Handling HTTP requests to port 25556 in Minecraft. This requires ProtocolLib (for now), but could be rewritten to avoid it.

BootstrapList.java

package com.comphenix.example;

import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.concurrent.Callable;

import com.google.common.collect.Lists;

import net.minecraft.util.io.netty.channel.Channel;

// Hopefully, CB won't version these as well
import net.minecraft.util.io.netty.channel.ChannelFuture;
import net.minecraft.util.io.netty.channel.ChannelHandler;

class BootstrapList implements List<Object> {
    private List<Object> delegate;
    private ChannelHandler handler;
    
    /**
     * Construct a new bootstrap list.
     * @param delegate - the delegate.
     * @param handler - the channel handler to add.
     */
    public BootstrapList(List<Object> delegate, ChannelHandler handler) {
        this.delegate = delegate;
        this.handler = handler;
        
        // Process all existing bootstraps
        for (Object item : this) {
            processElement(item);
        }
    }

    @Override
    public synchronized boolean add(Object element) {
        processElement(element);
        return delegate.add(element);
    }
    
    @Override
    public synchronized boolean addAll(Collection<? extends Object> collection) {
        List<Object> copy = Lists.newArrayList(collection);
        
        // Process the collection before we pass it on
        for (Object element : copy) {
            processElement(element);
        }
        return delegate.addAll(copy);
    }
    
    @Override
    public synchronized Object set(int index, Object element) {
        Object old = delegate.set(index, element);
        
        // Handle the old future, and the newly inserted future
        if (old != element) {
            unprocessElement(old);
            processElement(element);
        }
        return old;
    }
    
    /**
     * Process a single element.
     * @param element - the element.
     */
    protected void processElement(Object element) {
        if (element instanceof ChannelFuture) {
            processBootstrap((ChannelFuture) element);
        }
    }
    
    /**
     * Unprocess a single element.
     * @param element - the element to unprocess.
     */
    protected void unprocessElement(Object element) {
        if (element instanceof ChannelFuture) {
            unprocessBootstrap((ChannelFuture) element);
        }
    }
    
    /**
     * Process a single channel future.
     * @param future - the future.
     */
    protected void processBootstrap(ChannelFuture future) {
        // Important: Must be addFirst()
        future.channel().pipeline().addFirst(handler);
    }
    
    /**
     * Revert any changes we made to the channel future.
     * @param future - the future.
     */
    protected void unprocessBootstrap(ChannelFuture future) {
        final Channel channel = future.channel();
        
        // For thread safety - see ChannelInjector.close()
        channel.eventLoop().submit(new Callable<Object>() {
            @Override
            public Object call() throws Exception {
                channel.pipeline().remove(handler);
                return null;
            }
        });
    }
    
    /**
     * Close and revert all changes.
     */
    public synchronized void close() {
        for (Object element : this)
            unprocessElement(element);
    }

    // Boiler plate
    public synchronized int size() {
        return delegate.size();
    }

    public synchronized boolean isEmpty() {
        return delegate.isEmpty();
    }

    public boolean contains(Object o) {
        return delegate.contains(o);
    }

    public synchronized Iterator<Object> iterator() {
        return delegate.iterator();
    }

    public synchronized Object[] toArray() {
        return delegate.toArray();
    }

    public synchronized <T> T[] toArray(T[] a) {
        return delegate.toArray(a);
    }

    public synchronized boolean remove(Object o) {
        return delegate.remove(o);
    }

    public synchronized boolean containsAll(Collection<?> c) {
        return delegate.containsAll(c);
    }

    public synchronized boolean addAll(int index, Collection<? extends Object> c) {
        return delegate.addAll(index, c);
    }

    public synchronized boolean removeAll(Collection<?> c) {
        return delegate.removeAll(c);
    }

    public synchronized boolean retainAll(Collection<?> c) {
        return delegate.retainAll(c);
    }

    public synchronized void clear() {
        delegate.clear();
    }

    public synchronized Object get(int index) {
        return delegate.get(index);
    }

    public synchronized void add(int index, Object element) {
        delegate.add(index, element);
    }

    public synchronized Object remove(int index) {
        return delegate.remove(index);
    }

    public synchronized int indexOf(Object o) {
        return delegate.indexOf(o);
    }

    public synchronized int lastIndexOf(Object o) {
        return delegate.lastIndexOf(o);
    }

    public synchronized ListIterator<Object> listIterator() {
        return delegate.listIterator();
    }

    public synchronized ListIterator<Object> listIterator(int index) {
        return delegate.listIterator(index);
    }

    public synchronized List<Object> subList(int fromIndex, int toIndex) {
        return delegate.subList(fromIndex, toIndex);
    }
    // End boiler plate
}

