Netty4学习笔记（3）-- ServerBootstrap

转自：http://blog.csdn.net/zxhoo/article/details/17532857

这篇文章接着上一篇，分析一下Netty4的ServerBootstrp是如何工作的。

EchoServer

先看看Netty自带的EchoServer例子：

 /**
  * Echoes back any received data from a client.
  */
 public class EchoServer {

     private final int port;

     public EchoServer(int port) {
         this.port = port;
     }

     public void run() throws Exception {
         // Configure the server.
         EventLoopGroup bossGroup = new NioEventLoopGroup();
         EventLoopGroup workerGroup = new NioEventLoopGroup();
         try {
             ServerBootstrap b = new ServerBootstrap();
             b.group(bossGroup, workerGroup)
              .channel(NioServerSocketChannel.class)
              .option(ChannelOption.SO_BACKLOG, 100)
              .handler(new LoggingHandler(LogLevel.INFO))
              .childHandler(new ChannelInitializer<SocketChannel>() {
                  @Override
                  public void initChannel(SocketChannel ch) throws Exception {
                      ch.pipeline().addLast(
                              //new LoggingHandler(LogLevel.INFO),
                              new EchoServerHandler());
                  }
              });

             // Start the server.
             ChannelFuture f = b.bind(port).sync();

             // Wait until the server socket is closed.
             f.channel().closeFuture().sync();
         } finally {
             // Shut down all event loops to terminate all threads.
             bossGroup.shutdownGracefully();
             workerGroup.shutdownGracefully();
         }
     }

     public static void main(String[] args) throws Exception {
         int port;
         if (args.length > 0) {
             port = Integer.parseInt(args[0]);
         } else {
             port = 8080;
         }
         new EchoServer(port).run();
     }
 }

可以看出，用法和Bootstrap差不多。

作为Builder的ServerBootstrap

 public final class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {

     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;
     // ...
 }

看代码可以知道，ServerBootstrap比它的超类多了四个Part，如下图所示：

从bind()方法入手

bind()方法实际上是在AbstractBootstrap里定义的，bind()先调用validate()方法检查各个Part是否准备就绪，然后把工作交给了doBind()方法。doBind()方法首先调用initAndRegister()方法，然后把工作交给doBind0()。initAndRegister()会调用模板方法init()来初始化Channel，initAndRegister()方法的细节上篇文章分析过了，这里不再复述。下面是整个方法调用过程的示意图：

init()方法

 @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));
         }
     });
 }

ServerBootstrap类的init()方法虽然很长，但是却不难理解。首先是给Channel设置options和attrs，然后把User提供的针对NioServerSocketChannel的Handler添加到Pipeline的末尾。接下来复制childOptions和childAttrs，最后实例化一个ChannelInitializer，添加到Pipeline的末尾。init()方法执行完毕之后，AbstractBootstrap的initAndRegister()方法会将NioServerSocketChannel注册到group里。到此为止，NioServerSocketChannel的状态如下图所示：

ChannelInitializer在channelRegistered事件触发后会调用initChannel()方法，随后把自己从Pipeline里删除：

 @Sharable
 public abstract class ChannelInitializer<C extends Channel> extends ChannelInboundHandlerAdapter {

     private static final InternalLogger logger = InternalLoggerFactory.getInstance(ChannelInitializer.class);

     /**
      * This method will be called once the {@link Channel} was registered. After the method returns this instance
      * will be removed from the {@link ChannelPipeline} of the {@link Channel}.
      *
      * @param ch            the {@link Channel} which was registered.
      * @throws Exception    is thrown if an error occurs. In that case the {@link Channel} will be closed.
      */
     protected abstract void initChannel(C ch) throws Exception;

     @SuppressWarnings("unchecked")
     @Override
     public final void channelRegistered(ChannelHandlerContext ctx)
             throws Exception {
         boolean removed = false;
         boolean success = false;
         try {
             initChannel((C) ctx.channel());
             ctx.pipeline().remove(this);
             removed = true;
             ctx.fireChannelRegistered();
             success = true;
         } catch (Throwable t) {
             logger.warn("Failed to initialize a channel. Closing: " + ctx.channel(), t);
         } finally {
             if (!removed) {
                 ctx.pipeline().remove(this);
             }
             if (!success) {
                 ctx.close();
             }
         }
     }
 }

所以注册之后的NioServerSocketChannel状态如下图所示：

ServerBootstrapAcceptor

 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);
         }
     }
     // ...
 }

ServerBootstrapAcceptor在channelRead事件触发的时候（也就有客户端连接的时候），把childHandler加到childChannel Pipeline的末尾，设置childHandler的options和attrs，最后把childHandler注册进childGroup，如下图所示：