Netty4学习笔记（2）-- Bootstrap

Netty4的代码比我想象的要复杂的多，不过Netty4很好的将这种复杂性隐藏了起来，暴露出来的，是一个相对容易使用的接口。Bootstrap就是Netty试图隐藏这种复杂性的一个例子。

bootstrap包

bootstrap包是Netty4代码里最简单的一个包，总共只有4个类：

Bootstrap继承结构

AbstractBootstrap是抽象类，有两个具体的实现，Bootstrap和ServerBootstrap：

Bootstrap例子

Netty4自带的例子里有一个EchoClient，看看它是如何使用Bootstrap启动一个客户端程序的：

public class EchoClient {

    private final String host;
    private final int port;
    private final int firstMessageSize;

    public EchoClient(String host, int port, int firstMessageSize) {
        this.host = host;
        this.port = port;
        this.firstMessageSize = firstMessageSize;
    }

    public void run() throws Exception {
        // Configure the client.
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group)
             .channel(NioSocketChannel.class)
             .option(ChannelOption.TCP_NODELAY, true)
             .handler(new ChannelInitializer() {
                 @Override
                 public void initChannel(SocketChannel ch) throws Exception {
                     ch.pipeline().addLast(
                             //new LoggingHandler(LogLevel.INFO),
                             new EchoClientHandler(firstMessageSize));
                 }
             });

            // Start the client.
            ChannelFuture f = b.connect(host, port).sync();

            // Wait until the connection is closed.
            f.channel().closeFuture().sync();
        } finally {
            // Shut down the event loop to terminate all threads.
            group.shutdownGracefully();
        }
    }

    public static void main(String[] args) throws Exception {
    // ...
    }
}

Builder模式？

看上面的例子，Bootstrap的使用很像Builder模式，Bootstrap就是Builder，EventLoopGroup、Channel和Handler等是各种Part。稍有不同的是，准备好各种Part后，并不是直接build出一个Product来，而是直接通过connect()方法使用这个Product。

AbstractBootstrap

为了弄清楚Bootstrap如何工作，我们先从AbstractBootstrap入手：

public abstract class AbstractBootstrap, C extends Channel> implements Cloneable {

    private volatile EventLoopGroup group;
    private volatile ChannelFactory channelFactory;
    private volatile SocketAddress localAddress;
    private final Map, Object> options = new LinkedHashMap, Object>();
    private final Map, Object> attrs = new LinkedHashMap, Object>();
    private volatile ChannelHandler handler;
    // ...
}

可以看到，AbstractBootstrap这个抽象Builder一共需要有6个Part，如下图所示：

设置各个Part

AbstractBootstrap有一组方法用来设置各个Part，例如下面这些：

public B group(EventLoopGroup group)
public B channel(Class channelClass)
public B channelFactory(ChannelFactory channelFactory)
public B localAddress(SocketAddress localAddress)
public  B option(ChannelOption option, T value)
public  B attr(AttributeKey key, T value)
public B handler(ChannelHandler handler)

还有一组对应方法获得各个Part，如下：

final SocketAddress localAddress()
final ChannelFactory channelFactory()
final ChannelHandler handler()
public final EventLoopGroup group()
final Map, Object> options()
final Map, Object> attrs()

ChannelFactory

AbstractBootstrap通过ChannelFactory创建Channel实例，channel(channelClass)方法看起来好像是设置了一个Channel，但实际上只是设置了默认的ChannelFactory实现：

    public B channel(Class channelClass) {
        if (channelClass == null) {
            throw new NullPointerException("channelClass");
        }
        return channelFactory(new BootstrapChannelFactory(channelClass));
    }

默认的ChannelFactory实现使用反射创建Channel实例：

    private static final class BootstrapChannelFactory implements ChannelFactory {
        private final Class clazz;

        BootstrapChannelFactory(Class 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);
            }
        }
    }

Bootstrap.connect()

再来看Bootstrap类的connect()方法：

    public ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress) {
        if (remoteAddress == null) {
            throw new NullPointerException("remoteAddress");
        }
        validate();
        return doConnect(remoteAddress, localAddress);
    }

connect()方法调用validate()方法看各个Part是否准备就绪，然后调用doConnect()方法：

    private ChannelFuture doConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {
            return regFuture;
        }

        final ChannelPromise promise = channel.newPromise();
        if (regFuture.isDone()) {
            doConnect0(regFuture, channel, remoteAddress, localAddress, promise);
        } else {
            regFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    doConnect0(regFuture, channel, remoteAddress, localAddress, promise);
                }
            });
        }

        return promise;
    }

doConnect()方法首先调用了initAndRegister()方法，然后又调用了doConnect0()方法，方法调用示意图如下：

AbstractBootstrap.initAndRegister()

    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);
        // ...
    }

initAndRegister()方法用ChannelFactory创建了一个Channel的实例，然后调用init()方法初始化Channel，最后将Channel注册到EventLoopGroup上：

而Channel在实例化的时候已经自动关联了Pipeline，这点从AbstractChannel的构造函数可以看出：

    protected AbstractChannel(Channel parent) {
        this.parent = parent;
        unsafe = newUnsafe();
        pipeline = new DefaultChannelPipeline(this);
    }

 

Bootstrap.init()方法

    void init(Channel channel) throws Exception {
        ChannelPipeline p = channel.pipeline();
        p.addLast(handler());
        // ...
    }

Bootstrap.init()方法把Handler添加到了Pipeline的末尾，到这里，Channel就准备就绪了：

继续Bootstrap.doConnect()

initAndRegister()方法结束之后，doConnect()方法紧接着调用了doConnect0()方法，doConnect0()方法继而调用了Channel.connect()方法，这样Channel就接通服务器，可以收发消息了！