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Practical 3 – Networking

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CS5001 Object-Oriented Modelling, Design and
Programming
Practical 3 – Networking

MMS is the definitive source for deadlines and weightings.
In this practical, you are required to write a simple Java web server which can respond to some
HTTP/1.1 requests [1].
Setup
For this practical, you may develop your code in any IDE or editor of your choice. However, please
ensure that you create a suitable assignment directory CS5001-p3-networking on the computer
you’re using. All your source code should be in a src directory inside your assignment directory,
CS5001-p3-networking/src and your main method should be in a WebServerMain class in
WebServerMain.java .
You should probably copy the www directory from StudRes (containing some sample html
pages and images) to your assignment directory. This path can be found on the host machines,
which you can access remotely in the usual way. You can download the whole directory using
sftp and the get -r command. You can then add more files to your local www directory
when testing your server if you want to.
Basic Requirements
At a minimum your webserver program should satisfy the following basic requirements.
• The main method for your server should take two command-line arguments: the directory
from which your server will serve documents to clients; and the port on which your server
should listen.
• The server should support and respond correctly to HEAD requests.
• The server should support and respond correctly to GET requests.
• The server must be able to return HTML documents requested by a client.
• The server should respond with appropriate error messages when non-existent services
or resources are requested.
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Advanced Requirements
For the highest grades, your program should also support one or more of the following advanced
requirements.
• Returning of binary images (GIF, JPEG and PNG).
• Multithreading – support multiple concurrent client connection requests up to a specified
limit.
• Logging – each time requests are made, log them to a file, indicating date/time request
type, response code etc.
• Supporting other methods in addition to GET and HEAD .
Implementation
You must use sockets when writing your own server for this practical and should not rely on
existing HTTP library code from Oracle or other third parties. The server should be able to serve
documents from a specified document root and listen on a specified port. When it receives an
incoming request, the server will have to read text over the socket from the client and check
whether the textual request corresponds to a HEAD or GET request. If it does, it should try to
serve the requested response header for a HEAD request, and for a GET request, the response
header plus body (the latter containing the requested file data) from its document root over the
opened socket connection to the client (browser).
More specifically, your server should listen for incoming connection requests. When a client
connects and sends a request to the server, it should examine the text of the request. If your
server does not support the received request, it should send back an appropriate response
header (as indicated in lectures).
For HEAD requests, your server should send back an appropriate response header containing
the information about the resource identified in the request (if the file exists at the specified
location in the document root). For a GET request, your server should similarly send back an
appropriate response header (as for the HEAD request), followed by the content of the requested
file (assuming the file exists at the specified location in the document root). In either case, if the
requested file cannot be found, your server should send back an appropriate response header,
potentially followed by some error page (the latter for a GET request only).
Note that both response header and content must be formatted as indicated in lectures or in
[1], and must be sent back over the socket connection to the web browser, not merely printed to
System.out . That said, you may wish to also include System.out.println messages to help
you debug your server as you develop it.
Once your server has responded, it should flush and close the connection to the client and listen
for further requests. That is, your server is not required to keep connections alive.
Compiling and Running
For your program to be compatible with the automated checker:
• It must be possible to compile all your source code from within the src directory using
the simple command:
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javac *.java
• It must be possible to get your program to serve pages from a directory and port specified
on the command line that are passed to your main method in the String[] args . For
example, executing the following command from within the src directory should permit
your program to serve pages from the CS5001-p3-networking/www directory (i.e. one
level up from the src directory) and listen on port 12345:
java WebServerMain ../www 12345
• If your server is started without supplying the command-line arguments, it should simply
print the usage message indicated below and exit:
Usage: java WebServerMain <document_root> <port>
Running the Automated Checker
As for previous practicals, we have provided some basic tests. In order to run the automated
checker on your program, upload your solution to a directory on the School server, log into the
lab machines through SSH, change directory to your CS5001-p3-networking directory, and
execute the following command:
stacscheck /cs/studres/CS5001/Practicals/p3-networking/Tests
If you use a Linux or Mac machine, you can also install stacscheck on your own computer.
