netty Bootstrap解析

netty 抽象BootStrap定义：http://donald-draper.iteye.com/blog/2392492
netty ServerBootStrap解析：http://donald-draper.iteye.com/blog/2392572
引言：
上面一篇文章我们看了ServerBootstrap，先来回顾一下：
      服务端Bootstrap虽然继承与抽象Bootstrap，但他有自己的child通道选项及属性集，事件循环组和通道处理器，这些是用于配置，当Server通道接收客户端的请求，创建与客户端交互的通道。当构造Server引导配置时，如果传递一个事件循环，则Server通道监听器和客户端交互的通道公用一个事件循环组，否则parentGroup事件循环组用于监听器ServerChannel接受连接,childGroup事件循环组用于处理与客户端交互的通道相关事件和IO操作。
      Server引导配置绑定socket地址，首先初始化通道，对于Server引导配置，这个通道为NioServerSocketChannel，初始化通道，即初始化Server通道；初始化Server通道，首先将Server引导配置的父类抽象Bootstrap的选项和属性配置给Server通道，然后添加ServerBootstrapAcceptor到Server通道内部的Channel管道内，然后将Server通道注册到事件循环组parentGroup中，然后通过Server通道#bind方法完成实际socket地址；Server引导配置监听器实际为一个Inbound通道处理器，每当有客户端连接请求时，则创建一个与客户端交互的通道，将child通道选项及属性配置给通道，并将通道注册到childGroup事件循环组，然后将通道处理器添加到与客户端交互的通道内部的Channel管道中。
今天我们来看客户端Bootstrap：

package io.netty.bootstrap;

import io.netty.channel.Channel;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelOption;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.ChannelPromise;
import io.netty.channel.EventLoop;
import io.netty.channel.EventLoopGroup;
import io.netty.resolver.AddressResolver;
import io.netty.resolver.DefaultAddressResolverGroup;
import io.netty.resolver.NameResolver;
import io.netty.resolver.AddressResolverGroup;
import io.netty.util.AttributeKey;
import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.FutureListener;
import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.util.Map;
import java.util.Map.Entry;

/**
 * A {@link Bootstrap} that makes it easy to bootstrap a {@link Channel} to use
 * for clients.
 引导程序Bootstrap使客户端很容易启动一个通道。
 *
 * The {@link #bind()} methods are useful in combination with connectionless transports such as datagram (UDP).
 * For regular TCP connections, please use the provided {@link #connect()} methods.

 绑定方法#bind对无连接的报文通信UDP非常有用。对于Socket连接TCP，可以使用#connect连接方法。
 */
public class Bootstrap extends AbstractBootstrap<Bootstrap, Channel> {

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

    private static final AddressResolverGroup<?> DEFAULT_RESOLVER = DefaultAddressResolverGroup.INSTANCE;
   //引导配置
    private final BootstrapConfig config = new BootstrapConfig(this);

    @SuppressWarnings("unchecked")
    private volatile AddressResolverGroup<SocketAddress> resolver =
            (AddressResolverGroup<SocketAddress>) DEFAULT_RESOLVER;
    private volatile SocketAddress remoteAddress;//远端socket地址
     public Bootstrap() { }

    private Bootstrap(Bootstrap bootstrap) {
        super(bootstrap);
        resolver = bootstrap.resolver;
        remoteAddress = bootstrap.remoteAddress;
    }
}

由于客户端引导配置Bootstrap，不像Server引导配置需要创建交互通道，所以可以看到客户端引导配置没有在配置
子事件循环。在客户端实例中有这么几句：

EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
	//Bootstrap，用于配置客户端，一般为Socket通道
	Bootstrap bootstrap = new Bootstrap();
	bootstrap.group(workerGroup)
	 .channel(NioSocketChannel.class)
	 .handler(new ChannelInitializer<SocketChannel>() {
	     @Override
	     protected void initChannel(SocketChannel ch) throws Exception {
		 //添加安全套接字处理器和通道处理器到
		 ChannelPipeline pipeline = ch.pipeline();
		 if (sslCtx != null) {
			 pipeline.addLast(sslCtx.newHandler(ch.alloc(), ip, port));
		 }
		 pipeline.addLast(new LoggingHandler(LogLevel.INFO));
		 pipeline.addLast(new EchoClientHandler());
	     }
	 });
	 ...
	//连接socket地址
	ChannelFuture f = bootstrap.connect(inetSocketAddress).sync();
...
}

