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Java Network Programming Elliotte Rusty Harold elharo@metalab.unc.edu http://metalab.unc.edu/javafaq/slides/ We will learn how Java handles Internet Addresses URLs CGI Sockets Server Sockets UDP I assume you Understand basic Java syntax and I/O Have a user’s view of the Internet

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java network programming

Java Network Programming

Elliotte Rusty Harold

elharo@metalab.unc.edu

http://metalab.unc.edu/javafaq/slides/

we will learn how java handles
We will learn how Java handles
  • Internet Addresses
  • URLs
  • CGI
  • Sockets
  • Server Sockets
  • UDP
i assume you
I assume you
  • Understand basic Java syntax and I/O
  • Have a user’s view of the Internet
  • No prior network programming experience
applet network security restrictions
Applet Network Security Restrictions
  • Applets may:
    • send data to the code base
    • receive data from the code base
  • Applets may not:
    • send data to hosts other than the code base
    • receive data from hosts other than the code base
some background
Some Background
  • Hosts
  • Internet Addresses
  • Ports
  • Protocols
hosts
Hosts
  • Devices connected to the Internet are called hosts
  • Most hosts are computers, but hosts also include routers, printers, fax machines, soda machines, bat houses, etc.
internet addresses
Internet addresses
  • Every host on the Internet is identified by a unique, four-byte Internet Protocol (IP) address.
  • This is written in dotted quadformat like 199.1.32.90 where each byte is an unsigned integer between 0 and 255.
  • There are about four billion unique IP addresses, but they aren’t very efficiently allocated
domain name system dns
Domain Name System (DNS)
  • Numeric addresses are mapped to names like www.blackstar.com or star.blackstar.com by DNS.
  • Each site runs domain name server software that translates names to IP addresses and vice versa
  • DNS is a distributed system
the inetaddress class
The InetAddress Class
  • The java.net.InetAddress class represents an IP address.
  • It converts numeric addresses to host names and host names to numeric addresses.
  • It is used by other network classes like Socket and ServerSocket to identify hosts
creating inetaddresses
Creating InetAddresses
  • There are no public InetAddress() constructors. Arbitrary addresses may not be created.
  • All addresses that are created must be checked with DNS
the getbyname factory method
The getByName() factory method

public static InetAddress getByName(String host) throws UnknownHostException

InetAddress utopia, duke;

try {

utopia = InetAddress.getByName("utopia.poly.edu");

duke = InetAddress.getByName("128.238.2.92");

}

catch (UnknownHostException e) {

System.err.println(e);

}

other ways to create inetaddress objects
Other ways to create InetAddress objects

public static InetAddress[] getAllByName(String host) throws UnknownHostException

public static InetAddress getLocalHost() throws UnknownHostException

getter methods
Getter Methods
  • public boolean isMulticastAddress()
  • public String getHostName()
  • public byte[] getAddress()
  • public String getHostAddress()
utility methods
Utility Methods
  • public int hashCode()
  • public boolean equals(Object o)
  • public String toString()
ports
Ports
  • In general a host has only one Internet address
  • This address is subdivided into 65,536 ports
  • Ports are logical abstractions that allow one host to communicate simultaneously with many other hosts
  • Many services run on well-known ports. For example, http tends to run on port 80
protocols
Protocols
  • A protocol defines how two hosts talk to each other.
  • The daytime protocol, RFC 867, specifies an ASCII representation for the time that's legible to humans.
  • The time protocol, RFC 868, specifies a binary representation, for the time that's legible to computers.
  • There are thousands of protocols, standard and non-standard
ietf rfcs
IETF RFCs
  • Requests For Comment
  • Document how much of the Internet works
  • Various status levels from obsolete to required to informational
  • TCP/IP, telnet, SMTP, MIME, HTTP, and more
  • http://www.faqs.org/rfc/
w3c standards
W3C Standards
  • IETF is based on “rough consensus and running code”
  • W3C tries to run ahead of implementation
  • IETF is an informal organization open to participation by anyone
  • W3C is a vendor consortium open only to companies
w3c standards19
W3C Standards
  • HTTP
  • HTML
  • XML
  • RDF
  • MathML
  • SMIL
  • P3P
slide20
URLs
  • A URL, short for "Uniform Resource Locator", is a way to unambiguously identify the location of a resource on the Internet.
example urls
Example URLs

http://java.sun.com/

file:///Macintosh%20HD/Java/Docs/JDK%201.1.1%20docs/api/java.net.InetAddress.html#_top_

http://www.macintouch.com:80/newsrecent.shtml

ftp://ftp.info.apple.com/pub/

mailto:elharo@metalab.unc.edu

telnet://utopia.poly.edu

ftp://mp3:mp3@138.247.121.61:21000/c%3a/stuff/mp3/

http://elharo@java.oreilly.com/

http://metalab.unc.edu/nywc/comps.phtml?category=Choral+Works

the pieces of a url
The Pieces of a URL
  • the protocol, aka scheme
  • the authority
    • user info
      • user name
      • password
    • host name or address
    • port
  • the path, a.k.a. file
  • the ref, a.k.a. section or anchor
  • the query string
the java net url class
The java.net.URL class
  • A URL object represents a URL.
  • The URL class contains methods to
    • create new URLs
    • parse the different parts of a URL
    • get an input stream from a URL so you can read data from a server
    • get content from the server as a Java object
content and protocol handlers
Content and Protocol Handlers
  • Content and protocol handlers separate the data being downloaded from the the protocol used to download it.
  • The protocol handler negotiates with the server and parses any headers. It gives the content handler only the actual data of the requested resource.
  • The content handler translates those bytes into a Java object like an InputStream or ImageProducer.
finding protocol handlers
Finding Protocol Handlers
  • When the virtual machine creates a URL object, it looks for a protocol handler that understands the protocol part of the URL such as "http" or "mailto".
  • If no such handler is found, the constructor throws a MalformedURLException.
supported protocols
Supported Protocols
  • The exact protocols that Java supports vary from implementation to implementation though http and file are supported pretty much everywhere. Sun's JDK 1.1 understands ten:
    • file
    • ftp
    • gopher
    • http
    • mailto
  • appletresource
  • doc
  • netdoc
  • systemresource
  • verbatim
url constructors
URL Constructors
  • There are four (six in 1.2) constructors in the java.net.URL class.