When you are ready to submit, make sure that you also run the checker on the archive you are
preparing to submit, by calling, for example:
stacscheck –archive p3.zip /cs/studres/CS5001/Practicals/p3-networking/Tests
As in previous practicals, if the automated checker doesn’t run, or the build fails, or all comparison tests fail, you may have mistyped a command or not have followed the instructions
above.
Tests 1–14 check basic operation of your server. The first comparison test checks whether the
correct usage message is displayed when the command-line arguments are not supplied to your
server. The remaining tests check response header and content for a number of GET and HEAD
requests and one unsupported request. If one or more of these fails, then you are possibly not
sending back the expected header or content in your response where appropriate. In case of
failures, the tests should indicate what your program has sent back over the socket and what we
expected. Possible areas to examine include:
• opening, sending data on, flushing, and closing connections properly;
• use of <CR> and <LF> to delimit header fields and header from content (as indicated in
lectures);
• use of appropriate header fields and values in the response header;
• sending back the correct file content for the given request.
The final test runs the style checker over your source code.
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Test and Debug
In order to test your web server program manually, you should be able to use any browser on
the local machine and try to access http://localhost:12345/index.html to access index.html from
your server listening on port 12345 on the same machine.
Take care to put sufficient debug statements (using e.g. System.out.println) into your code so
as to help you locate errors in your code and e.g. print out the incoming request strings on the
server.
If you run into problems, you may find it useful to use simple client programs to send simple
requests to your server and print out the reply. You could use Curl & Telnet (on Linux) or
Curl & Putty in raw telnet mode (on Windows – see https://curl.haxx.se/download.html &
https://www.putty.org/) as simple client programs to test your server. On Linux, you could for
example type
curl -s -I -X GET localhost:12345/index.html
curl -s localhost:12345/index.html
to send a GET request for index.html to a server listening on port 12345 and display the response
header and body respectively. You can send a HEAD request by replacing GET with HEAD in
the first curl command above. You could also for example type
telnet localhost 12345
in a Terminal window to connect to a server you have listening on port 12345 on the same
machine. If your server program accepts the connection, you can then manually type in a
request into Telnet (such as GET /index.html HTTP/1.1 ) and check the responses from your
server program and its debug output.
In your own code, take care to flush sockets and close them cleanly or else you may not see
the responses made by the server on the client and you may have issues running your server
program due to sockets being left open.
You might also consider performing more rigorous automated testing. If you decide to adopt
this approach, it is probably a good idea to start by looking at the tests we have made available
to you at /cs/studres/CS5001/Practicals/p3-networking/Tests . You can create new tests
in a local sub-directory in your assignment directory and pass the directory of your own tests
to stacscheck when you run it from your assignment directory. Also, you should look at the
documentation for the automated checker at
https://studres.cs.st-andrews.ac.uk/Library/stacscheck/
Deliverables – Software (and Readme)
Along with your source code, you must include a short readme file which lists any of the advanced
requirements you have implemented in your assignment directory ( CS5001-p3-networking ).
You should hand in a zip archive of your assignment directory (containing all your source code,
any sub-directories, and readme), via MMS as usual.
You do not need to include a readme if you have not attempted any advanced requirements.
However, you should include a readme if there is anything else in your solution that you want
to draw our attention to.
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Marking
A very good attempt at satisfying the basic requirements above in an object-oriented fashion can
achieve a mark of 14–16. This means you should produce very good, re-usable code which makes
proper use of inheritance, association, and encapsulation with very good method decomposition.
To achieve a 17 or above, your code must in addition make a very good attempt at some of the
advanced requirements. Take care to test your solution exhaustively. See the standard mark
descriptors in the School Student Handbook:
https://info.cs.st-andrews.ac.uk/student-handbook/learning-teaching/feedback.html#Mark_
Descriptors
Lateness
The standard penalty for late submission applies (Scheme B: 1 mark per 8-hour period, or part
thereof):
https://info.cs.st-andrews.ac.uk/student-handbook/learning-teaching/assessment.html#
lateness-penalties
Good Academic Practice
The University policy on Good Academic Practice applies:
https://www.st-andrews.ac.uk/students/rules/academicpractice/
References
[1]: Berners-Lee, T., Fielding, R., Frystyk, H. et al. (1999), “Hypertext Transfer Protocol –
HTTP/1.1”, RFC 2616, available from: https://www.w3.org/Protocols/rfc2616/rfc2616.html
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