Problem Set 4: Multiplayer Minesweeper
The purpose of this problem set is to explore multithreaded programming with a shared mutable data type, which you should protect using one or more thread safety strategies.
Design Freedom and Restrictions
On this problem set, you have substantial design freedom.
change the name, signature, or specification of the
; you also should not change the implementation of
change any of the other provided code in
Also note that the axis definition of your board must match what is defined in Protocol and specification . The (x, y) coordinates start at (0, 0) in the top-left corner, extend horizontally to the right in the x direction, and extend vertically downwards in the y direction.
Your code will be tested against the specification. Changing these interfaces or axes will cause the tests to fail, and you will receive 0 points for the submission. It is your responsibility to examine Didit feedback and make sure your code compiles and runs for grading, but you must do your own testing to ensure correctness.
Beyond these requirements you have complete design freedom. For example, you can add new methods, classes, and packages; rewrite or delete other pieces of provided code; etc.
Get the code
You will start with some minimal server code and implement a server and thread-safe data structure for playing a multiplayer variant of the classic computer game “Minesweeper.”
You can review the rules of traditional single-player Minesweeper on Wikipedia .
You can also play traditional single-player Minesweeper online .
(You may notice that the implementation in the latter link above does something subtle. It ensures that there’s never a bomb where you make your first click of the game. You should not implement this for the assignment. It would be in conflict with giving the option to pass in a pre-designed board, for example.)
Your final product will consist of a server and no client; it should be fully playable using the
utility to send text commands directly over a network connection as described below.
Multiplayer Minesweeper server
We will refer to the board as a grid of squares. Each square is either flagged , dug , or untouched . Each square also either contains a bomb, or does not contain a bomb.
Our variant works very similarly to standard Minesweeper, but with multiple players simultaneously playing on a single board.
In both versions, players lose when they try to dig an untouched square that happens to contain a bomb.
Whereas a standard Minesweeper game would end at this point, in our version, the game keeps going for the other players.
In our version, when one player blows up a bomb, they still lose, and the game ends for them (the server ends their connection), but the other players may continue playing.
The square where the bomb was blown up is now a
square with no bomb.
The player who lost may reconnect to the same game again via
to start playing again.
Note that there are some tricky cases of user-level concurrency. For example, say user A has just modified the game state (i.e. by digging in one or more squares) such that square i,j obviously has a bomb. Meanwhile, user B has not observed the board state since this update has taken place, so user B goes ahead and digs in square i,j . Your program should allow the user to dig in that square — a user of Multiplayer Minesweeper must accept this kind of risk.
We are not specifically defining, or asking you to implement, any kind of “win condition” for the game.
is a utility that allows you to make a direct network connection to a listening server and communicate with it via a terminal interface.
Before starting this problem set, please ensure that you have
*nix operating systems (including Mac OS X) should have
installed by default.
Windows users should first check if telnet is installed by running the command
on the command line.
If you do not have
telnet, you can install it via Control Panel → Programs and Features → Turn Windows features on/off → Telnet client. However, this version of
telnetmay be very hard to use. If it does not show you what you’re typing, you will need to turn on the localecho option .
A better alternative is PuTTY: download
putty.exe. To connect using PuTTY, enter a hostname and port, select Connection type: Raw, and Close window on exit: Never. The last option will prevent the window from disappearing as soon as the server closes its end of the connection.
You can have
connect to a host and port (for example,
) from the command line with:
telnet web.mit.edu 80
Since port 80 is usually used for HTTP, we can now make HTTP requests through
. If you now type (where ↵ indicates a newline):
GET /↵ ↵
should retrieve the HTML for the webpage at
If you want to connect to your own machine, you can use
as the hostname and whatever port your server is listening on.
With the default port of 4444 in this problem set, you can connect to your Minesweeper server with:
telnet localhost 4444
The server may close the network connection at any time, at which point
To close a connection from the client, note the first output printed by telnet when it connects:
Trying 220.127.116.11... Connected to web.mit.edu. Escape character is '^]'.