public URL(String u) throws MalformedURLException

public URL(String protocol, String host, String file) throws MalformedURLException

public URL(String protocol, String host, int port, String file) throws MalformedURLException

public URL(URL context, String url) throws MalformedURLException

public URL(String protocol, String host, int port, String file, URLStreamHandler handler) throws MalformedURLException

public URL(URL context, String url, URLStreamHandler handler) throws MalformedURLException

constructing url objects
Constructing URL Objects
  • An absolute URL like http://www.poly.edu/fall97/grad.html#cs

try {

URL u = new URL( "http://www.poly.edu/fall97/grad.html#cs");

}

catch (MalformedURLException e) {}

constructing url objects in pieces
Constructing URL Objects in Pieces
  • You can also construct the URL by passing its pieces to the constructor, like this:
      • URL u = null;
      • try {
      • u = new URL("http", "www.poly.edu", "/schedule/fall97/bgrad.html#cs");
      • }
      • catch (MalformedURLException e) {}
including the port
Including the Port
  • URL u = null;
  • try {
  • u = new URL("http", "www.poly.edu", 8000, "/fall97/grad.html#cs");
  • }
  • catch (MalformedURLException e) {}
relative urls
Relative URLs
  • Many HTML files contain relative URLs.
  • Consider the page http://metalab.unc.edu/javafaq/index.html
  • On this page a link to “books.html" refers to http://metalab.unc.edu/javafaq/books.html.
constructing relative urls
Constructing Relative URLs
  • The fourth constructor creates URLs relative to a given URL. For example,

try {

URL u1 = new URL("http://metalab.unc.edu/index.html");

URL u2 = new URL(u1, "books.html");

}

catch (MalformedURLException e) {}

  • This is particularly useful when parsing HTML.
parsing urls
Parsing URLs
  • The java.net.URL class has five methods to split a URL into its component parts. These are:

public String getProtocol()

public String getHost()

public int getPort()

public String getFile()

public String getRef()

for example
For example,
  • try {
  • URL u = new URL("http://www.poly.edu/fall97/grad.html#cs ");
  • System.out.println("The protocol is " + u.getProtocol());
  • System.out.println("The host is " + u.getHost());
  • System.out.println("The port is " + u.getPort());
  • System.out.println("The file is " + u.getFile());
  • System.out.println("The anchor is " + u.getRef());
  • }
  • catch (MalformedURLException e) { }
parsing urls35
Parsing URLs
  • JDK 1.3 adds three more:

public String getAuthority()

public String getUserInfo()

public String getQuery()

missing pieces
Missing Pieces
  • If a port is not explicitly specified in the URL it's set to -1. This means the default port.
  • If the ref doesn't exist, it's just null, so watch out for NullPointerExceptions. Better yet, test to see that it's non-null before using it.
  • If the file is left off completely, e.g. http://java.sun.com, then it's set to "/".
reading data from a url
Reading Data from a URL
  • The openStream() method connects to the server specified in the URL and returns an InputStream object fed by the data from that connection.
      • public final InputStream openStream() throws IOException
  • Any headers that precede the actual data are stripped off before the stream is opened.
  • Network connections are less reliable and slower than files. Buffer with a BufferedReader or a BufferedInputStream.
webcat
Webcat

import java.net.*;

import java.io.*;

public class Webcat {

public static void main(String[] args) {

for (int i = 0; i < args.length; i++) {

try {

URL u = new URL(args[i]);

InputStream in = u.openStream();

InputStreamReader isr = new InputStreamReader(in);

BufferedReader br = new BufferedReader(isr);

String theLine;

while ((theLine = br.readLine()) != null) {

System.out.println(theLine);

}

} catch (IOException e) { System.err.println(e);}

}

}

}

the bug in readline
The Bug in readLine()
  • What readLine() does:
    • Sees a carriage return, waits to see if next character is a line feed before returning
  • What readLine() should do:
    • Sees a carriage return, return, throw away next character if it's a linefeed
webcat40
Webcat

import java.net.*;

import java.io.*;

public class Webcat {

public static void main(String[] args) {

for (int i = 0; i < args.length; i++) {

try {

URL u = new URL(args[i]);

InputStream in = u.openStream();

InputStreamReader isr = new InputStreamReader(in);

BufferedReader br = new BufferedReader(isr);

int c;

while ((c = br.read()) != -1) {

System.out.write(c);

