BootStrap是客户端的启动类，其主要功能就是设置必要的参数然后启动客户端。
实现如下：
Bootstrap b = new Bootstrap();
b.group(group)
.channel(NioSocketChannel.class)
.option(ChannelOption.TCP_NODELAY, true)
.handler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline p = ch.pipeline();
p.addLast("decoder", new StringDecoder());
p.addLast("encoder", new StringEncoder());
p.addLast(new ClientHandler());
}
});
ChannelFuture future = b.connect(HOST, PORT).sync();
首先是创建实例，然后设置group、channel、option和handler。
接下来就是连接服务器，主要的操作都在这部分进行操作。
public ChannelFuture connect(String inetHost, int inetPort) {
return connect(InetSocketAddress.createUnresolved(inetHost, inetPort));
}
public ChannelFuture connect(InetAddress inetHost, int inetPort) {
return connect(new InetSocketAddress(inetHost, inetPort));
}
public ChannelFuture connect(SocketAddress remoteAddress) {
if (remoteAddress == null) {
throw new NullPointerException("remoteAddress");
}
validate();
return doResolveAndConnect(remoteAddress, config.localAddress());
}
接下来的操作是在doResolveAndConnect中，主要有两个重要的操作：一是调用父类的initAndregister函数将NioServerSocketChannel注册到Selector中，具体实现是在AbstractNioChannel的doRegister函数中，二是调用自身函数doConnect连接到Server，并在线程池中执行。具体操作如下：
private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();
final Channel channel = regFuture.channel();
if (regFuture.isDone()) {
if (!regFuture.isSuccess()) {
return regFuture;
}
return doResolveAndConnect0(channel, remoteAddress, localAddress, channel.newPromise());
} 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 {
// Direclty obtain the cause and do a null check so we only need one volatile read in case of a
// failure.
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();
doResolveAndConnect0(channel, remoteAddress, localAddress, promise);
}
}
});
return promise;
}
}
首先是执行final ChannelFuture regFuture = initAndRegister();调用父类的initAndRegister()函数，实现将Channel注册到Selector中，实现如下，调用config().group().register(channel)，实现将Channel注册到Selector中。
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
channel = channelFactory.newChannel();
init(channel);
} catch (Throwable t) {
if (channel != null) {
channel.unsafe().closeForcibly();
}
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();
}
}
return regFuture;
}
接下的操作是将NioSocketChannel连接到Server端，实现的函数是doResolveAndConnect0，看如下代码，连接的具体实现在doContect中。
private ChannelFuture doResolveAndConnect0(final Channel channel, SocketAddress remoteAddress,
final SocketAddress localAddress, final ChannelPromise promise) {
try {
final EventLoop eventLoop = channel.eventLoop();
final AddressResolver<SocketAddress> resolver = this.resolver.getResolver(eventLoop);
if (!resolver.isSupported(remoteAddress) || resolver.isResolved(remoteAddress)) {
// Resolver has no idea about what to do with the specified remote address or it's resolved already.
doConnect(remoteAddress, localAddress, promise);
return promise;
}
final Future<SocketAddress> resolveFuture = resolver.resolve(remoteAddress);
if (resolveFuture.isDone()) {
final Throwable resolveFailureCause = resolveFuture.cause();
if (resolveFailureCause != null) {
// Failed to resolve immediately
channel.close();
promise.setFailure(resolveFailureCause);
} else {
// Succeeded to resolve immediately; cached? (or did a blocking lookup)
doConnect(resolveFuture.getNow(), localAddress, promise);
}
return promise;
}
// Wait until the name resolution is finished.
resolveFuture.addListener(new FutureListener<SocketAddress>() {
@Override
public void operationComplete(Future<SocketAddress> future) throws Exception {
if (future.cause() != null) {
channel.close();
promise.setFailure(future.cause());
} else {
doConnect(future.getNow(), localAddress, promise);
}
}
});
} catch (Throwable cause) {
promise.tryFailure(cause);
}
return promise;
}
客户端连接到服务端的操作在doConnect中，发现在EventLoop线程池中执行线程，Channel去连接服务器。
private static void doConnect(
final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise connectPromise) {
// This method is invoked before channelRegistered() is triggered. Give user handlers a chance to set up
// the pipeline in its channelRegistered() implementation.
final Channel channel = connectPromise.channel();
channel.eventLoop().execute(new Runnable() {
@Override
public void run() {
if (localAddress == null) {
channel.connect(remoteAddress, connectPromise);
} else {
channel.connect(remoteAddress, localAddress, connectPromise);
}
connectPromise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
}
});
}
BootStrap总结起来有两个主要的功能，
（1）将调用父类AbstractBootStrap中的initAndRegister函数将NioSocketChannel注册到Selector，具体实现是在AbstractNioChannel的doRegister函数中。
（2）调用自身函数doConnect连接到Server，并在线程池中执行。