, so press that.
to close the connection and exit telnet.
Protocol and specification
You must implement the following protocol for communication between the user and the server.
The messages in this protocol are described precisely and comprehensively using a pair of grammars. Your server must accept any incoming message that satisfies the user-to-server grammar, react appropriately, and generate only outgoing messages that satisfy the server-to-user grammar.
Messages from the user to the server
Each message starts with a message type and arguments to the message are seperated by a single SPACE character and the message ends with a NEWLINE.
The NEWLINE can be a single character
or the two-character sequence
the same definition used by
The user can send the following messages to the server:
Messages from the server to the user
There are no message types in the messages sent by the server to the user. The server sends the HELLO message as soon as it establishes a connection to the user. After that, for any message it receives that matches the user-to-server message format, other than a BYE message, the server should always return either a BOARD message, a BOOM message, or a HELP message.
For any message from the user which does not match the user-to-server message format , discard the incoming message and send a HELP message as the reply to the user.
The action to take for each different kind of message is as follows:
Problem 1: set up the server to deal with multiple clients
, we have provided you with a single-threaded server which can accept connections with one client at a time, and which includes code to parse the input according to the client-server protocol above.
Modify the server so it can maintain multiple client connections simultaneously. Each client connection should be maintained by its own thread. You may wish to add another class to do this.
As always, all your code should be safe from bugs, easy to understand, and ready for change. Be sure the code to implement multiple concurrent clients is written clearly and commented if necessary.
You may continue to do nothing with the parsed user input for now.
When you write tests in
, you may write tests similar to the
we provide, and you may also write tests using
stubs as described in
Sockets & Networking
Problem 2: design a data structure for Minesweeper
, we have provided a starting file for this problem.
Specify, test, and implement a data structure for representing the Minesweeper board (as a Java type, without using sockets or threads).
ADT as well as any additional classes you might need to accomplish this.
You may ignore thread safety for now.
class (and other classes) must have specifications for all methods; abstraction function, rep invariant, and safety from rep exposure written as comments; and the rep invariant implemented as a
method called by your code.
Problem 3: make the entire system thread-safe
Come up with a strategy to make your system thread safe. As we’ve learned, there are multiple ways to make a system thread safe. For example, this can be done by:
- using immutable or thread-safe data structures
- using a queue to send messages that will be processed sequentially by a single thread
- using synchronized methods or the synchronized keyword at the right places
- or combination of these techniques
The specification for
Board(and any other classes) should state whether that type is thead-safe.
In addition to the other internal documentation,
Board(and any other classes) must document their thread-safety argument. Depending on your design, the board (or other classes) may not be thread-safe. If so, document that.
You will also document your system thread safety argument in
MinesweeperServer.javain problem 5.
SFB, ETU, RFC: be sure any code to implement thread safety is written clearly and commented if necessary.
Problem 4: initialize the board based on command-line options
We want our server to be able to accept some command-line options.
The exact specification is given in the Javadoc for
, which is excerpted below:
MinesweeperServer [--debug | --no-debug] [--port PORT] [--size SIZE_X,SIZE_Y | --file FILE]
--debugargument means the server should run in debug mode. The server should disconnect a client after a BOOM message if and only if the
--debugflag was NOT given. Using
--no-debugis the same as using no flag at all.
MinesweeperServer --debugstarts the server in debug mode.
PORTis an optional integer in the range 0 to 65535 inclusive, specifying the port the server should be listening on for incoming connections.
MinesweeperServer --port 1234starts the server listening on port 1234.
SIZE_Yare optional positive integer arguments specifying that a random board of size SIZE_X × SIZE_Y should be generated.
MinesweeperServer --size 42,58starts the server initialized with a random board of size 42 × 58.
FILEis an optional argument specifying a file pathname where a board has been stored. If this argument is given, the stored board should be loaded as the starting board.
