1. Introduction to Java Network Programming and HTTP
FG Telekooperation
Dirk Bradler, Julian Schröder-Bernhardi :

2. Outline Hypertext Transfer Protocol – HTTP Java network programming
Different versions
Requests and responses
Java network programming
Opening sockets in Java
Reading and writing with sockets :

3. HyperText Transfer Protocol – HTTP
Origins in early 1990’s
Three versions of HTTP
HTTP/0.9 (name not official)
The original protocol, never standardized
Very simple, only one type of requests
HTTP/1.0
First “real” HTTP protocol version, RFC 1945
Widely used by the first browsers
Still quite a simple protocol
HTTP/1.1
Current standardized version, RFC 2616
Extends HTTP 1.0 in many ways, backwards compatible :

4. HTTP Connections Non-persistent connections (HTTP/1.0)
Use a new TCP connection for each object
Example: HTML page with 10 images  11 TCP conns!
Performance not good (TCP 3-way handshake, 2 extra RTTs)
TCP slow start for each connection
Persistent connections (HTTP/1.1)
Keep connection open for several requests  no slow start
Issue new request when old download finishes (extra RTT!)
No slow start, except at very beginning
Further improvement: Pipelining
Send several requests without waiting for reply
Continuous download!
Most efficient, but not always widely supported/implemented :

5. HTTP Requests Example request: GET /index.html HTTP/1.1
Host:
Connection: close
User-Agent: Mozilla 1.6
<CR><LF>
Request line, header lines, possible body :

6. HTTP Request Format Method <sp> URL <sp> Version
<cr>
<lf>
Request line
Header field
:<sp>
Value
<cr>
<lf>
...
Header lines
Header field
:<sp>
Value
<cr>
<lf>
<cr>
<lf>
Entity body :

7. HTTP Requests Method: GET, POST, HEAD, ... (see RFC 2616)
Normal requests GET
Retrieving web pages, images, etc.
Simple forms also processed with GET
Entity body in POST
POST used for complicated forms (lot of info to handle)
Contents of form in entity body
Parameters from forms separated by &
Example: field1=value1&field2=value2&... :

8. HTTP Responses Example response: HTTP/1.1 200 OK
Date: Thu, 06 Aug :00:15 GMT
Connection: close
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun :23:24 GMT
Content-Length: 6821
Content-Type: text/html
<CR><LF>
(data data data data data)
Status line, header lines, requested document :

9. The BufferedReader Problem
Version
<sp>
Status
<sp>
Phrase
<cr>
<lf>
Status line
Header field
:<sp>
Value
<cr>
<lf>
...
Header lines
Header field
:<sp>
Value
<cr>
<lf>
<cr>
<lf>
Entity body :

10. HTTP Responses Status code gives result
Phrase for humans, only code is important!
Typical status codes with standard phrases:
200 OK: Everything went fine, information follows
301 Moved Permanently: Document moved, new location in Location-header in response
400 Bad Request: Error in request
404 Not Found: Document does not exist on server
505 HTTP Version Not Supported: Requested protocol version not supported by server :

11. More about HTTP Above describes basic HTTP/1.0
Read RFC for information about different header fields and their meaning
HTTP/1.1 adds lot of new headers and features :

12. Java Network Programming
Network programming in Java in general much easier than in other languages...
...except some advanced things which are harder 
E.g. setting socket options
Java supports both TCP and UDP sockets
Many different ways to read/write sockets
Differentiates between text and binary 
Often several correct ways to handle socket
TIMTOWTDI: There Is More Than One Way To Do It :

13. Network Programming Guidelines
Robustness Principle
Be conservative in what you do; be liberal in what you accept from others.
Key ingredient for interoperability
Reformulation
Follow the standards for things you send
You may receive things that does not fully follow the standard
But: If you receive crap, you should return a well-defined error, don‘t try to understand it :

14. Using TCP Sockets Client side: String host = host to contact
Socket sock = new Socket(host, port);
String host = host to contact
Host can also be InetAddress instead of String
int port = port to be used (e.g. 80 for HTTP)
Server side
ServerSocket sock = new ServerSocket(port);
Listen for incoming connections
Socket client = sock.accept(); :

15. Using UDP Sockets Same for client and server
DatagramSocket sock = new DatagramSocket();
For server, give port number as argument
Send packets with send()
Receive packets with receive()
UDP packets implemented in DatagramPacket-class :

16. Reading and Writing TCP Sockets
Socket has InputStream and OutputStream
Need to wrap other streams around them
Some wrappers implement buffers
Java has many different I/O Streams
See Java API for others (e.g., reading files)
Relevant for sockets:
InputStreamReader, OutputStreamWriter
BufferedReader, BufferedWriter
DataInputStream, DataOutputStream :

17. Reading from a Socket Prossible code:
InputStream is = socket.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
Read text by calling br.readLine()
Can be used only for reading text!
Possible solution: Implement a Reader on your own without buffering to mix and match text and binary data! :

18. Writing to a Socket Typical code
OutputStream os = socket.getOutputStream();
OutputStreamWriter osw = new OutputStreamWriter(os);
BufferedWriter bw = new BufferedWriter(osw);
Write by calling one of many write()-functions
See the different classes for different possibilities
Can also write directly to OutputStream
For Streams, Strings need to be converted to bytes with getBytes()
Writer are only for text output! :

19. DataInputStream DataInputStream can read binary data from socket
Also can send primitive data types
Typical code
InputStream is = socket.getInputStream();
DataInputStream dis = new DataInputStream(is);
Read binary data with read() (see API for details)
Bonus functionality: Read text with readLine()
But: deprecated (does not work correctly in some cases), use in Praktikum results in a worse grade :

20. DataOutputStream DataOutputStream can be used to write Typical code:
OutputStream os = socket.getOutputStream();
DataOutputStream dos = new DataOutputStream(os);
DataOutputStream can also write text and binary
Has writeBytes()-function
 no need for String.getBytes() :

21. Differences Between Output Streams?!?
What is the difference between DataOutputStream and normal OutputStream wrapped with BufferedWriter?
Answer: There is almost no difference in practice
Some subtleties:
Possible problems with conversion between 8-bit and 16-bit characters (e.g., DataInputStream.readLine())
Possible text/binary data issues
Possible problems with buffering (use flush())
dos.writeBytes(str) vs. bw.write(str.getBytes())
No “correct” way, use either as long as it works
Be careful not to get confused! :

22. Assignment 1
Java Network Programming :

23. Assignment Details Assignment sheet on the Web page of the Praktikum
Milestones:
TCP client and server
Simple Web server
Web server improvements
Important: pay attention to error-semantics, be conservative (i.e. follow the RFC) in what you send
Deadline for returning: at 15:00
To be returned: Code with comments, test cases used
Send at :

24. Questions? :