Description
1 Objective
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 a
web object specified by its URL.
2 Specification
2.1 Overview
In this assignment, you will implement an HTTP client called HttpClient 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 in the same directory where your program is running.
2.2 Implementation
A high-level description of the internal operation of HttpClient is presented in Program 1.
Program 1 HttpClient
Input Parameter: url
1: Parse the input url to extract server address and object path
2: Establish a TCP connection with the server
3: Send a GET request for the specified object
4: Read the server response status and header lines
5: if response status is OK then
6: Create the local file with the object name
7: while not end of input stream do
8: Read from the socket and write to the local file
9: end while
10: end if
11: Clean up (e.g., close the streams and socket)
The client program HttpClient should operate in non-persistent HTTP mode. This means that
once the object is downloaded, the client closes the underlying TCP connection. The program
takes as input the URL of the object to be downloaded. The input URL is a properly formatted
URL with the following syntax:
http://hostname[:port]/[pathname]
where,
• [:port] is an optional part which specifies the server port. If no port is specified, use
the default port 80
2
Assignment 2 CPSC 441
• [pathname] is an optional part which specifies the object path on the server. If no
pathname is specified, use the default name index.html
2.3 Sending GET Request
To send an HTTP GET request, you should establish a TCP connection to the server on the
specified port number. Then create a request using the pathname and send the request to the
server. For writing ASCII formatted data to a socket, e.g., HTTP headers, you can create a string
object and then call String.getBytes(“US-ASCII”) to convert the string to a sequence of
bytes that can be written to the socket. Make sure to flush the output stream so that the data is
actually written to the socket.
Once the request is submitted, start reading the HTTP response from the socket. The local file
name of the downloaded object should match the file name specified by the URL. For example,
if the input URL is
http://ict746x.cs.ucalgary.ca/files/medium.pdf
then the object should be saved in file medium.pdf in the working directory of the client program.
2.4 Constructing GET Request
To be compliant with what most web servers expect from a well-behaving web client, include
the following header lines in your GET request:
• Host:
• Connection: close
For the protocol version, you can specify HTTP/1.1.
2.5 Reading Server Response
You can use method InputStream.read() on the socket input stream to read an array of
bytes that can be converted to a String object for parsing. Remember that each line of the response header is terminated with character sequence “\r\n”. Once you have read the entire
header, you know the length of the body of the response, and subsequently can use method
InputStream.read(byte[]) to read the body of the response.
3 Software Interfaces
3.1 Method Signatures
The required method signatures for class HttpClient are provided to you in the source file
HttpClient.java. There is only one method that you need to implement, namely method
3
Assignment 2 CPSC 441
get(). Refer to the Javadoc documentation provided in the source file for more information on
this method.
3.2 Exception Handling
Your implementation should include exception handling code to deal with all checked exceptions in your program. Print exception messages (the stack trace) to the standard system output.
3.3 Running Your Code
A driver class named HttpDriver is provided on D2L to demonstrate how we are going to
run your code. Read the inline documentation in the source file HttpDriver.java for detailed information on how to use the driver class. There are also a number of test URLs in the
driver class that can be used to test the functionality of your program with a web server on
ict746x.cs.ucalgary.ca, although you should be able to use any HTTP web server (e.g.,
python -m http.server) for testing purposes.
3.4 Console Output
Add print statements (using System.out.println) in your code to print the following information on the console:
1. HTTP request (request line and header lines)
2. HTTP response (status line and header lines only)
Do not directly print anything else to the console beyond exception messages and the above
information. For debugging purposes, you can use the global logger object defined in the
driver class, whose level can be set using the command line option -v. Refer to the driver class
source file for more information. The logger can be used to write messages to console during
code development.
3.5 Design Document
Prepare and submit a design document (one page) to describe how you read and parse the
server response. Explain what stream you used and how you read header lines and the object
body. Follow the design document formatting requirements described on D2L.
Restrictions
• You are not allowed to modify the method signatures provided to you. However, you can
implement additional methods and classes as needed in your implementation.
• You are not allowed to use package java.net.http, and URL, URI, URLConnection
classes or their subclasses for this assignment. Ask the instructor if you are in doubt about
any specific Java classes that you want to use in your program.
4