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Networking with
Java
Introduction to Networking
Protocols
Hi
Hi
TCP connection
request
Client
Got the
time?
TCP connection
Server
reply
GET http://www.cs.huji.ac.il/~dbi
2:00
<file>
time
Internet Architecture Model
DATA
Application (HTTP, FTP)
HEADER
DATA
HEADER
HEADER
DATA
HEADER
HEADER
DATA
Transport (TCP,UDP)
Network (IP)
Link (LINK)
HEADER
TCP (Transmission-Control Protocol)
• Enables symmetric byte-stream transmission between
two endpoints (applications)
• Reliable communication channel
• TCP perform these tasks:
- connection establishment by handshake (relatively slow)
- division to numbered packets (transferred by IP)
- error correction of packets (checksum)
- acknowledgement and retransmission of packets
- connection termination by handshake
UDP (User Datagram Protocol)
• Enables direct datagram (packet) transmission from one
endpoint to another
• No reliability (except for data correction)
- sender does not wait for acknowledgements
- arrival order is not guaranteed
- arrival is not guaranteed
• Used when speed is essential, even in cost of reliability
- e.g., streaming media, games, Internet telephony, etc.
Ports
• A computer may have several applications that
communicate with applications on remote computers
through the same physical connection to the network
• When receiving a packet, how can the computer tell
which application is the destination?
• Solution: each channel endpoint is assigned a unique
port that is known to both the computer and the other
endpoint
Ports (cont)
• Thus, an endpoint application on the Internet is
identified by
- A host name → 32 bits IP-address
- A 16 bits port
• Q: Why don’t we specify the port in a Web
browser?
Known Ports
Client
Application
• Some known ports are
- 20, 21: FTP
mail client
- 23: TELNET
web browser
- 25: SMTP
- 110: POP3
- 80: HTTP
- 119: NNTP
21
23
25
110 80 119
Sockets
• A socket is a construct that represents one end-point of a
two-way communication channel between two programs
running on the network
• Using sockets, the OS provides processes a file-like
access to the channel
- i.e., sockets are allocated a file descriptor, and processes can
access (read/write) the socket by specifying that descriptor
• A specific socket is identified by the machine's IP and a
port within that machine
Sockets (cont)
• A socket stores the IP and port number of the other end-
point computer of the channel
• When writing to a socket, the written bytes are sent to
the other computer and port (e.g., over TCP/IP)
- That is, remote IP and port are attached to the packets
• When OS receives packets on the network, it uses their
destination port to decide which socket should get the
received bytes
Client-Server Model
• A common paradigm for distributed applications
• Asymmetry in connection establishment:
- Server waits for client requests (daemon) at a well known
address (IP+port)
- Connection is established upon client request
• Once the connection is made, it can be either symmetric
(TELNET) or asymmetric (HTTP)
• For example: Web servers and browsers
Client-Server Interaction
Client
80
Server
Client
Client-Server Interaction
6945
Client
80
Server
Client
Client-Server Interaction
6945
1932
Client
80
Server
Client
Client-Server Interaction
6945
1932
Client
80
Server
8002
Client
Client-Server Interaction
6945
1932
Client
80
2341
Server
8002
Client
Java Sockets
Low-Level Networking
Java Sockets
• Java wraps OS sockets (over TCP) by the objects of
class java.net.Socket
• new Socket(String remoteHost, int remotePort) creates a
TCP socket and connects it to the remote host on the
remote port (hand shake)
• Write and read using streams:
- InputStream getInputStream()
- OutputStream getOutputStream()
A Socket Example
import java.net.*;
import java.io.*;
public class SimpleSocket {
public static void main(String[] args) throws IOException {
... next slide ...