ExampleMod.java

package com.comphenix.example;

import net.minecraft.util.io.netty.channel.Channel;

import org.bukkit.event.Listener;
import org.bukkit.plugin.java.JavaPlugin;

public class ExampleMod extends JavaPlugin implements Listener {
    private NettyInjector injector = new NettyInjector() {
        @Override
        protected void injectChannel(Channel channel) {
            channel.pipeline().addFirst(new JSONAPIChannelDecoder());
        }
    };
    
    @Override
    public void onEnable() {
        injector.inject();
    }
    
    @Override
    public void onDisable() {
        injector.close();
    }
}

JSONAPIChannelDecoder.java

package com.comphenix.example;

import java.util.List;
import java.util.NoSuchElementException;

import net.minecraft.util.io.netty.buffer.ByteBuf;
import net.minecraft.util.io.netty.channel.ChannelHandlerContext;
import net.minecraft.util.io.netty.channel.ChannelOption;
import net.minecraft.util.io.netty.channel.ChannelPipeline;
import net.minecraft.util.io.netty.handler.codec.ByteToMessageDecoder;
import net.minecraft.util.io.netty.handler.codec.http.HttpRequestDecoder;
import net.minecraft.util.io.netty.handler.codec.http.HttpResponseEncoder;

public class JSONAPIChannelDecoder extends ByteToMessageDecoder {
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf buf, List<Object> list) throws Exception {
        // use 4 bytes to detect HTTP or abort
        if (buf.readableBytes() < 4) {
            return;
        }
        buf.retain(2);

        final int magic1 = buf.getUnsignedByte(buf.readerIndex());
        final int magic2 = buf.getUnsignedByte(buf.readerIndex() + 1);
        final int magic3 = buf.getUnsignedByte(buf.readerIndex() + 2);
        final int magic4 = buf.getUnsignedByte(buf.readerIndex() + 3);
        ChannelPipeline p = ctx.channel().pipeline();
        
        if (isHttp(magic1, magic2, magic3, magic4)) {
            ByteBuf copy = buf.copy();
            ctx.channel().config().setOption(ChannelOption.TCP_NODELAY, true);

            try {
                while (p.removeLast() != null);
            } catch (NoSuchElementException e) {

            }
            
            p.addLast("decoder", new HttpRequestDecoder());
            p.addLast("encoder", new HttpResponseEncoder());
            p.addLast("handler", new JSONAPIHandler()); 
           
            p.fireChannelRead(copy);
            buf.release();
            buf.release();
        } else {
            try {
                p.remove(this);
            } catch (NoSuchElementException e) {
                // probably okay, it just needs to be off
                System.out.println("NoSuchElementException");
            }

            buf.release();
            buf.release();
        }
    }

    private boolean isHttp(int magic1, int magic2, int magic3, int magic4) {
        return magic1 == 'G' && magic2 == 'E' && magic3 == 'T' && magic4 == ' ' || // GET
                magic1 == 'P' && magic2 == 'O' && magic3 == 'S' && magic4 == 'T' || // POST
                magic1 == 'P' && magic2 == 'U' && magic3 == 'T' && magic4 == ' ' || // PUT
                magic1 == 'H' && magic2 == 'E' && magic3 == 'A' && magic4 == 'D' || // HEAD
                magic1 == 'O' && magic2 == 'P' && magic3 == 'T' && magic4 == 'I' || // OPTIONS
                magic1 == 'P' && magic2 == 'A' && magic3 == 'T' && magic4 == 'C' || // PATCH
                magic1 == 'D' && magic2 == 'E' && magic3 == 'L' && magic4 == 'E' || // DELETE
                magic1 == 'T' && magic2 == 'R' && magic3 == 'C' && magic4 == 'C' || // TRACE
                magic1 == 'C' && magic2 == 'O' && magic3 == 'N' && magic4 == 'N'; // CONNECT
    }
}

JSONAPIHandler.java

package com.comphenix.example;

import com.google.common.base.Charsets;

import net.minecraft.util.io.netty.channel.ChannelFuture;
import net.minecraft.util.io.netty.channel.ChannelFutureListener;
import net.minecraft.util.io.netty.channel.ChannelHandlerContext;
import net.minecraft.util.io.netty.channel.ChannelPromise;
import net.minecraft.util.io.netty.channel.SimpleChannelInboundHandler;
import net.minecraft.util.io.netty.handler.codec.http.DefaultFullHttpResponse;
import net.minecraft.util.io.netty.handler.codec.http.FullHttpResponse;
import net.minecraft.util.io.netty.handler.codec.http.HttpRequest;
import net.minecraft.util.io.netty.handler.codec.http.HttpResponseStatus;
import net.minecraft.util.io.netty.handler.codec.http.HttpVersion;

class JSONAPIHandler extends SimpleChannelInboundHandler<HttpRequest> {
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, HttpRequest request) throws Exception {
        FullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
        ChannelPromise promise = ctx.channel().newPromise();
        
        response.content().writeBytes(Charsets.UTF_8.encode("Hello, world!"));
        
        // Write the response, and close the channel
        ctx.channel().writeAndFlush(response, promise);
        promise.addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws Exception {
                future.channel().close();
            }
        });
    }
}