}

} catch (IOException e) { System.err.println(e);}

}

}

}

slide41
CGI
  • Common Gateway Interface
  • A lot is written about writing server side CGI. I’m going to show you client side CGI.
  • We’ll need to explore HTTP a little deeper to do this
normal web surfing uses these two steps
Normal web surfing uses these two steps:
    • The browser requests a page
    • The server sends the page
  • Data flows primarily from the server to the client.
forms
Forms
  • There are times when the server needs to get data from the client rather than the other way around. The common way to do this is with a form like this one:
slide44
CGI
  • The user types data into a form and hits the submit button.
  • The browser sends the data to the server using the Common Gateway Interface, CGI.
  • CGI uses the HTTP protocol to transmit the data, either as part of the query string or as separate data following the MIME header.
get and post
GET and POST
  • When the data is sent as a query string included with the file request, this is called CGI GET.
  • When the data is sent as data attached to the request following the MIME header, this is called CGI POST
slide46
HTTP
  • Web browsers communicate with web servers through a standard protocol known as HTTP, an acronym for HyperText Transfer Protocol.
  • This protocol defines
    • how a browser requests a file from a web server
    • how a browser sends additional data along with the request (e.g. the data formats it can accept),
    • how the server sends data back to the client
    • response codes
a typical http connection
A Typical HTTP Connection
  • Client opens a socket to port 80 on the server.
  • Client sends a GET request including the name and path of the file it wants and the version of the HTTP protocol it supports.
  • The client sends a MIME header.
  • The client sends a blank line.
  • The server sends a MIME header
  • The server sends the data in the file.
  • The server closes the connection.
what the client sends to the server
What the client sends to the server

GET /javafaq/images/cup.gif HTTP/1.0

Connection: Keep-Alive

User-Agent: Mozilla/3.01 (Macintosh; I; PPC)

Host: www.oreilly.com:80

Accept: image/gif, image/x-xbitmap, image/jpeg, */*

slide49
MIME
  • MIME is an acronym for "Multipurpose Internet Mail Extensions".
  • an Internet standard defined in RFCs 2045 through 2049
  • originally intended for use with email messages, but has been been adopted for use in HTTP.
browser request mime header
Browser Request MIME Header
  • When the browser sends a request to a web server, it also sends a MIME header.
  • MIME headers contain name-value pairs, essentially a name followed by a colon and a space, followed by a value.

Connection: Keep-Alive

User-Agent: Mozilla/3.01 (Macintosh; I; PPC)

Host: www.digitalthink.com:80

Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*

server response mime header
Server Response MIME Header
  • When a web server responds to a web browser it sends a response message and a MIME header along with the response that looks something like this:
      • HTTP/1.0 200 OK
      • Server: Netscape-Enterprise/2.01
      • Date: Sat, 02 Aug 1997 07:52:46 GMT
      • Accept-ranges: bytes
      • Last-modified: Tue, 29 Jul 1997 15:06:46 GMT
      • Content-length: 2810
      • Content-type: text/html
query strings
Query Strings
  • CGI GET data is sent in URL encoded query strings
  • a query string is a set of name=value pairs separated by ampersands
      • Author=Sadie, Julie&Title=Women Composers
  • separated from rest of URL by a question mark
url encoding
URL Encoding
  • Alphanumeric ASCII characters (a-z, A-Z, and 0-9) and the $-_.!*'(), punctuation symbols are left unchanged.
  • The space character is converted into a plus sign (+).
  • Other characters (e.g. &, =, ^, #, %, ^, {, and so on) are translated into a percent sign followed by the two hexadecimal digits corresponding to their numeric value.
for example54
For example,
  • The comma is ASCII character 44 (decimal) or 2C (hex). Therefore if the comma appears as part of a URL it is encoded as %2C.
  • The query string "Author=Sadie, Julie&Title=Women Composers" is encoded as:

Author=Sadie%2C+Julie&Title=Women+Composers

the urlencoder class
The URLEncoder class
  • The java.net.URLEncoder class contains a single static method which encodes strings in x-www-form-url-encoded format

URLEncoder.encode(String s)

for example56
For example,

String qs = "Author=Sadie, Julie&Title=Women Composers";

String eqs = URLEncoder.encode(qs);

System.out.println(eqs);

  • This prints:
      • Author%3dSadie%2c+Julie%26Title%3dWomen+Composers
slide57
String eqs = "Author=" + URLEncoder.encode("Sadie, Julie");
      • eqs += "&";
      • eqs += "Title=";
      • eqs += URLEncoder.encode("Women Composers");
  • This prints the properly encoded query string:
      • Author=Sadie%2c+Julie&Title=Women+Composers
the urldecoder class
The URLDecoder class
  • In Java 1.2 the java.net.URLDecoder class contains a single static method which decodes strings in x-www-form-url-encoded format

URLEncoder.decode(String s)

get urls
GET URLs

String eqs =

"Author=" + URLEncoder.encode("Sadie, Julie");

eqs += "&";

eqs += "Title=";

eqs += URLEncoder.encode("Women Composers");

try {

URL u = new URL("http://www.superbooks.com/search.cgi?" + eqs);

InputStream in = u.openStream();

//...

}

catch (IOException e) { //...

urlconnections
URLConnections
  • The java.net.URLConnection class is an abstract class that handles communication with different kinds of servers like ftp servers and web servers.
  • Protocol specific subclasses of URLConnection handle different kinds of servers.
  • By default, connections to HTTP URLs use the GET method.
urlconnections vs urls
URLConnections vs. URLs
  • Can send output as well as read input
  • Can post data to CGIs
  • Can read headers from a connection
urlconnection five steps
URLConnection five steps:

1. The URL is constructed.

2. The URL’s openConnection() method creates the URLConnection object.

3. The parameters for the connection and the request properties that the client sends to the server are set up.

4. The connect() method makes the connection to the server. (optional)

5. The response header information is read using getHeaderField().

i o across a urlconnection
I/O Across a URLConnection
  • Data may be read from the connection in one of two ways
    • raw by using the input stream returned by getInputStream()
    • through a content handler with getContent().
  • Data can be sent to the server using the output stream provided by getOutputStream().
for example64
For example,
  • try {
  • URL u = new URL("http://www.wwwac.org/");
  • URLConnection uc = u.openConnection();
  • uc.connect();
  • InputStream in = uc.getInputStream();
  • // read the data...
  • }
  • catch (IOException e) { //...
reading header data
Reading Header Data
  • The getHeaderField(String name) method returns the string value of a named header field.
  • Names are case-insensitive.
  • If the requested field is not present, null is returned.

