Lab 2 Primer

1
Lab 2 Primer

• Assignment 3 Structure
• File I/O
• More parsing and HTTP Formatting

2
Assignment Reviewso far

• We explored sockets
• We looked at how to connect to them, how to
  communicate (I/O) through them, how to parse the
  data through them, and briefly how data is
  interpreted through them
• We explored the idea of clients and servers
• We looked at connecting to a server through
  telnet, making a client to connect via sockets,
  protocol schemes (how they understand each
  other), extracting information from sent
  messages, and multiple simultaneous user support
  (threads and why we need them)

3
Now

• We will now extend the knowledge weve gained
  with more practice at the same principles
• We will add file I/O to the mix
• We will continue with string parsing
• We will add to our use of the HTTP protocol

4
Assignment 3 Structure

• We need to add
• File I/O to serve html files
• Parsing to get key-value pairs from either the
  query string, body or both
• Parsing to get requested resource from request
• Proper HTTP responses (200 OK, 404 File Not
  Found, etc.)
• Content-Length and Content-Type headers

5
Parsing the Query String and Requested File

• Use same structure as getContentLength() from
  Assignment 2 to find whole path/filequery
• Use .indexOf( , 5) to find the first space
  after GET or POST /
• Use indexOf(?) to find the query string (if
  any)
• If there is no match, indexOf() returns -1
• Use .substring to get file request and query
  string

6
Parsing the Key-Value pairs

• We need to make 2 arrays one for keys and one
  for values
• A key-value pair should be indexed with the same
  value

Index i0 i1 i2 i3
key 1
value 1
key 2
value 2
key 3
value 3
key 4
value 4
7
Parsing the Key-Value pairs

• New method .split()
• Splits a String into an array of strings based on
  a delimiter
• New class StringTokenizer(String, delimiter)
• Breaks a String into tokens based on delimiters
• Can iterate through tokens in string

8
Parsing the Key-Value pairs

• String temp "GET /index.html/?Thisisequalsstr
  inguserfoo"
• String keystemp.split(")
• int length temp2.length
• String values new Stringlength
• StringTokenizer st new StringTokenizer(temp,
  )
• int i
• for(i0 iltlength i)
• keysi st.nextToken()
• valuesi st.nextToken()

9
File Input/Output

• Java is designed for simple I/O
• ALL I/O works the same keyboard, monitor, files,
  sockets, etc.
• All I/O can be cast to higher level objects
• An input/output stream can be cast to a
  reader/writer or a buffered I/O class
• If you understand socket I/O, file I/O will be
  very simple
• There are 2 file I/O classes we will use
• File fin new File(/html)
• FileReader fileIn new FileReader(filename)

10
File I/O Classes

• File
• Opens a local file
• Can be used to quickly find length of a text file
• .length()
• File fin new File (/path/to/file)
• System.out.print(fin.length())

11
File I/O Classes

• FileReader
• Opens the file specified in filename throws
  FileNotFoundException if the file cannot be
  opened
• Use with BufferedReader
• BufferedReader fileIn new BufferedReader(new
  FileReader(File))

12
File I/O Basics

• Much like a socket, keyboard or monitor, file I/O
  can use the read(), readLine(), print(),
  println() methods to get data in and out of a
  file
• When reading from a file, the end can be
  determined
• When the input with readLine() is null
• When the input with read() is 1
• A file pointer keeps track of where in the file
  we are currently located (see example on next
  page)
• If we want to write a line where the file pointer
  currently is located, we will overwrite the line
  following the pointer. If the pointer is at the
  end of the file, we will write a new line

13
Final Thoughts

• Feel free to either refer to the sample solution,
  or to use it as the basis for the next
  assignment. Thats what it is there for.
• If you have questions, ask! There is nothing to
  gain from silence (except a worse mark)!
• Try this early! You have plenty of time to finish
  this. Do not wait until the last minute!