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();
}
}
// 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 beause 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 regPromise;
}
在这里，我们看到了我们之前介绍event时说的register函数，它就是用于将Channel注册到eventloop中去的。eventloop经过层层调用，最终调用了SingleThreadEventLoop类中的register函数，下面是它的定义：
@Override
public ChannelFuture register(final Channel channel, final ChannelPromise promise) {
if (channel == null) {
throw new NullPointerException("channel");
}
if (promise == null) {
throw new NullPointerException("promise");
}
channel.unsafe().register(this, promise);
return promise;
}
还是逃离不了unsafe对象的调用，前面说了那么多的unsafe，这个函数猜都可以猜测出执行过程了，这里就不细细的列举代码了。
还有一个init函数，这里需要说明一下，代码如下：
@Override
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));
}
});
}
它就是用来处理channel 的pipeline，并添加一个ServerBootstrapAcceptor的handler，继续看看这个handler的定义，我们就会明白它的意图。
private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
private final EventLoopGroup childGroup;
private final ChannelHandler childHandler;
private final Entry<ChannelOption<?>, Object>[] childOptions;
private final Entry<AttributeKey<?>, Object>[] childAttrs;
@SuppressWarnings("unchecked")
ServerBootstrapAcceptor(
EventLoopGroup childGroup, ChannelHandler childHandler,
Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
this.childGroup = childGroup;
this.childHandler = childHandler;
this.childOptions = childOptions;
this.childAttrs = childAttrs;
}
@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
Channel child = (Channel) msg;
child.pipeline().addLast(childHandler);
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);
}
}
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
try {
childGroup.register(child);
} catch (Throwable t) {
child.unsafe().closeForcibly();
logger.warn("Failed to register an accepted channel: " + child, t);
}
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
final ChannelConfig config = ctx.channel().config();
if (config.isAutoRead()) {
// stop accept new connections for 1 second to allow the channel to recover
// See https://github.com/netty/netty/issues/1328
config.setAutoRead(false);
ctx.channel().eventLoop().schedule(new Runnable() {
@Override
public void run() {
config.setAutoRead(true);
}
}, 1, TimeUnit.SECONDS);
}
// still let the exceptionCaught event flow through the pipeline to give the user
// a chance to do something with it
ctx.fireExceptionCaught(cause);
}
}
上面就是这个handler的全部代码，它重写了ChannelRead函数，它的目的其实是想将server端accept的channel注册到ChildGroup的eventloop中，这样就可以理解，服务端代码workerGroup这个eventloop的作用了，它终于在这里体现出了它的作用了。
3、总结
这篇文章主要是分析了serverbootstrap的全过程，通过对这个的分析，我们清晰的看到了平时编写socket服务端代码时对bind，register事件，以及accept channel等的处理。