我们需要关注的是这一句：

bootstrap.connect(inetSocketAddress).sync();

引导配置连接远端socket地址：

/**
 * Connect a {@link Channel} to the remote peer.
 */
public ChannelFuture connect(String inetHost, int inetPort) {
    return connect(InetSocketAddress.createUnresolved(inetHost, inetPort));
}

/**
 * Connect a {@link Channel} to the remote peer.
 */
public ChannelFuture connect(InetAddress inetHost, int inetPort) {
    return connect(new InetSocketAddress(inetHost, inetPort));
}

/**
 * Connect a {@link Channel} to the remote peer.
 */
public ChannelFuture connect(SocketAddress remoteAddress) {
    if (remoteAddress == null) {
        throw new NullPointerException("remoteAddress");
    }

    validate();
    return doResolveAndConnect(remoteAddress, config.localAddress());
}


/**
 * Connect a {@link Channel} to the remote peer.
 */
public ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress) {
    if (remoteAddress == null) {
        throw new NullPointerException("remoteAddress");
    }
    validate();
    return doResolveAndConnect(remoteAddress, localAddress);
}

从上面来了，连接远端socket地址，实际委托给doResolveAndConnect方法：

/**
 * @see #connect()
 连接操作
 */
private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
    //初始化通道，并注册通道到事件循环组
    final ChannelFuture regFuture = initAndRegister();
    final Channel channel = regFuture.channel();

    if (regFuture.isDone()) {
        if (!regFuture.isSuccess()) {
            return regFuture;
        }
	//初始化成功，则委托给doResolveAndConnect0完成实际连接操作
        return doResolveAndConnect0(channel, remoteAddress, localAddress, channel.newPromise());
    } else {
       //如果初始化和注册工作没有完成，添加任务结果监听器，待完成时，更新注册状态，完成实际连接操作
        // 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 {
                // Directly 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;
    }
}

下面我们先看一下初始化通道方法，再来看实际连接完成方法doResolveAndConnect0。
//初始化通道

@Override
@SuppressWarnings("unchecked")
void init(Channel channel) throws Exception {
   //添加通道处理器到通道内存的Channel管道中
    ChannelPipeline p = channel.pipeline();
    p.addLast(config.handler());

   //配置通道选项和属性
    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()) {
            channel.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
        }
    }
}

//实际连接操作
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.
	    //如果地址解析器不知道如何应对远端地址，或已经解决则连接远端地址，完成实际连接
            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 {
	         //远端地址解析完毕，并支持，完成实际连接
                // Succeeded to resolve immediately; cached? (or did a blocking lookup)
                doConnect(resolveFuture.getNow(), localAddress, promise);
            }
            return promise;
        }
        //如果地址解析任务没有完成，添加监听器，待任务完成时，完成连接操作
        // 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;
}

//连接远端地址
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();
    //创建一个任务线程完成实际连接远端地址操作，实际委托给通道的连接方法，任务线程交由通道所在的事件循环去执行
    channel.eventLoop().execute(new Runnable() {
        @Override
        public void run() {
            if (localAddress == null) {
                channel.connect(remoteAddress, connectPromise);
            } else {
	       //实际委托给通道的连接方法
                channel.connect(remoteAddress, localAddress, connectPromise);
            }
            connectPromise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
        }
    });
}

我们来理一下，客户端引导配置的连接操作，首先初始化通道，主要是配置通道的，选型和属性，将通道处理器添加到通道内部的Channel管道中，注册通道到事件循环组，然后委托通道完成实际的连接操作。
再来看其他方法：

//配置远端socket地址
/**
 * The {@link SocketAddress} to connect to once the {@link #connect()} method
 * is called.
 */
public Bootstrap remoteAddress(SocketAddress remoteAddress) {
    this.remoteAddress = remoteAddress;
    return this;
}