String lm = uc.getHeaderField("Last-modified");

getheaderfieldkey
getHeaderFieldKey()
  • The keys of the header fields are returned by the getHeaderFieldKey(int n) method.
  • The first field is 1.
  • If a numbered key is not found, null is returned.
  • You can use this in combination with getHeaderField() to loop through the complete header
for example67
For example
  • String key = null;
  • for (int i=1; (key = uc.getHeaderFieldKey(i))!=null); i++) {
  • System.out.println(key + ": " + uc.getHeaderField(key));
  • }
getheaderfieldint and getheaderfielddate
getHeaderFieldInt() and getHeaderFieldDate()
  • Utility methods that read a named header and convert its value into an int and a long respectively.

public int getHeaderFieldInt(String name, int default)

public long getHeaderFieldDate(String name, long default)

slide69
The long returned by getHeaderFieldDate() can be converted into a Date object using a Date() constructor like this:
      • long lm = uc.getHeaderFieldDate("Last-modified", 0);
      • Date lastModified = new Date(lm);
six convenience methods
Six Convenience Methods
  • These return the values of six particularly common header fields:
      • public int getContentLength()
      • public String getContentType()
      • public String getContentEncoding()
      • public long getExpiration()
      • public long getDate()
      • public long getLastModified()
slide71
try {

URL u = new URL("http://www.sdexpo.com/");

URLConnection uc = u.openConnection();

uc.connect();

String key=null;

for (int n = 1;

(key=uc.getHeaderFieldKey(n)) != null;

n++) {

System.out.println(key + ": " + uc.getHeaderField(key));

}

}

catch (IOException e) {

System.err.println(e);

}

writing data to a urlconnection
Writing data to a URLConnection
  • Similar to reading data from a URLConnection.
  • First inform the URLConnection that you plan to use it for output
  • Before getting the connection's input stream, get the connection's output stream and write to it.
  • Commonly used to talk to CGIs that use the POST method
eight steps
Eight Steps:

1. Construct the URL.

2. Call the URL’s openConnection() method to create the URLConnection object.

3. Pass true to the URLConnection’s setDoOutput() method

4. Create the data you want to send, preferably as a byte array.

slide74
5. Call getOutputStream() to get an output stream object.

6. Write the byte array calculated in step 5 onto the stream.

7. Close the output stream.

8. Call getInputStream() to get an input stream object. Read from it as usual.

post cgis
POST CGIs
  • A typical POST request to a CGI looks like this:
      • POST /cgi-bin/booksearch.pl HTTP/1.0
      • Referer: http://www.macfaq.com/sampleform.html
      • User-Agent: Mozilla/3.01 (Macintosh; I; PPC)
      • Content-length: 60
      • Content-type: text/x-www-form-urlencoded
      • Host: utopia.poly.edu:56435
      • username=Sadie%2C+Julie&realname=Women+Composers
a post request includes
A POST request includes
  • the POST line
  • a MIME header which must include
    • content type
    • content length
  • a blank line that signals the end of the MIME header
  • the actual data of the form, encoded in x-www-form-urlencoded format.
slide77
A URLConnection for an http URL will set up the request line and the MIME header for you as long as you set its doOutput field to true by invoking setDoOutput(true).
  • If you also want to read from the connection, you should set doInput to true with setDoInput(true) too.
for example78
For example,
  • URLConnection uc = u.openConnection();
  • uc.setDoOutput(true);
  • uc.setDoInput(true);
slide79
The request line and MIME header are sent as soon as the URLConnection connects. Then getOutputStream() returns an output stream on which you can write the x-www-form-urlencoded name-value pairs.
httpurlconnection
HttpURLConnection
  • java.net.HttpURLConnection is an abstract subclass of URLConnection that provides some additional methods specific to the HTTP protocol.
  • URL connection objects that are returned by an http URL will be instances of java.net.HttpURLConnection.
recall
Recall
  • a typical HTTP response from a web server begins like this:
      • HTTP/1.0 200 OK
      • Server: Netscape-Enterprise/2.01
      • Date: Sat, 02 Aug 1997 07:52:46 GMT
      • Accept-ranges: bytes
      • Last-modified: Tue, 29 Jul 1997 15:06:46 GMT
      • Content-length: 2810
      • Content-type: text/html
response codes
Response Codes
  • The getHeaderField() and getHeaderFieldKey() don't return the HTTP response code
  • After you've connected, you can retrieve the numeric response code--200 in the above example--with the getResponseCode() method and the message associated with it--OK in the above example--with the getResponseMessage() method.
http protocols
HTTP Protocols
  • Java 1.0 only supports GET and POST requests to HTTP servers
  • Java 1.1/1.2 supports GET, POST, HEAD, OPTIONS, PUT, DELETE, and TRACE.
  • The protocol is chosen with the setRequestMethod(String method) method.
  • A java.net.ProtocolException, a subclass of IOException, is thrown if an unknown protocol is specified.
getrequestmethod
getRequestMethod()
  • The getRequestMethod() method returns the string form of the request method currently set for the URLConnection. GET is the default method.
disconnect
disconnect()
  • The disconnect() method of the HttpURLConnection class closes the connection to the web server.
  • Needed for HTTP/1.1 Keep-alive
for example86
For example,
  • try {
  • URL u = new URL( "http://www.metalab.unc.edu/javafaq/books.html" );
  • HttpURLConnection huc = (HttpURLConnection) u.openConnection();
  • huc.setRequestMethod("PUT");
  • huc.connect();
  • OutputStream os = huc.getOutputStream();
  • int code = huc.getResponseCode();
  • if (code >= 200 && < 300) {
  • // put the data...
  • }
  • huc.disconnect();
  • }
  • catch (IOException e) { //...
usingproxy
usingProxy
  • The boolean usingProxy() method returns true if web connections are being funneled through a proxy server, false if they're not.
redirect instructions
Redirect Instructions
  • Most web servers can be configured to automatically redirect browsers to the new location of a page that's moved.
  • To redirect browsers, a server sends a 300 level response and a Location header that specifies the new location of the requested page.
slide89
GET /~elharo/macfaq/index.html HTTP/1.0

