Introduction to Java Network Programming and HTTP

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

2. Outline • Hypertext Transfer Protocol – HTTP • 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: www.google.com Connection: close User-Agent: Mozilla 1.6 <CR><LF> • Request line, header lines, possible body :

6. HTTP Request Format <sp> URL <sp> Version <cr> <lf> Method 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 1998 12:00:15 GMT Connection: close Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 09:23:24 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 <sp> Status <sp> Phrase <cr> <lf> Version 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: • 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: 09.05.2007 at 15:00 • To be returned: Code with comments, test cases used • Send at bradler@tk.informatik.tu-darmstadt.de :

24. Questions? :