/**
 * @see #remoteAddress(SocketAddress)
 */
public Bootstrap remoteAddress(String inetHost, int inetPort) {
    remoteAddress = InetSocketAddress.createUnresolved(inetHost, inetPort);
    return this;
}

/**
 * @see #remoteAddress(SocketAddress)
 */
public Bootstrap remoteAddress(InetAddress inetHost, int inetPort) {
    remoteAddress = new InetSocketAddress(inetHost, inetPort);
    return this;
}

/**
 * Connect a {@link Channel} to the remote peer.
 连接远端peer通道
 */
public ChannelFuture connect() {
    validate();
    SocketAddress remoteAddress = this.remoteAddress;
    if (remoteAddress == null) {
        throw new IllegalStateException("remoteAddress not set");
    }
    return doResolveAndConnect(remoteAddress, config.localAddress());
}

//验证引导配置是否有效
@Override
public Bootstrap validate() {
    super.validate();
    if (config.handler() == null) {
        throw new IllegalStateException("handler not set");
    }
    return this;
}
//克隆配置
@Override
@SuppressWarnings("CloneDoesntCallSuperClone")
public Bootstrap clone() {
    return new Bootstrap(this);
}

/**
 * Returns a deep clone of this bootstrap which has the identical configuration except that it uses
 * the given {@link EventLoopGroup}. This method is useful when making multiple {@link Channel}s with similar
 * settings.
 */
public Bootstrap clone(EventLoopGroup group) {
    Bootstrap bs = new Bootstrap(this);
    bs.group = group;
    return bs;
}
//获取引导配置
@Override
public final BootstrapConfig config() {
    return config;
}
//获取远端socket地址
final SocketAddress remoteAddress() {
    return remoteAddress;
}
//获取地址解析组
final AddressResolverGroup<?> resolver() {
    return resolver;
}

总结：
客户端引导配置的连接操作，首先初始化通道，主要是配置通道的，选型和属性，将通道处理器添加到通道内部的Channel管道中，注册通道到事件循环组，然后委托通道完成实际的连接操作。

附：
在客户端引导配置有一个地址解析组配置

private static final AddressResolverGroup<?> DEFAULT_RESOLVER = DefaultAddressResolverGroup.INSTANCE;

我们来简单看一下：
//DefaultAddressResolverGroup,默认地址解析组

package io.netty.resolver;

import io.netty.util.concurrent.EventExecutor;
import io.netty.util.internal.UnstableApi;

import java.net.InetSocketAddress;

/**
 * A {@link AddressResolverGroup} of {@link DefaultNameResolver}s.
 */
@UnstableApi
public final class DefaultAddressResolverGroup extends AddressResolverGroup<InetSocketAddress> {

    public static final DefaultAddressResolverGroup INSTANCE = new DefaultAddressResolverGroup();
    private DefaultAddressResolverGroup() { }
    //创建地址解析器
    @Override
    protected AddressResolver<InetSocketAddress> newResolver(EventExecutor executor) throws Exception {
        return new DefaultNameResolver(executor).asAddressResolver();
    }
}

//DefaultNameResolver，默认命名解析器

package io.netty.resolver;

import io.netty.util.internal.SocketUtils;
import io.netty.util.concurrent.EventExecutor;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.UnstableApi;

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
import java.util.List;

/**
 * A {@link InetNameResolver} that resolves using JDK's built-in domain name lookup mechanism.
 * Note that this resolver performs a blocking name lookup from the caller thread.
 */
@UnstableApi
public class DefaultNameResolver extends InetNameResolver {

    public DefaultNameResolver(EventExecutor executor) {
        super(executor);
    }

    @Override
    protected void doResolve(String inetHost, Promise<InetAddress> promise) throws Exception {
        try {
            promise.setSuccess(SocketUtils.addressByName(inetHost));
        } catch (UnknownHostException e) {
            promise.setFailure(e);
        }
    }