MinesweeperServer --file boardfile.txtstarts the server initialized with the board stored in
We provide you with the code required to parse these command-line arguments in the
method already existing in
. You should not change this method.
Instead, you should change
to handle each of the two different ways a board can be initialized: either by random, or through input from a file.
(You’ll deal with the debug flag in Problem 5.)
argument: if the passed-in size X,Y > 0, the server’s
instance should be randomly generated and should have size equal to X by Y.
To randomly generate your board, you should assign each square to contain a bomb with probability .25 and otherwise no bomb.
All squares’ states should be set to
argument: If a file exists at the given path, read the the corresponding file, and if it is properly formatted, deterministically create the
The file format for input is:
In a properly formatted file matching the FILE grammar, the first line specifies the board size, and it must be followed by exactly Y lines, where each line must contain exactly X values.
If the file is properly formatted, the
should be instantiated such that square
has a bomb if and only if the
’th VAL in LINE
of the input is 1.
If the file is improperly formatted, your program should throw an unchecked exception (either
or define your own).
The following example file describes a board with 4 columns and 3 rows with a single bomb at the location
is given, generate a random board of size 10 × 10.
may not be specified simultaneously.
Reading from a file: see tips for reading and writing below.
method so that it handles the two possible ways to initialize a board (randomly or by loading a file).
Running the server on the command line:
this is easiest to do from the
directory where your code is compiled.
Open a command prompt,
Here are some examples:
cd bin java minesweeper.server.MinesweeperServer --debug java minesweeper.server.MinesweeperServer --port 1234 java minesweeper.server.MinesweeperServer --size 123,234 java minesweeper.server.MinesweeperServer --file ../testBoard java minesweeper.server.MinesweeperServer --debug --port 1234 --size 20,14
You can specify command-line arguments in Eclipse by clicking on the drop-down arrow next to “run,” clicking on “Run Configurations…”, and selecting the “Arguments” tab. Type your arguments into the text box.
Problem 5: putting it all together
Modify your server so that it implements our protocols and specifications, using a single instance of
Note that when we send a BOOM message to the user, we should terminate their connection if and only if the debug flag (see Problem 4) is missing.
The tips for reading and writing below may be useful for reading from and writing to socket streams.
Near the top of
MinesweeperServer, include a substantial comment with an argument for why your system is thread-safe.
SFB, ETU, RFC: be sure the code to implement thread safety is written clearly and commented if necessary.
Commit to Git. Pushing your code to your Git repository on Athena will cause Didit to run a single staff-provided test. You can read and download the source code for the published test .
You are strongly encouraged to download and run the published test on your own machine.
Do not attempt to debug failures on Didit before you can run and pass the published test locally on your own machine.
Tips for reading and writing
BufferedReader.readLine()reads a line of text from an input stream, where line is considered to be terminated by any one of a line feed (
\n), a carriage return (
\r), or a carriage return followed immediately by a linefeed. This behavior matches the NEWLINE productions in our grammars.
PrintWriter.println()prints formatted text to an output stream, and terminates the current line by writing the line separator string. It always writes a line separator, even if the input string already ends with one. The line separator is defined by the system property
line.separator; you can obtain it using
System.lineSeparator(). It is
\non *nix systems and
\r\non Windows. Both of these line endings are allowed by the NEWLINE productions in our grammars.
You are now done with the problem set, but you could imagine not having to use
to play Minesweeper.
A GUI would do a lot to bring your program to life.
Here is GUI client from past student
that you can use to play your Minesweeper game:
This GUI waits until you do a LOOK or other operation before updating the view.
It has UI for spawning clients, so you can actually just run the jar once and then use the UI to spawn a number of clients.
It also displays a printout of all the protocol I/O which is going on, making it particularly helpful for debugging.
You should have your server running before trying to start a client connection from this GUI.
Note: This GUI is entirely unofficial! You may find bugs or incorrect behavior. Do not consider it sufficient for testing your code.
If you want practice writing GUIs, we encourage you to write your own Minesweeper GUI and email the source to the staff.