Assignment 1: application layer protocols

Assignment 1
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The objective of this assignment is to practice network programming and learn about application
layer protocols. Specifically, you will implement an HTTP client program to download web
objects from various web servers on the Internet.
2 Overview
In this assignment, you will implement a simple HTTP client from scratch. You have to write
code to establish a TCP connection to a given server and send and receive HTTP messages. Your
program should be able to download both binary and text objects. All downloaded objects are
stored locally.
Your program should maintain a list of the objects (called catalog) that are stored in the local storage.
The catalog will be used to check if an object needs to be downloaded from the Internet or
can be served from the local storage. In case a copy of the object already exists locally, your program
should request a new copy from the origin web server only if the object has been recently
updated on the server. This is achieved by implementing the conditional GET as described in
class. In case the requested object has been modified, your program should replace the local
copy with the new copy received from the web server. You should not blindly download every
object from the origin web server again.
To implement conditional GET, the Last-Modified information for the object being requested is
required. Thus, in addition to object URLs, the catalog should store the Last-Modified value
for each object as well. While there are a number of ways to implement a catalog, a simple Java
HashMap might be a good choice (you are free to choose other structures including a simple text
file). Make sure that every time an object is downloaded, the catalog is updated with the object’s
information (i.e., its URL and Last-Modified). The catalog should be saved in a file so that its
information is preserved between different runs of the program. For simplicity, every time the
catalog changes, save it in a file in the local directory where your program is running.
Skeleton code for a Java class called UrlCache is provided to you. You are asked to write code
to complete the implementation of this class. There are three methods in the class that you need
to implement. A description of these methods follows:
• UrlCache() throws UrlCacheException
This is the no-argument constructor that may include code for initialization. You should
also check to find out if a catalog file already exists. If so, initialize your catalog with the
information read from the saved catalog file.
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Assignment 1 CPSC 441
• void getObject(String url) throws UrlCacheException
This is the main method for downloading objects from the Internet. The parameter url is
a properly formatted URL that specifies the object to be downloaded. It has the following
format:
hostname[:port]/pathname
where, [:port] is an optional part which specifies the server port. If no server port is
specified, use the default port 80.
To download an object, you should establish a TCP connection to the server on the specified
port number. Then create an HTTP request using the object pathname and send the
request to the server. Once the request is submitted, start reading the HTTP response
from the socket and save the received object to a file. The file name should match the
hostname/pathname of the URL with the same directory structure being created locally inside
the directory where your program is running. You may find class File useful for this
purpose. Once a URL is downloaded, you need to record its Last-Modified value in the
catalog, which can be obtained from the header part of the HTTP response.
As mentioned earlier, your program should download a new copy of an object only if the
local copy is out of date by implementing conditional GET.
• long getLastModified(String url) throws UrlCacheException
This method simply returns the Last-Modified time associated with the object specified
by the parameter url. If the object is not in the catalog (i.e., does not exist locally) then
an exception is thrown. Otherwise, the time is returned in millisecond as in the method
getTime() in Java class Date. You may find class SimpleDateFormat with pattern "EEE,
dd MMM yyyy hh:mm:ss zzz" useful for this purpose.
The source code for the exception class UrlCacheException is provided to you on the assignment
page on D2L. The aforementioned methods should catch all possible exceptions/errors and in
turn throw UrlCacheException.
Restrictions
• You are not allowed to change the signature of the methods provided to you.
• You are not allowed to use the class URL or URLConnection for this assignment. Ask the
instructor if you are in doubt about any specific Java classes that you want to use in your
program.
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