    @Override
    protected void doResolveAll(String inetHost, Promise<List<InetAddress>> promise) throws Exception {
        try {
            promise.setSuccess(Arrays.asList(SocketUtils.allAddressesByName(inetHost)));
        } catch (UnknownHostException e) {
            promise.setFailure(e);
        }
    }
}

//命名解析器InetNameResolver

package io.netty.resolver;

import io.netty.util.concurrent.EventExecutor;
import io.netty.util.concurrent.Future;
import io.netty.util.internal.UnstableApi;

import java.net.InetAddress;
import java.net.InetSocketAddress;

/**
 * A skeletal {@link NameResolver} implementation that resolves {@link InetAddress}.
 */
@UnstableApi
public abstract class InetNameResolver extends SimpleNameResolver<InetAddress> {
    private volatile AddressResolver<InetSocketAddress> addressResolver;//地址解析器

    /**
     * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
     *                 by {@link #resolve(String)}
     */
    protected InetNameResolver(EventExecutor executor) {
        super(executor);
    }

    /**
     * Return a {@link AddressResolver} that will use this name resolver underneath.
     * It's cached internally, so the same instance is always returned.
     这个方法时我们要找的
     */
    public AddressResolver<InetSocketAddress> asAddressResolver() {
        AddressResolver<InetSocketAddress> result = addressResolver;
        if (result == null) {
            synchronized (this) {
                result = addressResolver;
                if (result == null) {
		    //如果内部地址解析器为空，则创建一个socket地址解析器
                    addressResolver = result = new InetSocketAddressResolver(executor(), this);
                }
            }
        }
        return result;
    }
}

先把InetNameResolver的父类SimpleNameResolver看一下，在看socket地址解析器InetSocketAddressResolver

//SimpleNameResolver
package io.netty.resolver;

import io.netty.util.concurrent.EventExecutor;
import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.UnstableApi;

import java.util.List;

import static io.netty.util.internal.ObjectUtil.*;

/**
 * A skeletal {@link NameResolver} implementation.
 */
@UnstableApi
public abstract class SimpleNameResolver<T> implements NameResolver<T> {

    private final EventExecutor executor;

    /**
     * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
     *                 by {@link #resolve(String)}
     */
    protected SimpleNameResolver(EventExecutor executor) {
        this.executor = checkNotNull(executor, "executor");
    }

    /**
     * Returns the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
     * by {@link #resolve(String)}.
     */
    protected EventExecutor executor() {
        return executor;
    }

    @Override
    public final Future<T> resolve(String inetHost) {
        final Promise<T> promise = executor().newPromise();
        return resolve(inetHost, promise);
    }

    @Override
    public Future<T> resolve(String inetHost, Promise<T> promise) {
        checkNotNull(promise, "promise");

        try {
            doResolve(inetHost, promise);
            return promise;
        } catch (Exception e) {
            return promise.setFailure(e);
        }
    }

    @Override
    public final Future<List<T>> resolveAll(String inetHost) {
        final Promise<List<T>> promise = executor().newPromise();
        return resolveAll(inetHost, promise);
    }

    @Override
    public Future<List<T>> resolveAll(String inetHost, Promise<List<T>> promise) {
        checkNotNull(promise, "promise");

        try {
            doResolveAll(inetHost, promise);
            return promise;
        } catch (Exception e) {
            return promise.setFailure(e);
        }
    }

    /**
     * Invoked by {@link #resolve(String)} to perform the actual name resolution.
     */
    protected abstract void doResolve(String inetHost, Promise<T> promise) throws Exception;

    /**
     * Invoked by {@link #resolveAll(String)} to perform the actual name resolution.
     */
    protected abstract void doResolveAll(String inetHost, Promise<List<T>> promise) throws Exception;

    @Override
    public void close() { }
}

//NameResolver

package io.netty.resolver;

import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.UnstableApi;

import java.io.Closeable;
import java.util.List;

/**
 * Resolves an arbitrary string that represents the name of an endpoint into an address.
 */
@UnstableApi
public interface NameResolver<T> extends Closeable {

    /**
     * Resolves the specified name into an address.
     *
     * @param inetHost the name to resolve
     *
     * @return the address as the result of the resolution
     */
    Future<T> resolve(String inetHost);