}
}
Socket socket = new Socket("www.cs.huji.ac.il", 80);
InputStream istream = socket.getInputStream();
OutputStream ostream = socket.getOutputStream();
String request =
"GET /~dbi/admin.html HTTP/1.1\r\n" +
"Host: www.cs.huji.ac.il\r\n" +
"Connection: close\r\n\r\n";
ostream.write(request.getBytes());
byte[] response = new byte[4096]; int bytesRead = -1;
while ((bytesRead = istream.read(response)) >= 0) {
System.out.write(response, 0, bytesRead);
}
socket.close();
Java ServerSocket
• ServerSocket represents a socket that listens and waits
for requests from clients
• Construction:
- new ServerSocket(int port)
- Why do want to specify the port?
• Listen and accept incoming connections
- Socket accept()
- returns a new socket (with a new port) for the new channel
- blocks until connection is made
public class EchoServer {
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(8000);
Socket socket = null;
while (true) {
try {
... next slide ...
}
catch (IOException exp) { ... }
finally {
try {if (!socket.isClosed()) socket.close(); }
catch (IOException e) {} }
}}}
socket = serverSocket.accept();
String clientName = socket.getInetAddress().getHostName();
BufferedReader reader = new BufferedReader(
new InputStreamReader(socket.getInputStream()));
PrintStream writer = new
PrintStream(socket.getOutputStream());
writer.println("Hello " + clientName + "!"); writer.flush();
String lineRead = null;
while ((lineRead = reader.readLine()) != null) {
writer.println("You wrote: " + lineRead);
writer.flush();
}
Accepting Connections
• Usually, the accept() method is executed within
an infinite loop
- i.e., while(true){...}
• Whenever accept() returns, a new thread is
launched to handle that interaction
• Hence, the server can handle several requests
concurrently
Timeout
• You can set timeout values to these blocking
methods:
- read() of Socket
- accept() of ServerSocket
• Use the method setSoTimeout(milliseconds)
• If timeout is reached before the method returns,
java.net.SocketTimeoutException is thrown
Java Sockets and HTTP
HTTP Message Structure
• A HTTP message has the following structure:
Request/Status-Line \r\n
Header1: value1 \r\n
Header2: value2 \r\n
...
HeaderN: valueN \r\n
\r\n
Message-Body
Reading HTTP Messages
• Several ways to interpret the bytes of the body
- Binary: images, compressed files, class files, ...
- Text: ASCII, Latin-1, UTF-8, ...
• Commonly, applications parse the headers of the
message, and process the body according to the
information supplied by the headers
- E.g., Content-Type, Content-Encoding, TransferEncoding
An Example
Parsing the Headers
• So how are the headers themselves are represented?
• Answer: The headers of a HTTP message must be in
US-ASCII format (1 byte per character)
• However, in order to parse the headers, it may be
impossible to use readLine() of BufferedReader,
since the body should be later read as binary bytes
• Hence, we convert bytes to characters
Example: Extracting the Headers
Socket socket = new Socket(argv[0], 80);
InputStream istream = socket.getInputStream();
OutputStream ostream = socket.getOutputStream();
String request = "GET / HTTP/1.1\r\n" +
"Host: " + argv[0] + "\r\n" +
"Connection: close\r\n\r\n";
ostream.write(request.getBytes());
StringBuffer headers = new StringBuffer(); int byteRead = 0;
while ( !endOfHeaders(headers) &&
(byteRead = istream.read()) >= 0) {
headers.append((char) byteRead);
}
System.out.print(headers);
socket.close();
Example: Extracting the Headers
(cont)
public static boolean endOfHeaders(StringBuffer headers) {
int lastIndex = headers.length() - 1;
if (lastIndex < 3 || headers.charAt(lastIndex) != '\n')
return false;
return ( headers.substring(lastIndex - 3, lastIndex + 1)
.equals("\r\n\r\n"));
}
Persistent Connections
• According to HTTP/1.1, a server does not have to close
the connection after fulfilling your request
• One connection (socket) can be used for several requests
and responses send more requests
- even while earlier responses are being transferred (pipelining)
• Question: how can the client know when one response
ends and a new one begins?