HTTP/1.1 302 Moved Temporarily

Date: Mon, 04 Aug 1997 14:21:27 GMT

Server: Apache/1.2b7

Location: http://www.macfaq.com/macfaq/index.html

Connection: close

Content-type: text/html

<HTML><HEAD>

<TITLE>302 Moved Temporarily</TITLE>

</HEAD><BODY>

<H1>Moved Temporarily</H1>

The document has moved <A HREF="http://www.macfaq.com/macfaq/index.html">here</A>.<P>

</BODY></HTML>

slide90
HTML is returned for browsers that don't understand redirects, but most modern browsers jump straight to the page specified in the Location header instead.
  • Because redirects can change the site which a user is connecting without their knowledge so redirects are not arbitrarily followed by URLConnections.
following redirects
Following Redirects

HttpURLConnection.setFollowRedirects(true) method says that connections will follow redirect instructions from the web server.

Untrusted applets are not allowed to set this.

HttpURLConnection.getFollowRedirects() returns true if redirect requests are honored, false if they're not.

datagrams
Datagrams
  • Before data is sent across the Internet from one host to another using TCP/IP, it is split into packets of varying but finite size called datagrams.
  • Datagrams range in size from a few dozen bytes to about 60,000 bytes.
  • Packets larger than this, and often smaller than this, must be split into smaller pieces before they can be transmitted.
packets allow error correction
Packets Allow Error Correction
  • If one packet is lost, it can be retransmitted without requiring redelivery of all other packets.
  • If packets arrive out of order they can be reordered at the receiving end of the connection.
abstraction
Abstraction
  • Datagrams are mostly hidden from the Java programmer.
  • The host's native networking software transparently splits data into packets on the sending end of a connection, and then reassembles packets on the receiving end.
  • Instead, the Java programmer is presented with a higher level abstraction called a socket.
sockets
Sockets
  • A socket is a reliable connection for the transmission of data between two hosts.
  • Sockets isolate programmers from the details of packet encodings, lost and retransmitted packets, and packets that arrive out of order.
  • There are limits. Sockets are more likely to throw IOExceptions than files, for example.
socket operations
Socket Operations
  • There are four fundamental operations a socket performs. These are:

1. Connect to a remote machine

2. Send data

3. Receive data

4. Close the connection

  • A socket may not be connected to more than one host at a time.
  • A socket may not reconnect after it's closed.
the java net socket class
The java.net.Socket class
  • The java.net.Socket class allows you to create socket objects that perform all four fundamental socket operations.
  • You can connect to remote machines; you can send data; you can receive data; you can close the connection.
  • Each Socket object is associated with exactly one remote host. To connect to a different host, you must create a new Socket object.
constructing a socket
Constructing a Socket
  • Connection is accomplished through the constructors.
      • public Socket(String host, int port) throws UnknownHostException, IOException
      • public Socket(InetAddress address, int port) throws IOException
      • public Socket(String host, int port, InetAddress localAddr, int localPort) throws IOException
      • public Socket(InetAddress address, int port, InetAddress localAddr, int localPort) throws IOException
opening sockets
Opening Sockets
  • The Socket() constructors do not just create a Socket object. They also attempt to connect the underlying socket to the remote server.
  • All the constructors throw an IOException if the connection can't be made for any reason.
slide100
You must at least specify the remote host and port to connect to.
  • The host may be specified as either a string like "utopia.poly.edu" or as an InetAddress object.
  • The port should be an int between 1 and 65535.
      • Socket webMetalab = new Socket("metalab.unc.edu", 80);
slide101
You cannot just connect to any port on any host. The remote host must actually be listening for connections on that port.
  • You can use the constructors to determine which ports on a host are listening for connections.
slide102
public static void scan(InetAddress remote) {

String hostname = remote.getHostName();

for (int port = 0; port < 65536; port++) {

try {

Socket s = new Socket(remote, port);

System.out.println("There is a server on port "

+ port + " of " + hostname);

s.close();

}

catch (IOException e) {

// The remote host is not listening on this port

}

}

}

picking an ip address
Picking an IP address
  • The last two constructors also specify the host and port you're connecting from.
  • On a system with multiple IP addresses, like many web servers, this allows you to pick your network interface and IP address.
choosing a local port
Choosing a Local Port
  • You can also specify a local port number,
  • Setting the port to 0 tells the system to randomly choose an available port.
  • If you need to know the port you're connecting from, you can always get it with getLocalPort().

