Also, the cache is a to be persistant across shutdowns. This means the data about which objects are cached is to be stored in some file when the proxy cache is not executing. If the file does not exist when the proxy cache begins execution, the cache is assumed to be empty. Otherwise, all web objects that have information stored in the cache from previous executions should result in a cache hit during the current execution.
In this lab you will develop a small web proxy server which is also able to cache web pages. This is a very simple proxy server which only understands simple GET-requests, but is able to handle all kinds of objects, not just HTML pages, but also images.
The code is divided into three classes as follows:
ProxyCacheholds the start-up code for the proxy and code for handling the requests.
HttpRequestcontains the routines for parsing and processing the incoming requests from clients.
HttpResponsetakes care of reading the replies from servers and processing them.
Your work will be to complete the proxy so that it is able to
receive requests, forward them, read replies, and return those to the
clients. You will need to complete the classes
HttpResponse. The places where you need to fill in code
are marked with /* Fill in */. Each place may require one or
more lines of code.
NOTE: As explained below, the proxy uses DataInputStreams for processing the replies from servers. This is because the replies are a mixture of textual and binary data and the only input streams in Java which allow treating both at the same time are DataInputStreams. To get the code to compile, you must use the -deprecation argument for the compiler as follows:
javac -deprecation *.java
If you do not use the -deprecation flag, the compiler will refuse to compile your code!
Running the proxy is as follows:
java ProxyCache portwhere port is the port number on which you want the proxy to listen for incoming connections from clients.
You will also need to configure your web browser to use your proxy. This depends on your browser. In Internet Explorer, you can set the proxy in "Internet Options" in the Connections tab under LAN Settings. In Netscape (and derived browsers, such as Mozilla), you can set the proxy in Edit->Preferences and then select Advanced and Proxies.
In both cases you need to give the address of the proxy and the port number which you gave when you started the proxy. You can run the proxy and browser on the same computer without any problems.
The proxy works as follows.
Your task is to complete the code which handles the above process. Most of the error handling in the proxy is very simple and it does not inform the client about errors. When there are errors, the proxy will simply stop processing the request and the client will eventually get a timeout.
Some browsers also send their requests one at a time, without using parallel connections. Especially in pages with lot of inlined images, this may cause the page to load very slowly.
Caching the responses in the proxy is left as an optional exercise, since it demands a significant amount of additional work. The basic functionality of caching goes as follows.
In practice, the proxy must verify that the cached responses are still valid and that they are the correct response to the client's request. You can read more about caching and how it is handled in HTTP in RFC 2068. For this lab, it is sufficient to implement the above simple policy.
Most of the code you need to write relates to processing HTTP requests and responses as well as handling Java sockets.
One point worth noting is the processing of replies from the server. In an HTTP response, the headers are sent as ASCII lines, separated by CRLF character sequences. The headers are followed by an empty line and the response body, which can be binary data in the case of images, for example.
Java separates the input streams according to whether they are text-based or binary, which presents a small problem in this case. Only DataInputStreams are able to handle both text and binary data simultaneously; all other streams are either pure text (e.g., BufferedReader), or pure binary (e.g., BufferedInputStream), and mixing them on the same socket does not generally work.
The DataInputStream has a small gotcha, because it is not able to guarantee that the data it reads can be correctly converted to the correct characters on every platform (DataInputStream.readLine() function). In the case of this lab, the conversion usually works, but the compiler will flag the DataInputStream.readLine()-method as deprecated and will refuse to compile without the -deprecation flag.
It is highly recommended that you use the DataInputStream for reading the response.
When you have finished the basic exercises, you can try the following optional exercises.