ServerBootstrap与Bootstrap类似，只不过这个是用于服务端的启动。下面看下具体的使用：
public class TimeServer {
public void bind(int port) throws Exception {
// 配置服务端的NIO线程组
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 1024)
.childHandler(new ChildChannelHandler());
// 绑定端口，同步等待成功
ChannelFuture f = b.bind(port).sync();
// 等待服务端监听端口关闭
f.channel().closeFuture().sync();
} finally {
// 优雅退出，释放线程池资源
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
private class ChildChannelHandler extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel arg0) throws Exception {
arg0.pipeline().addLast(new TimeServerHandler());
}
}
/**
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
int port = 8080;
if (args != null && args.length > 0) {
try {
port = Integer.valueOf(args[0]);
} catch (NumberFormatException e) {
// 采用默认值
}
}
new TimeServer().bind(port);
}
}
与上篇文章介绍的Bootstrap不同，这里传入了两个EventLoopGroup，其中bossGroup用于接收请求，workerGroup用于处理IO事件。Netty是Reactor模式的实现，有关Reactor模式请参考NIO技术概览。
这里传入的Channel类型是NioServerSocketChannel，与Bootstrap一样，在bind方法中会调用init方法，下面看下ServerBootstrap中init方法的实现：
@Override
void init(Channel channel) throws Exception {
final Map<ChannelOption<?>, Object> options = options0();
synchronized (options) {
setChannelOptions(channel, options, logger);
}
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()));
}
// 向pipeLine中添加Handler
p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(final Channel ch) throws Exception {
final ChannelPipeline pipeline = ch.pipeline();
ChannelHandler handler = config.handler();
if (handler != null) {
pipeline.addLast(handler);
}
// 使用eventLoop来执行任务
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
}
注意一下这段代码：
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
这段代码向pipeLine中添加了一个ServerBootstrapAcceptor对象，ServerBootstrapAcceptor对象也是一个Handler，看下ServerBootstrapAcceptor的定义：
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;
private final Runnable enableAutoReadTask;
ServerBootstrapAcceptor(
final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
this.childGroup = childGroup;
this.childHandler = childHandler;
this.childOptions = childOptions;
this.childAttrs = childAttrs;
// Task which is scheduled to re-enable auto-read.
// It's important to create this Runnable before we try to submit it as otherwise the URLClassLoader may
// not be able to load the class because of the file limit it already reached.
//
// See https://github.com/netty/netty/issues/1328
enableAutoReadTask = new Runnable() {
@Override
public void run() {
channel.config().setAutoRead(true);
}
};
}
@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel) msg;
child.pipeline().addLast(childHandler);
setChannelOptions(child, childOptions, logger);
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
try {
childGroup.register(child).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (!future.isSuccess()) {
forceClose(child, future.cause());
}
}
});
} catch (Throwable t) {
forceClose(child, t);
}
}
private static void forceClose(Channel child, 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(enableAutoReadTask, 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);
}
}
ServerBootstrapAcceptor继承自ChannelInboundHandlerAdapter，用于处理Inbound事件，该类的主要功能就是在服务器端接收到请求之后，会返回一个NioSocketChannel对象作为参数msg传入channelRead方法中，用变量child来表示；然后把在TimeServer中传入的childHandler添加到child所对应的pipeLine中，然后把child注册到childGroup中，也就是TimeServer中定义的workerGroup。
接收请求的操作是在NioServerSocketChannel中的doReadMessages方法中实现的：
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = SocketUtils.accept(javaChannel());
try {
if (ch != null) {
buf.add(new NioSocketChannel(this, ch));
return 1;
}
} catch (Throwable t) {
logger.warn("Failed to create a new channel from an accepted socket.", t);
try {
ch.close();
} catch (Throwable t2) {
logger.warn("Failed to close a socket.", t2);
}
}
return 0;
}
doReadMessages方法在NioMessageUnsafe中的read方法中调用，然后会执行pipeline.fireChannelRead(readBuf.get(i));将SocketChannel对象传入，也就是channelRead方法中的msg参数。至此，一个请求已经接收完毕，通道之间的通信就会交给childGroup，也就是TimeServer中的workerGroup来处理。