Socket webMetalab = new Socket("metalab.unc.edu", 80, "calzone.oit.unc.edu", 0);

sending and receiving data
Sending and Receiving Data
  • Data is sent and received with output and input streams.
  • There are methods to get an input stream for a socket and an output stream for the socket.

public InputStream getInputStream() throws IOException

public OutputStream getOutputStream() throws IOException

  • There's also a method to close a socket.

public synchronized void close() throws IOException

reading input from a socket
Reading Input from a Socket
  • The getInputStream() method returns an InputStream which reads data from the socket.
  • You can use all the normal methods of the InputStream class to read this data.
  • Most of the time you'll chain the input stream to some other input stream or reader object to more easily handle the data.
for example107
For example
  • The following code fragment connects to the daytime server on port 13 of metalab.unc.edu, and displays the data it sends.

try {

Socket s = new Socket("metalab.unc.edu", 13);

InputStream in = s.getInputStream();

InputStreamReader isr = new InputStreamReader(in);

BufferedReader br = new BufferedReader(isr);

String theTime = br.readLine();

System.out.println(theTime);

}

catch (IOException e) {

return (new Date()).toString();

}

writing output to a socket
Writing Output to a Socket
  • The getOutputStream() method returns an output stream which writes data to the socket.
  • Most of the time you'll chain the raw output stream to some other output stream or writer class to more easily handle the data.
discard
Discard

byte[] b = new byte[128];

try {

Socket s = new Socket("metalab.unc.edu", 9);

OutputStream theOutput = s.getOutputStream();

while (true) {

int n = theInput.available();

if (n > b.length) n = b.length;

int m = theInput.read(b, 0, n);

if (m == -1) break;

theOutput.write(b, 0, n);

}

s.close();

}

catch (IOException e) {}

reading and writing to a socket
Reading and Writing to a Socket
  • It's unusual to only read from a socket. It's even more unusual to only write to a socket.
  • Most protocols require the client to both read and write.
slide111
Some protocols require the reads and the writes to be interlaced. That is:
    • write
    • read
    • write
    • read
    • write
    • read
slide112
Other protocols, such as HTTP 1.0, have multiple writes, followed by multiple reads, like this:
    • write
    • write
    • write
    • read
    • read
    • read
    • read
slide113
Other protocols don't care and allow client requests and server responses to be freely intermixed.
  • Java places no restrictions on reading and writing to sockets.
  • One thread can read from a socket while another thread writes to the socket at the same time.
slide114
try {

URL u = new URL(args[i]);

if (u.getPort() != -1) port = u.getPort();

if (!(u.getProtocol().equalsIgnoreCase("http"))) {

System.err.println("I only understand http.");

}

Socket s = new Socket(u.getHost(), u.getPort());

OutputStream theOutput = s.getOutputStream();

PrintWriter pw = new PrintWriter(theOutput, false);

pw.print("GET " + u.getFile() + " HTTP/1.0\r\n");

pw.print("Accept: text/plain, text/html, text/*\r\n");

pw.print("\r\n");

pw.flush();

InputStream in = s.getInputStream();

InputStreamReader isr = new InputStreamReader(in);

BufferedReader br = new BufferedReader(isr);

int c;

while ((c = br.read()) != -1) {

System.out.write(c);

}

}

catch (MalformedURLException e) {

System.err.println(args[i] + " is not a valid URL");

}

catch (IOException e) {

System.err.println(e);

}

socket options
Socket Options
  • Several methods set various socket options. Most of the time the defaults are fine.

public void setTcpNoDelay(boolean on) throws SocketException

public boolean getTcpNoDelay() throws SocketException

public void setSoLinger(boolean on, int val) throws SocketException

public int getSoLinger() throws SocketException

public synchronized void setSoTimeout(int timeout) throws SocketException

public synchronized int getSoTimeout() throws SocketException

slide116
These methods to return information about the socket:

public InetAddress getInetAddress()

public InetAddress getLocalAddress()

public int getPort()

public int getLocalPort()

  • Finally there's the usual toString() method:

public String toString()

servers
Servers
  • There are two ends to each connection: the client, that is the host that initiates the connection, and the server, that is the host that responds to the connection.
  • Clients and servers are connected by sockets.
  • A server, rather than connecting to a remote host, a program waits for other hosts to connect to it.
server sockets
Server Sockets
  • A server socket binds to a particular port on the local machine.
  • Once it has successfully bound to a port, it listens for incoming connection attempts.
  • When a server detects a connection attempt, it accepts the connection. This creates a socket between the client and the server over which the client and the server communicate.
multiple clients
Multiple Clients
  • Multiple clients can connect to the same port on the server at the same time.
  • Incoming data is distinguished by the port to which it is addressed and the client host and port from which it came.
  • The server can tell for which service (like http or ftp) the data is intended by inspecting the port.
  • It can tell which open socket on that service the data is intended for by looking at the client address and port stored with the data.
queueing
Queueing
  • Incoming connections are stored in a queue until the server can accept them.
  • On most systems the default queue length is between 5 and 50.
  • Once the queue fills up further incoming connections are refused until space in the queue opens up.
the java net serversocket class
The java.net.ServerSocket Class
  • The java.net.ServerSocket class represents a server socket.
  • A ServerSocket object is constructed on a particular local port. Then it calls accept() to listen for incoming connections.
  • accept() blocks until a connection is detected. Then accept() returns a java.net.Socket object that performs the actual communication with the client.
constructors
Constructors
  • There are three constructors that let you specify the port to bind to, the queue length for incoming connections, and the IP address to bind to:

public ServerSocket(int port) throws IOException

public ServerSocket(int port, int backlog) throws IOException

public ServerSocket(int port, int backlog, InetAddress networkInterface) throws IOException

constructing server sockets
Constructing Server Sockets
  • Normally you only specify the port you want to listen on, like this:

try {

ServerSocket ss = new ServerSocket(80);

