Introduction to Java Network Programming and HTTP

Telecooperation

Technische Universität Darmstadt

Copyrighted material; for TUD student use only

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Introduction to Java Network Programming and HTTP

FG TelekooperationProf. Max MühlhäuserDirk Bradler, Julian Schröder-Bernhardi

Prof. Dr. M. MühlhäuserTelekooperation

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Outline

• Hypertext Transfer Protocol – HTTP– Different versions– Requests and responses

• Java network programming– Opening sockets in Java– Reading and writing with sockets


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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


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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


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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

(HTTP/1.2) PEP?- Discontinued- Protocol Extension Protocol


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<cr> <lf>

Header field

:<sp> <cr> <lf>Value

<sp>

HTTP Request Format

Method <sp>

URL Version

<cr> <lf>

...

Entity body

Header lines

Request line

Header field



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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&...


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HTTP Responses

• Example response:

HTTP/1.1 200 OKDate: Thu, 06 Aug 1998 12:00:15 GMTConnection: closeServer: Apache/1.3.0 (Unix)Last-Modified: Mon, 22 Jun 1998 09:23:24 GMTContent-Length: 6821Content-Type: text/html<CR><LF>(data data data data data)

• Status line, header lines, requested document


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<cr> <lf>

Header field


<sp>

The BufferedReader Problem

Version <sp>

Status

Phrase <cr> <lf>

...

Entity body

Header lines

Status line

Header field



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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


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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


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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


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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


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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();


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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


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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


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Reading from a Socket

• Possible 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!


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Writing to a Socket

• Typical codeOutputStream 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!


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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


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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()


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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!


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Assignment 1

• Java Network Programming


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Assignment Details

• Assignment sheet on the Web page of the Praktikum• Milestones:

1. TCP client and server2. Simple Web server3. Web server improvements

• Important: pay attention to error-semantics, be conservative (i.e. follow the RFC) in what you send

• Deadline for returning: 05.11.2008 at 12:00• To be returned: Code with comments, test cases

used– Send at [email protected]– CC at [email protected]


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Questions?

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Introduction to Java Network Programming and HTTP