    /**
     * Resolves the specified name into an address.
     *
     * @param inetHost the name to resolve
     * @param promise the {@link Promise} which will be fulfilled when the name resolution is finished
     *
     * @return the address as the result of the resolution
     */
    Future<T> resolve(String inetHost, Promise<T> promise);

    /**
     * Resolves the specified host name and port into a list of address.
     *
     * @param inetHost the name to resolve
     *
     * @return the list of the address as the result of the resolution
     */
    Future<List<T>> resolveAll(String inetHost);

    /**
     * Resolves the specified host name and port into a list of address.
     *
     * @param inetHost the name to resolve
     * @param promise the {@link Promise} which will be fulfilled when the name resolution is finished
     *
     * @return the list of the address as the result of the resolution
     */
    Future<List<T>> resolveAll(String inetHost, Promise<List<T>> promise);

    /**
     * Closes all the resources allocated and used by this resolver.
     */
    @Override
    void close();
}

回到InetNameResolver的asAddressResolver方法的这一句

//如果内部地址解析器为空，则创建一个socket地址解析器
addressResolver = result = new InetSocketAddressResolver(executor(), this);

//socket地址解析器,InetSocketAddressResolver

package io.netty.resolver;

import io.netty.util.concurrent.EventExecutor;
import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.FutureListener;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.UnstableApi;

import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.util.ArrayList;
import java.util.List;

/**
 * A {@link AbstractAddressResolver} that resolves {@link InetSocketAddress}.
 */
@UnstableApi
public class InetSocketAddressResolver extends AbstractAddressResolver<InetSocketAddress> {

    final NameResolver<InetAddress> nameResolver;//命名解决器，默认为DefaultNameResolver

    /**
     * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
     *                 by {@link #resolve(java.net.SocketAddress)}
     * @param nameResolver the {@link NameResolver} used for name resolution
     */
    public InetSocketAddressResolver(EventExecutor executor, NameResolver<InetAddress> nameResolver) {
        super(executor, InetSocketAddress.class);
        this.nameResolver = nameResolver;
    }
    @Override
    public void close() {
	  nameResolver.close();
    }
}

来看地址解析方法，

//地址是否可以解析
@Override
protected boolean doIsResolved(InetSocketAddress address) {
    return !address.isUnresolved();
}

//InetSocketAddress

/**
  * Checks whether the address has been resolved or not.
  *
  * @return <code>true</code> if the hostname couldn't be resolved into
  *          an <code>InetAddress</code>.
  */
 public final boolean isUnresolved() {
     return holder.isUnresolved();
 }

再看另外两个地址解决方法，

@Override
protected void doResolve(final InetSocketAddress unresolvedAddress, final Promise<InetSocketAddress> promise)
        throws Exception {
    // Note that InetSocketAddress.getHostName() will never incur a reverse lookup here,
    // because an unresolved address always has a host name.
    //委托给命令解决器
    nameResolver.resolve(unresolvedAddress.getHostName())
            .addListener(new FutureListener<InetAddress>() {
                @Override
                public void operationComplete(Future<InetAddress> future) throws Exception {
                    if (future.isSuccess()) {
                        promise.setSuccess(new InetSocketAddress(future.getNow(), unresolvedAddress.getPort()));
                    } else {
                        promise.setFailure(future.cause());
                    }
                }
            });
}

@Override
protected void doResolveAll(final InetSocketAddress unresolvedAddress,
                            final Promise<List<InetSocketAddress>> promise) throws Exception {
    // Note that InetSocketAddress.getHostName() will never incur a reverse lookup here,
    // because an unresolved address always has a host name.
     //委托给命令解决器
    nameResolver.resolveAll(unresolvedAddress.getHostName())
            .addListener(new FutureListener<List<InetAddress>>() {
                @Override
                public void operationComplete(Future<List<InetAddress>> future) throws Exception {
                    if (future.isSuccess()) {
                        List<InetAddress> inetAddresses = future.getNow();
                        List<InetSocketAddress> socketAddresses =
                                new ArrayList<InetSocketAddress>(inetAddresses.size());
                        for (InetAddress inetAddress : inetAddresses) {
                            socketAddresses.add(new InetSocketAddress(inetAddress, unresolvedAddress.getPort()));
                        }
                        promise.setSuccess(socketAddresses);
                    } else {
                        promise.setFailure(future.cause());
                    }
                }
            });
}