}

catch (IOException e) {

System.err.println(e);

}

slide124
When a ServerSocket object is created, it attempts to bind to the port on the local host given by the port argument.
  • If another server socket is already listening to the port, then a java.net.BindException, a subclass of IOException, is thrown.
  • No more than one process or thread can listen to a particular port at a time. This includes non-Java processes or threads.
  • For example, if there's already an HTTP server running on port 80, you won't be able to bind to port 80.
slide125
On Unix systems (but not Windows or the Mac) your program must be running as root to bind to a port between 1 and 1023.
  • 0 is a special port number. It tells Java to pick an available port.
  • The getLocalPort() method tells you what port the server socket is listening on. This is useful if the client and the server have already established a separate channel of communication over which the chosen port number can be communicated.
  • FTP
expanding the queue
Expanding the Queue
  • If you think you aren't going to be processing connections very quickly you may wish to expand the queue when you construct the server socket. For example,
      • try {
      • ServerSocket httpd = new ServerSocket(80, 50);
      • }
      • catch (IOException e) {
      • System.err.println(e);
      • }
choosing an ip address
Choosing an IP address
  • Many hosts have more than one IP address.
  • By default, a server socket binds to all available IP addresses on a given port.
  • You can modify that behavior with this constructor:
      • public ServerSocket(int port, int backlog, InetAddress bindAddr)throws IOException
example
Example
  • try {
  • InetAddress ia = InetAddress.getByName("199.1.32.90");
  • ServerSocket ss = new ServerSocket(80, 50, ia);
  • }
  • catch (IOException e) {
  • System.err.println(e);
  • }
slide129
On a server with multiple IP addresses, the getInetAddress() method tells you which one this server socket is listening to.

public InetAddress getInetAddress()

  • The getLocalPort() method tells you which port you're listening to.

public int getLocalPort()

slide130
The accept() and close() methods provide the basic functionality of a server socket.
      • public Socket accept() throws IOException
      • public void close() throws IOException
  • A server socket can’t be reopened after it’s closed
reading data with a serversocket
Reading Data with a ServerSocket
  • ServerSocket objects use their accept() method to connect to a client.
      • public Socket accept() throws IOException
  • There are no getInputStream() or getOutputStream() methods for ServerSocket.
  • accept() returns a Socket object, and its getInputStream() and getOutputStream() methods provide streams.
example132
Example
  • try {
  • ServerSocket ss = new ServerSocket(2345);
  • Socket s = ss.accept();
  • PrintWriter pw = new
  • PrintWriter(s.getOutputStream());
  • pw.println("Hello There!");
  • pw.println("Goodbye now.");
  • s.close();
  • }
  • catch (IOException e) {
  • System.err.println(e);
  • }
better example
Better Example
  • try {
  • ServerSocket ss = new ServerSocket(2345);
  • Socket s = ss.accept();
  • PrintWriter pw = new
  • PrintWriter(s.getOutputStream());
  • pw.print("Hello There!\r\n");
  • pw.print("Goodbye now.\r\n");
  • s.close();
  • }
  • catch (IOException e) {
  • System.err.println(e);
  • }
writing data to a client
Writing Data to a Client

try {

ServerSocket ss = new ServerSocket(port);

while (true) {

try {

Socket s = ss.accept();

PrintWriter pw = new PrintWriter(s.getOutputStream());

pw.print("Hello " + s.getInetAddress() + " on port "

+ s.getPort() + "\r\n");

pw.print("This is " + s.getLocalAddress() + " on port "

+ s.getLocalPort() + "\r\n");

pw.flush();

s.close();

}

catch (IOException e) {}

}

}

catch (IOException e) { System.err.println(e); }

interacting with a client
Interacting with a Client
  • More commonly, a server needs to both read a client request and write a response.
threading
Threading
  • No more than one server socket can listen to a particular port at one time.
  • Since a server may need to handle many connections at once, server programs tend to be heavily multi-threaded.
  • Generally the server socket passes off the actual processing of connections to a separate thread.
adding threading to a server
Adding Threading to a Server
  • It's better to make your server multi-threaded.
  • There should be a loop which continually accepts new connections.
  • Rather than handling the connection directly the socket should be passed to a Thread object that handles the connection.
adding a thread pool to a server
Adding a Thread Pool to a Server
  • Multi-threading is a good thing but it's still not a perfect solution.
  • Look at this accept loop:
      • while (true) {
      • try {
      • Socket s = ss.accept();
      • ThreadedEchoServer tes = new ThreadedEchoServer(s) tes.start();
      • }
      • catch (IOException e) {}
slide139
Every pass through this loop, a new thread gets created. Every time a connection is finished the thread is disposed of.
  • Spawning a new thread for each connection takes a non-trivial amount of time, especially on a heavily loaded server.
  • Too many simultaneous threads overload a VM.
thread pools
Thread Pools
  • Create a pool of threads when the server launches, store incoming connections in a queue, and have the threads in the pool progressively remove connections from the queue and process them.
  • The main change you need to make to implement this is to call accept() in the run() method rather than in the main() method.
setting server socket options
Setting Server Socket Options
  • There are three methods to set and get various options. The defaults are generally fine.
      • public synchronized void setSoTimeout(int timeout) throws SocketException
      • public synchronized int getSoTimeout() throws IOException
      • public static synchronized void setSocketFactory(SocketImplFactory fac) throws IOException
utility methods142
Utility Methods
  • Finally, there's the usual toString() method:
      • public String toString()
slide143
UDP
  • Unreliable Datagram Protocol
  • Packet Oriented, not stream oriented like TCP/IP
  • Much faster but no error correction
  • NFS, TFTP, and FSP use UDP/IP
  • Must fit data into packets of about 8K or less
the udp classes
The UDP Classes
  • Java's support for UDP is contained in two classes:

