Minecraft
An Introduction
Copyright © 2015 – Curt Hill
Introduction
• A curious phenomenon!
• A game with many modes
• It trades good graphics for incharacter environment creation
• May be played by very young but still
has interest for the mature
Copyright © 2015 – Curt Hill
History
• Original developer was Markus
Persson in 2009
• Inspired by several previous games
that it was transcended
• Mojang was the owning company
• Purchased by Microsoft for 2.5
billion dollars
• Is available on PCs, Macs, Android,
iPad, XBox, PlayStation and
numerous other platforms
Copyright © 2015 – Curt Hill
Characteristics
• Open world type
• First or third person
• Several modes of play
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–
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Survival
Creative
Adventure
Spectator
Demo
• Single and multiple player
Copyright © 2015 – Curt Hill
Modes
• Survival mode – Scavenger hunt to
gather resources
– Creating tools is part of game
• Creative – Create environments and
objects
– None of the limits of survival mode
• Adventure – Set down in a created
world
– Author limited creation and destruction
Copyright © 2015 – Curt Hill
Java
• Minecraft is a game that is
completely written in Java
• This is both good and bad
• The Good
– Nearly platform independent
– Lots of Java programmers
– Decompilable
• The Bad
– Somewhat slower
– Decompilable
Copyright © 2015 – Curt Hill
Decompilable
• The decompiler has been a dream in
CS for decades
• It takes an executable and produces
the high level language that
produced it
• In general this is a dream that
cannot be done
• However, in Java it is possible
Copyright © 2015 – Curt Hill
What makes Java different?
• There is only one compiler
– Makes it easy to see characteristic
code constructions and infer what
caused them
• Each class must be in a file that has
the same name
– Even when packed into a jar file
• The reflection feature
– ability to examine and modify the
structure and behavior of an object at
runtime
Copyright © 2015 – Curt Hill
Reflection Again
• Reflection gives us the ability to ask
a class if it has a particular method
• It also allows us to call that method
• To look for methods not knowing if
we will find them and execute them if
we do:
String s = …
Method mthd = foo.getClass()
.getMethod(s, null);
method.invoke(foo, null);
• We did not know what the name was
until we found
it
Copyright © 2015 – Curt Hill
Good and Bad?
• The decompilability issue is both
good and bad
• For the owners their code is
exposed
– They are somewhat protected by
copyright
– Derived code could be disguised and
marketed by a competitor
• For the modders we can change
anything
– More power than in most modifiable
games
Copyright © 2015 – Curt Hill
Obfuscation
• The owner’s answer is obfuscation
• The word means to confuse or
disguise
• The process generally involves:
– Transforming the names into
meaningless ones
– Refactoring classes
– Reorganizing flow of control
• To be useful in this case it must be
done and undone by program
Copyright © 2015 – Curt Hill
Process
• The author’s need to be able to
write, debug and maintain code in its
original, highly understandable form
• They then obfuscate by program
• The new Java code should execute
in a functionally equivalent way
– A bug reported in the production code
should also be present in the pristine
code
– Otherwise it is a obfuscation bug
Copyright © 2015 – Curt Hill
The Modder’s Problem
• How can we alter this obfuscated
code?
• Someone has figured this out
– There is a de-obfuscator that restores
reasonable names
• The MCP toolkit and Minecraft Forge
has the tools needed to de- and reobfuscate as well as decompilers
– Neither of these are officially
connected to Mojang
Copyright © 2015 – Curt Hill
Relationship
• Recently Mojang has been rather
cooperative with the modders
• In Curt’s Occasionally Humble
Opinion they have legal grounds to
eliminate these people or at least
harass them enough to make it not
worth while
– They have chosen not to do so
– This is likely that the modding
community has increased rather than
decreased revenues
Copyright © 2015 – Curt Hill
Java Again
• Recall that Java has dynamic
binding
• When a method call is executed for
the first time the class file is loaded
from disk or elsewhere
– It did not have to be in memory before
the call
• This allows objects to be
dynamically loaded into memory
without the program knowing
anything about it before it started
Copyright © 2015 – Curt Hill
Structure of the Game
• Like many multi-player games there
are two main components
– A server
– A client
• Both are Java programs and are
subject to our change
• They communicate using packets
over the internet
Copyright © 2015 – Curt Hill
Client and Server
• The client is interested in three main
things:
– Catching and interpreting keyboard
commands
– Displaying the current scene on the
monitor
– Communicating with the server on what
is going on
• The server is the communications
relay
– What has been reported by any client is
reported to all the rest
Copyright © 2015 – Curt Hill
Infinite Loops
• The client stays in an infinite loop
• The goal is to go through the loop 20
times a second
– This is a tick
• The tick based items include
updates to the world
– Communication to and from the server
– Monitoring the keyboard/mouse
• Rendering is not tick based
• All the server processing is tick
based
Copyright © 2015 – Curt Hill
Client Again
• The Minecraft client is actually
somewhat more than that
• A server was integrated into the
client
– This facilitates group play on a LAN
• Thus there are two programs
– The server without the client (stand
alone or dedicated)
– The client with the server
• We also have the issues of proxies
Copyright © 2015 – Curt Hill
Proxies
• Code in a mod could run in two
places: the integrated server (part
of the client) or the stand alone
server
• If this code references code in the
client then one of two things can
happen:
– In the integrated server all is good
– In the stand alone server it crashes
• We thus need classes with different
methods depending on the location
Copyright © 2015 – Curt Hill
Proxies Again
• A typical solution is to have a
common proxy
– This determines the needed method
signatures
• We take derivations of this for the
server proxy and client proxy
• An annotation will show the system
which one to load
• Usually client and server have same
signatures but do different things
– A rendering method would be empty on
the server Copyright © 2015 – Curt Hill
Classes
• Minecraft is largely constructed
from a few classes
• Most of these classes have many
derivations
• In this presentation we will mention
them
• Other presentations will discuss
them more fully
Copyright © 2015 – Curt Hill
• Block
Classes
– Describes how to draw and how it reacts
to events (behavior)
• Item
– Anything that a player can carry such as
a sword
• Entity
– Anything that can move around
• TileEntity
– Used when more data than a block is
needed
Copyright © 2015 – Curt Hill
Finally
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•
•
•
•
Plenty of more to consider
Licenses
Establishing the forge environment
Creating a simple mod
Lots more
Copyright © 2015 – Curt Hill