这个命名Resolver为DefaultNameResolver，可以回到DefaultNameResolver的定义在上面就不说了，

来看InetSocketAddressResolver的父类AbstractAddressResolver：

package io.netty.resolver;

import io.netty.util.concurrent.EventExecutor;
import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.TypeParameterMatcher;
import io.netty.util.internal.UnstableApi;

import java.net.SocketAddress;
import java.nio.channels.UnsupportedAddressTypeException;
import java.util.Collections;
import java.util.List;

import static io.netty.util.internal.ObjectUtil.checkNotNull;

/**
 * A skeletal {@link AddressResolver} implementation.
 */
@UnstableApi
public abstract class AbstractAddressResolver<T extends SocketAddress> implements AddressResolver<T> {

    private final EventExecutor executor;//事件执行器
    private final TypeParameterMatcher matcher;//socket地址匹配器

    /**
     * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
     *                 by {@link #resolve(SocketAddress)}
     */
    protected AbstractAddressResolver(EventExecutor executor) {
        this.executor = checkNotNull(executor, "executor");
        matcher = TypeParameterMatcher.find(this, AbstractAddressResolver.class, "T");
    }

    /**
     * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
     *                 by {@link #resolve(SocketAddress)}
     * @param addressType the type of the {@link SocketAddress} supported by this resolver
     */
    protected AbstractAddressResolver(EventExecutor executor, Class<? extends T> addressType) {
        this.executor = checkNotNull(executor, "executor");
        matcher = TypeParameterMatcher.get(addressType);
    }

    /**
     * Returns the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
     * by {@link #resolve(SocketAddress)}.
     */
    protected EventExecutor executor() {
        return executor;
    }
    //命名Resolver是否支持socket地址
    @Override
    public boolean isSupported(SocketAddress address) {
        return matcher.match(address);
    }
    //socket地址是否可以Resolve
    @Override
    public final boolean isResolved(SocketAddress address) {
        if (!isSupported(address)) {
            throw new UnsupportedAddressTypeException();
        }

        @SuppressWarnings("unchecked")
        final T castAddress = (T) address;
	//具体Resolve工作委托给doIsResolved方法
        return doIsResolved(castAddress);
    }

    /**
     * Invoked by {@link #isResolved(SocketAddress)} to check if the specified {@code address} has been resolved
     * already.
     */
    protected abstract boolean doIsResolved(T address);

    @Override
    public final Future<T> resolve(SocketAddress address) {
        if (!isSupported(checkNotNull(address, "address"))) {
            // Address type not supported by the resolver
            return executor().newFailedFuture(new UnsupportedAddressTypeException());
        }

        if (isResolved(address)) {
            // Resolved already; no need to perform a lookup
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
            return executor.newSucceededFuture(cast);
        }

        try {
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
            final Promise<T> promise = executor().newPromise();
	    //具体Resolve工作委托给doIsResolved方法
            doResolve(cast, promise);
            return promise;
        } catch (Exception e) {
            return executor().newFailedFuture(e);
        }
    }

    @Override
    public final Future<T> resolve(SocketAddress address, Promise<T> promise) {
        checkNotNull(address, "address");
        checkNotNull(promise, "promise");

        if (!isSupported(address)) {
            // Address type not supported by the resolver
            return promise.setFailure(new UnsupportedAddressTypeException());
        }

        if (isResolved(address)) {
            // Resolved already; no need to perform a lookup
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
            return promise.setSuccess(cast);
        }

        try {
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
	    //具体Resolve工作委托给doIsResolved方法
            doResolve(cast, promise);
            return promise;
        } catch (Exception e) {
            return promise.setFailure(e);
        }
    }