java.net.DatagramSocket

java.net.DatagramPacket

  • A datagram socket is used to send and receive datagram packets.
java net datagrampacket
java.net.DatagramPacket
  • a wrapper for an array of bytes from which data will be sent or into which data will be received.
  • also contains the address and port to which the packet will be sent.
java net datagramsocket
java.net.DatagramSocket
  • A DatagramSocket object is a local connection to a port that does the sending and receiving.
  • There is no distinction between a UDP socket and a UDP server socket.
  • Also unlike TCP sockets, a DatagramSocket can send to multiple, different addresses.
  • The address to which data goes is stored in the packet, not in the socket.
udp ports
UDP ports
  • Separate from TCP ports.
  • Each computer has 65,536 UDP ports as well as its 65,536 TCP ports.
  • A server socket can be bound to TCP port 20 at the same time as a datagram socket is bound to UDP port 20.
two datagrampacket constructors
Two DatagramPacket Constructors

public DatagramPacket(byte[] data, int length)

public DatagramPacket(byte[] data, int length, InetAddress remote, int port)

  • First is for receiving, second is for sending
for example149
For example,
  • String s = "My first UDP Packet";
  • byte[] b = s.getBytes();
  • DatagramPacket dp = new DatagramPacket(b, b.length);
with a destination
With a destination:
  • try {
  • InetAddress metalab = InetAddress.getByName("metalab.unc.edu");
  • int chargen = 19;
  • String s = "My second UDP Packet";
  • byte[] b = s.getBytes();
  • DatagramPacket dp = new DatagramPacket(b, b.length, metalab, chargen);
  • }
  • catch (UnknownHostException e) {
  • System.err.println(e);
  • }
datagrampackets are not immutable
DatagramPackets are not immutable.
      • public synchronized void setAddress(InetAddress remote)
      • public synchronized void setPort(int port)
      • public synchronized void setData(byte[] data)
      • public synchronized void setLength(int length)
      • public synchronized InetAddress getAddress()
      • public synchronized int getPort()
      • public synchronized byte[] getData()
      • public synchronized int getLength()
  • These methods are primarily useful when you're receiving datagrams.
java net datagramsocket152
java.net.DatagramSocket
      • public DatagramSocket() throws SocketException
      • public DatagramSocket(int port) throws SocketException
      • public DatagramSocket(int port, InetAddress laddr) throws SocketException
  • The first is for client datagram sockets; that is sockets that send datagrams before receiving any.
  • The second two are for server datagram sockets since they specify the port and optionally the IP address of the socket
sending udp datagrams
Sending UDP Datagrams
  • To send data to a particular server
    • Convert the data into byte array.
    • Pass this byte array, the length of the data in the array (most of the time this will be the length of the array) and the InetAddress and port to which you wish to send it into the DatagramPacket() constructor.
    • Next create a DatagramSocket and pass the packet to its send() method
for example154
For example,
  • InetAddress metalab = InetAddress.getByName("metalab.unc.edu");
  • int chargen = 19;
  • String s = "My second UDP Packet";
  • byte[] b = s.getBytes();
  • DatagramPacket dp = new DatagramPacket(b, b.length, ia, chargen);
  • DatagramSocket sender = new DatagramSocket();
  • sender.send(dp);
receiving udp datagrams
Receiving UDP Datagrams
  • Construct a DatagramSocket object on the port on which you want to listen.
  • Pass an empty DatagramPacket object to the DatagramSocket's receive() method.
      • public synchronized void receive(DatagramPacket dp) throws IOException
  • The calling thread blocks until a datagram is received.
slide156
dp is filled with the data from that datagram.
  • Use getPort() and and getAddress() to tell where the packet came from, getData() to retrieve the data, and getLength() to see how many bytes were in the data.
  • If the received packet was too long for the buffer, it's truncated to the length of the buffer.
  • Length is reset when packet is received
for example157
For example,
  • try {
  • byte buffer = new byte[65536];
  • DatagramPacket incoming = new DatagramPacket(buffer, buffer.length);
  • DatagramSocket ds = new DatagramSocket(2134);
  • ds.receive(incoming);
  • byte[] data = incoming.getData();
  • String s = new String(data, 0, data.getLength());
  • System.out.println("Port " + incoming.getPort() + " on " + incoming.getAddress() + " sent this message:");
  • System.out.println(s);
  • }
  • catch (IOException e) {
  • System.err.println(e);
  • }
receiving multiple packets
Receiving Multiple Packets
  • Watch out for strictly decreasing packet sizes
      • DatagramPacket incoming = new DatagramPacket(new byte[8192], 8192);
      • DatagramSocket ds = new DatagramSocket(2134);
      • while (true) {
      • try {
      • incoming.setLength(8192);
      • ds.receive(incoming);
      • byte[] data = incoming.getData();
      • String s = new String(data, 0, data.getLength());
      • System.out.println("Port " + incoming.getPort()
      • + " on " + incoming.getAddress()
      • + " sent this message:");
      • System.out.println(s);
      • }
      • catch (IOException e) {
      • System.err.println(e);
      • }
      • }
to learn more
To Learn More
  • Java Network Programming
    • O’Reilly & Associates, 1997
    • ISBN 1-56592-227-1
  • Java I/O
    • O’Reilly & Associates, 1999
    • ISBN 1-56592-485-1
  • Web Client Programming with Java
    • http://www.digitalthink.com/catalog/cs/cs308/index.html