NettyInjector.java

package com.comphenix.example;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.List;
import net.minecraft.util.io.netty.channel.Channel;
import net.minecraft.util.io.netty.channel.ChannelFuture;
import net.minecraft.util.io.netty.channel.ChannelHandler;
import net.minecraft.util.io.netty.channel.ChannelHandlerContext;
import net.minecraft.util.io.netty.channel.ChannelInboundHandler;
import net.minecraft.util.io.netty.channel.ChannelInboundHandlerAdapter;
import net.minecraft.util.io.netty.channel.ChannelInitializer;

import com.comphenix.protocol.reflect.FuzzyReflection;
import com.comphenix.protocol.reflect.VolatileField;
import com.comphenix.protocol.utility.MinecraftReflection;
import com.google.common.collect.Lists;

public abstract class NettyInjector {   
    // The temporary player factory
    private List<VolatileField> bootstrapFields = Lists.newArrayList();
   
    // List of network managers
    private volatile List<Object> networkManagers;
    private boolean injected;

    private boolean closed;

    /**
     * Inject into the spigot connection class.
     */
    @SuppressWarnings("unchecked")
    public synchronized void inject() {
        if (injected)
            throw new IllegalStateException("Cannot inject twice.");
        try {
            FuzzyReflection fuzzyServer = FuzzyReflection.fromClass(MinecraftReflection.getMinecraftServerClass());
            Method serverConnectionMethod = fuzzyServer.getMethodByParameters("getServerConnection", MinecraftReflection.getServerConnectionClass(), new Class[] {});
            
            // Get the server connection
            Object server = fuzzyServer.getSingleton();
            Object serverConnection = serverConnectionMethod.invoke(server);
            
            // Handle connected channels
            final ChannelInboundHandler endInitProtocol = new ChannelInitializer<Channel>() {
                @Override
                protected void initChannel(Channel channel) throws Exception {
                    try {
                        // This can take a while, so we need to stop the main thread from interfering
                        synchronized (networkManagers) {
                            injectChannel(channel);
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            };
            
            // This is executed before Minecraft's channel handler
            final ChannelInboundHandler beginInitProtocol = new ChannelInitializer<Channel>() {
                @Override
                protected void initChannel(Channel channel) throws Exception {
                    // Our only job is to add init protocol
                    channel.pipeline().addLast(endInitProtocol);
                }
            };
            
            // Add our handler to newly created channels
            final ChannelHandler connectionHandler = new ChannelInboundHandlerAdapter() {
                @Override
                public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
                    Channel channel = (Channel) msg;

                    // Prepare to initialize ths channel
                    channel.pipeline().addFirst(beginInitProtocol);
                    ctx.fireChannelRead(msg);
                }
            };
            
            // Get the current NetworkMananger list
            networkManagers = (List<Object>) FuzzyReflection.fromObject(serverConnection, true).
                invokeMethod(null, "getNetworkManagers", List.class, serverConnection);
            
            // Insert ProtocolLib's connection interceptor
            bootstrapFields = getBootstrapFields(serverConnection);
            
            for (VolatileField field : bootstrapFields) {
                final List<Object> list = (List<Object>) field.getValue();
     
                // We don't have to override this list
                if (list == networkManagers) {
                    continue;
                }
                
                // Synchronize with each list before we attempt to replace them.
                field.setValue(new BootstrapList(list, connectionHandler));
            }

            injected = true;
            
        } catch (Exception e) {
            throw new RuntimeException("Unable to inject channel futures.", e);
        }
    }
    
    /**
     * Invoked when a channel is ready to be injected.
     * @param channel - the channel to inject.
     */
    protected abstract void injectChannel(Channel channel);
    
    /**
     * Retrieve a list of every field with a list of channel futures.
     * @param serverConnection - the connection.
     * @return List of fields.
     */
    private List<VolatileField> getBootstrapFields(Object serverConnection) {
        List<VolatileField> result = Lists.newArrayList();
        
        // Find and (possibly) proxy every list
        for (Field field : FuzzyReflection.fromObject(serverConnection, true).getFieldListByType(List.class)) {
            VolatileField volatileField = new VolatileField(field, serverConnection, true).toSynchronized();
            
            @SuppressWarnings("unchecked")
            List<Object> list = (List<Object>) volatileField.getValue();
            
            if (list.size() == 0 || list.get(0) instanceof ChannelFuture) {
                result.add(volatileField);
            }
        }
        return result;
    }
    
    /**
     * Clean up any remaning injections.
     */
    public synchronized void close() {
        if (!closed) {
            closed = true;

            for (VolatileField field : bootstrapFields) {
                Object value = field.getValue();

                // Undo the processed channels, if any 
                if (value instanceof BootstrapList) {
                    ((BootstrapList) value).close();
                }
                field.revertValue();
            }
        }
    }
}

This is really, really useful to a lot of people, including myself. 😜