    @Override
    public final Future<List<T>> resolveAll(SocketAddress address) {
        if (!isSupported(checkNotNull(address, "address"))) {
            // Address type not supported by the resolver
            return executor().newFailedFuture(new UnsupportedAddressTypeException());
        }

        if (isResolved(address)) {
            // Resolved already; no need to perform a lookup
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
            return executor.newSucceededFuture(Collections.singletonList(cast));
        }

        try {
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
            final Promise<List<T>> promise = executor().newPromise();
	    //具体Resolve工作委托给doResolveAll方法
            doResolveAll(cast, promise);
            return promise;
        } catch (Exception e) {
            return executor().newFailedFuture(e);
        }
    }

    @Override
    public final Future<List<T>> resolveAll(SocketAddress address, Promise<List<T>> promise) {
        checkNotNull(address, "address");
        checkNotNull(promise, "promise");

        if (!isSupported(address)) {
            // Address type not supported by the resolver
            return promise.setFailure(new UnsupportedAddressTypeException());
        }

        if (isResolved(address)) {
            // Resolved already; no need to perform a lookup
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
            return promise.setSuccess(Collections.singletonList(cast));
        }

        try {
            @SuppressWarnings("unchecked")
            final T cast = (T) address;
	    //具体Resolve工作委托给doResolveAll方法
            doResolveAll(cast, promise);
            return promise;
        } catch (Exception e) {
            return promise.setFailure(e);
        }
    }
    //下面几个方法待子类实现,见InetSocketAddressResolver的定义
    /**
     * Invoked by {@link #resolve(SocketAddress)} to perform the actual name
     * resolution.
     */
    protected abstract void doResolve(T unresolvedAddress, Promise<T> promise) throws Exception;

    /**
     * Invoked by {@link #resolveAll(SocketAddress)} to perform the actual name
     * resolution.
     */
    protected abstract void doResolveAll(T unresolvedAddress, Promise<List<T>> promise) throws Exception;

    @Override
    public void close() { }
}

//AddressResolver接口定义，这个就不说，看看就行

package io.netty.resolver;

import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.UnstableApi;

import java.io.Closeable;
import java.net.SocketAddress;
import java.nio.channels.UnsupportedAddressTypeException;
import java.util.List;

/**
 * Resolves a possibility unresolved {@link SocketAddress}.
 */
@UnstableApi
public interface AddressResolver<T extends SocketAddress> extends Closeable {

  /**
   * Returns {@code true} if and only if the specified address is supported by this resolved.
   */
  boolean isSupported(SocketAddress address);

  /**
   * Returns {@code true} if and only if the specified address has been resolved.
   *
   * @throws UnsupportedAddressTypeException if the specified address is not supported by this resolver
   */
  boolean isResolved(SocketAddress address);

  /**
   * Resolves the specified address. If the specified address is resolved already, this method does nothing
   * but returning the original address.
   *
   * @param address the address to resolve
   *
   * @return the {@link SocketAddress} as the result of the resolution
   */
  Future<T> resolve(SocketAddress address);

  /**
   * Resolves the specified address. If the specified address is resolved already, this method does nothing
   * but returning the original address.
   *
   * @param address the address to resolve
   * @param promise the {@link Promise} which will be fulfilled when the name resolution is finished
   *
   * @return the {@link SocketAddress} as the result of the resolution
   */
  Future<T> resolve(SocketAddress address, Promise<T> promise);

  /**
   * Resolves the specified address. If the specified address is resolved already, this method does nothing
   * but returning the original address.
   *
   * @param address the address to resolve
   *
   * @return the list of the {@link SocketAddress}es as the result of the resolution
   */
  Future<List<T>> resolveAll(SocketAddress address);

  /**
   * Resolves the specified address. If the specified address is resolved already, this method does nothing
   * but returning the original address.
   *
   * @param address the address to resolve
   * @param promise the {@link Promise} which will be fulfilled when the name resolution is finished
   *
   * @return the list of the {@link SocketAddress}es as the result of the resolution
   */
  Future<List<T>> resolveAll(SocketAddress address, Promise<List<T>> promise);

  /**
   * Closes all the resources allocated and used by this resolver.
   */
  @Override
  void close();
}