你一定看得懂的 Netty 客户端启动源码分析！

public final class EchoClient {
    public static void main(String[] args) throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            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(new EchoClientHandler());
                 }
             });

            ChannelFuture f = b.connect(HOST, PORT).sync();

            f.channel().closeFuture().sync();
        } finally {
            group.shutdownGracefully();
        }
    }
}


EventLoopGroup group = new NioEventLoopGroup();

public NioEventLoopGroup() {
    this(0);
}

public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider) {
    this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
}

protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
    super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);
}


protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
                                            EventExecutorChooserFactory chooserFactory, Object... args) {
    if (executor == null) {
        executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
    }
    
    children = new EventExecutor[nThreads];
    
    for (int i = 0; i < nThreads; i ++) {
        children[i] = newChild(executor, args);
    }
    
    // ... 省略
}


public final class ThreadPerTaskExecutor implements Executor {
    private final ThreadFactory threadFactory;

    public ThreadPerTaskExecutor(ThreadFactory threadFactory) {
        if (threadFactory == null) {
            throw new NullPointerException("threadFactory");
        }
        this.threadFactory = threadFactory;
    }

    @Override
    public void execute(Runnable command) {
        threadFactory.newThread(command).start();
    }
}


public class DefaultThreadFactory implements ThreadFactory {
    @Override
    public Thread newThread(Runnable r) {
        Thread t = newThread(FastThreadLocalRunnable.wrap(r), prefix + nextId.incrementAndGet());
        return t;
    }

    protected Thread newThread(Runnable r, String name) {
        return new FastThreadLocalThread(threadGroup, r, name);
    }
}


protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
                                            EventExecutorChooserFactory chooserFactory, Object... args) {
    children = new EventExecutor[nThreads];
    for (int i = 0; i < nThreads; i ++) {
        children[i] = newChild(executor, args);
        // .... 省略部分代码
    }
}


@Override
protected EventLoop newChild(Executor executor, Object... args) throws Exception {
    return new NioEventLoop(this, executor, (SelectorProvider) args[0],
        ((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
}

NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
                 SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
    super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
    if (selectorProvider == null) {
        throw new NullPointerException("selectorProvider");
    }
    if (strategy == null) {
        throw new NullPointerException("selectStrategy");
    }
    provider = selectorProvider;
    final SelectorTuple selectorTuple = openSelector();
    selector = selectorTuple.selector;
    unwrappedSelector = selectorTuple.unwrappedSelector;
    selectStrategy = strategy;
}


public B channel(Class<? extends C> channelClass) {
    if (channelClass == null) {
        throw new NullPointerException("channelClass");
    }
    return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}

public ReflectiveChannelFactory(Class<? extends T> clazz) {
    if (clazz == null) {
        throw new NullPointerException("clazz");
    }
    this.clazz = clazz;
}

public B channelFactory(ChannelFactory<? extends C> channelFactory) {
    if (channelFactory == null) {
        throw new NullPointerException("channelFactory");
    }
    if (this.channelFactory != null) {
        throw new IllegalStateException("channelFactory set already");
    }

    this.channelFactory = channelFactory;
    return self();
}


ChannelFuture f = b.connect(HOST, PORT).sync();


final ChannelFuture initAndRegister() {
    Channel channel = channelFactory.newChannel();
    init(channel);
    
    ChannelFuture regFuture = config().group().register(channel);
    return regFuture;
}


@Override
public T newChannel() {
    try {
        return clazz.getConstructor().newInstance();
    } catch (Throwable t) {
        throw new ChannelException("Unable to create Channel from class " + clazz, t);
    }
}


public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
    public NioSocketChannel() {
        this(DEFAULT_SELECTOR_PROVIDER);
    }

    public NioSocketChannel(SelectorProvider provider) {
        this(newSocket(provider));
    }

    private static SocketChannel newSocket(SelectorProvider provider) {
        try {
            return provider.openSocketChannel();
        } catch (IOException e) {
            throw new ChannelException("Failed to open a socket.", e);
        }
    }
}


public abstract class AbstractNioChannel extends AbstractChannel {
    protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
        super(parent, ch, SelectionKey.OP_READ);
    }

    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        ch.configureBlocking(false);
    }
}


public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
    protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }
}


 public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {
    final ChannelFuture initAndRegister() {
        Channel channel = channelFactory.newChannel();
        init(channel);
        ChannelFuture regFuture = config().group().register(channel);
    }
}


@Override
protected void doRegister() throws Exception {
    boolean selected = false;
    for (;;) {
        selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
        return;
    }
}


protected AbstractChannel(Channel parent) {
    this.parent = parent;
    id = newId();
    unsafe = newUnsafe();
    pipeline = newChannelPipeline();
}


@Override
protected AbstractNioUnsafe newUnsafe() {
    return new NioSocketChannelUnsafe();
}


protected DefaultChannelPipeline newChannelPipeline() {
    return new DefaultChannelPipeline(this);
}

protected DefaultChannelPipeline(Channel channel) {
    this.channel = ObjectUtil.checkNotNull(channel, "channel");
    succeededFuture = new SucceededChannelFuture(channel, null);
    voidPromise =  new VoidChannelPromise(channel, true);

    tail = new TailContext(this);
    head = new HeadContext(this);

    head.next = tail;
    tail.prev = head;
}


public static boolean connect(final SocketChannel socketChannel, final SocketAddress remoteAddress)
            throws IOException {
    try {
        return AccessController.doPrivileged(new PrivilegedExceptionAction<Boolean>() {
            @Override
            public Boolean run() throws IOException {
                return socketChannel.connect(remoteAddress);
            }
        });
    } catch (PrivilegedActionException e) {
        throw (IOException) e.getCause();
    }
}