Handler.java

package de.batschkoto.mcping;

import com.google.common.base.Charsets;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.Channel;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;

public class Handler extends SimpleChannelInboundHandler<ByteBuf> {

    public Channel ch;

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        this.ch = ctx.channel();

        ByteBuf buf = Unpooled.buffer();

        this.writeVarInt(0x00, buf);
        this.writeVarInt(4, buf);
        this.writeString("localhost", buf);
        buf.writeShort(25565);
        this.writeVarInt(1, buf);

        this.send(buf);

        buf.clear();

        this.writeVarInt(0x00, buf);

        this.send(buf);
    }

    @Override
    protected void channelRead0(ChannelHandlerContext channelHandlerContext, ByteBuf msg) throws Exception {
        System.out.println(readString(msg));
        this.ch.close();
    }

    public synchronized void send(ByteBuf buf){
        ByteBuf out = Unpooled.buffer();

        this.writeVarInt(buf.readableBytes(), out);
        out.writeBytes(buf);

        this.ch.writeAndFlush(out);
    }

    public void writeString(String s, ByteBuf buf)
    {
        byte[] b = s.getBytes( Charsets.UTF_8 );
        writeVarInt( b.length, buf );
        buf.writeBytes( b );
    }

    public String readString(ByteBuf buf)
    {
        int len = readVarInt( buf );
        byte[] b = new byte[ len ];
        buf.readBytes( b );

        return new String( b, Charsets.UTF_8 );
    }

    public int readVarInt(ByteBuf input)
    {
        int out = 0;
        int bytes = 0;
        byte in;
        while ( true )
        {
            in = input.readByte();

            out |= ( in & 0x7F ) << ( bytes++ * 7 );

            if ( bytes > 32 )
            {
                throw new RuntimeException( "VarInt too big" );
            }

            if ( ( in & 0x80 ) != 0x80 )
            {
                break;
            }
        }

        return out;
    }

    public void writeVarInt(int value, ByteBuf output)
    {
        int part;
        while ( true )
        {
            part = value & 0x7F;

            value >>>= 7;
            if ( value != 0 )
            {
                part |= 0x80;
            }

            output.writeByte( part );

            if ( value == 0 )
            {
                break;
            }
        }
    }
}