• To avoid persistency, require explicitly by the header:
Connection: close
URL and URLConnection
High-Level Networking
Working with URLs
• URL (Uniform Resource Locator):
a reference (an address) to a resource on the Internet
http://www.cs.huji.ac.il:80/~dbi/main.html#notes
Protocol
Host
Name
Port
Number
File
Name
Reference
The Class URL
• The class URL is used for parsing URLs
• Constructing URLs:
- URL w3c1 = new URL("http://www.w3.org/TR/");
- URL w3c2 = new URL("http","www.w3.org",80,"TR/");
- URL w3c3 = new URL(w3c2, "xhtml1/");
• If the string is not an absolute URL, then it is considered
relative to the URL
Parsing URLs
• The following methods of URL can be used for
parsing URLs
getProtocol(), getHost(), getPort(), getPath(),
getFile(), getQuery(), getRef(), getQuery()
The class URLConnection
• To establish the actual resource, we can use the object
URLConnection obtained by url.openConnection()
• If the protocol of the URL is HTTP, the returned object
is of class HttpURLConnection
• This class encapsulates all socket management and
HTTP directions required to obtain the resource
public class ContentExtractor {
public static void main(String[] argv) throws Exception {
URL url = new URL(argv[0]);
System.out.println("Host: " + url.getHost());
System.out.println("Protocol: " + url.getProtocol());
System.out.println("----");
URLConnection con = url.openConnection();
InputStream stream = con.getInputStream();
}}}
byte[] data = new byte[4096]; int bytesRead = 0;
while((bytesRead=stream.read(data))>=0) {
System.out.write(data,0,bytesRead);
About URLConnection
• The life cycle of a URLConnection object has two
parts:
- Before actual connection establishment
• Connection configuration
- After actual connection establishment
• Content retrieval
• Passage from the first phase to the second is implicit
- A result of calling some committing methods, like getDate()
About HttpURLConnection
• The HttpURLConnection class encapsulates all HTTP
transaction over sockets, e.g.,
- Content decoding
- Redirection
- Proxy indirection
• However, you can control requests by its methods
- setRequestMethod, setFollowRedirects,
setRequestProperty, ...
Sending POST Requests
• In order to send POST requests with
HttpURLConnection, you have to do the following:
- Enable connection output: con.setDoOutput(true);
- Get the output stream: con.getOutputStream()
• This changes the method from GET to POST
- Write the message body into the stream
- close the stream (important!)
URL url = new URL("http://find.walla.co.il/");
URLConnection connection = url.openConnection();
connection.setDoOutput(true);
String query = "q=" + URLEncoder.encode(args[0], "UTF-8");
OutputStream out = connection.getOutputStream();
out.write(query.getBytes());
out.close();
InputStream in = connection.getInputStream();
int bytesRead = -1; byte[] response = new byte[4096];
while ((bytesRead = in.read(response)) >= 0)
System.out.write(response, 0, bytesRead);
in.close();
Sent Headers
java SearchWalla "Java networking"
POST / HTTP/1.1
User-Agent: Java/1.5.0_01
Host: find.walla.co.il
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
Connection: keep-alive
Content-type: application/x-www-form-urlencoded
Content-Length: 18
q=Java+networking
URLEncoder
• Contains a utility method encode for converting a string
into an encoded format (used in URLs)
• To convert a string, each character is examined in turn:
- Space is converted into a plus sign +
- a-z, A-Z, 0-9, ., -, * and _ remain the same.
- The bytes of all special characters are replaced by hexadecimal
numbers, preceded with %
• To decode an encoded string, use decode() of the class
URLDecoder
Defining Default Proxy
• For reading a URL using a proxy, we run java with the
environment variables for http.proxyHost and
http.proxyPort set properly:
java -DproxyHost=wwwproxy.huji.ac.il –DproxyPort=8080 ...
• Another option is to set the environment variables in the
program itself
System.getProperties().put( "proxyHost", "wwwproxy.huji.ac.il" );
System.getProperties().put( "proxyPort", "8080" );
Not Covered: UDP Connections
• The URL, URLConnection, Socket and
ServerSocket classes all use the TCP protocol to
communicate over the network
• Java programs can also use UDP protocol for
communication
• For that, use DatagramPacket, DatagramSocket,
and MulticastSocket classes