Java Network Programming

Java Network Programming
Elliotte Rusty Harold

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 No prior network programming experience

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 Hosts Internet Addresses Ports Protocols

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 Every host on the Internet is identified by a unique, four-byte Internet Protocol (IP) address. This is written in dotted quad format like 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)
Numeric addresses are mapped to names like 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 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
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
public static InetAddress getByName(String host) throws UnknownHostException InetAddress utopia, duke; try { utopia = InetAddress.getByName("utopia.poly.edu"); duke = InetAddress.getByName(" "); } catch (UnknownHostException e) { System.err.println(e);

Other ways to create InetAddress objects
public static InetAddress[] getAllByName(String host) throws UnknownHostException public static InetAddress getLocalHost() throws UnknownHostException

Getter Methods public boolean isMulticastAddress()
public String getHostName() public byte[] getAddress() public String getHostAddress()

Utility Methods public int hashCode() public boolean equals(Object o)
public String toString()

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 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 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

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 Standards HTTP HTML XML RDF MathML SMIL P3P

URLs A URL, short for "Uniform Resource Locator", is a way to unambiguously identify the location of a resource on the Internet.

Example URLs http://java.sun.com/
file:///Macintosh%20HD/Java/Docs/JDK% %20docs/api/java.net.InetAddress.html#_top_ ftp://ftp.info.apple.com/pub/ telnet://utopia.poly.edu

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 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 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
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 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 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
An absolute URL like try { URL u = new URL( " } catch (MalformedURLException e) {}

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", " "/schedule/fall97/bgrad.html#cs"); } catch (MalformedURLException e) {}

Including the Port URL u = null; try {
u = new URL("http", " 8000, "/fall97/grad.html#cs"); } catch (MalformedURLException e) {}

Relative URLs Many HTML files contain relative URLs.
Consider the page On this page a link to “books.html" refers to

Constructing Relative URLs
The fourth constructor creates URLs relative to a given URL. For example, try { URL u1 = new URL(" URL u2 = new URL(u1, "books.html"); } catch (MalformedURLException e) {} This is particularly useful when parsing HTML.

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, try { URL u = new URL(" "); 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 URLs JDK 1.3 adds three more: public String getAuthority()
public String getUserInfo() public String getQuery()

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. then it's set to "/".

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 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() What readLine() does: What readLine() should do:
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

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);}

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:
The browser requests a page The server sends the page Data flows primarily from the server to the client.

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:

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 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

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
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
GET /javafaq/images/cup.gif HTTP/1.0 Connection: Keep-Alive User-Agent: Mozilla/3.01 (Macintosh; I; PPC) Host: Accept: image/gif, image/x-xbitmap, image/jpeg, */*

MIME MIME is an acronym for "Multipurpose Internet Mail Extensions".
an Internet standard defined in RFCs 2045 through 2049 originally intended for use with messages, but has been been adopted for use in HTTP.

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

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/ OK Server: Netscape-Enterprise/2.01 Date: Sat, 02 Aug :52:46 GMT Accept-ranges: bytes Last-modified: Tue, 29 Jul :06:46 GMT Content-length: 2810 Content-type: text/html

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 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 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 java.net.URLEncoder class contains a single static method which encodes strings in x-www-form-url-encoded format URLEncoder.encode(String s)

For example, This prints:
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

This prints the properly encoded query string:
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 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 String eqs = "Author=" + URLEncoder.encode("Sadie, Julie");
eqs += "Title="; eqs += URLEncoder.encode("Women Composers"); try { URL u = new URL(" + eqs); InputStream in = u.openStream(); //... } catch (IOException e) { //...

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
Can send output as well as read input Can post data to CGIs Can read headers from a connection

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
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 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 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() 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 example String key = null;
for (int i=1; (key = uc.getHeaderFieldKey(i))!=null); i++) { System.out.println(key + ": " + uc.getHeaderField(key)); }

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)

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
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()

try { URL u = new URL(" 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
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: 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.

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 A typical POST request to a CGI looks like this:
POST /cgi-bin/booksearch.pl HTTP/1.0 Referer: 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
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.

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 example, URLConnection uc = u.openConnection();
uc.setDoOutput(true); uc.setDoInput(true);

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 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 a typical HTTP response from a web server begins like this:
HTTP/ OK Server: Netscape-Enterprise/2.01 Date: Sat, 02 Aug :52:46 GMT Accept-ranges: bytes Last-modified: Tue, 29 Jul :06:46 GMT Content-length: 2810 Content-type: text/html

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 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() The getRequestMethod() method returns the string form of the request method currently set for the URLConnection. GET is the default method.

disconnect() The disconnect() method of the HttpURLConnection class closes the connection to the web server. Needed for HTTP/1.1 Keep-alive

For example, try { URL u = new URL( " ); 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 The boolean usingProxy() method returns true if web connections are being funneled through a proxy server, false if they're not.

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.

GET /~elharo/macfaq/index.html HTTP/1.0
HTTP/ Moved Temporarily Date: Mon, 04 Aug :21:27 GMT Server: Apache/1.2b7 Location: 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=" </BODY></HTML>

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 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 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
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 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 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 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 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 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 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.

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

You cannot just connect to any port on any host
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.

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 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 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
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
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 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
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 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
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.

Some protocols require the reads and the writes to be interlaced
Some protocols require the reads and the writes to be interlaced. That is: write read

Other protocols, such as HTTP 1
Other protocols, such as HTTP 1.0, have multiple writes, followed by multiple reads, like this: write read

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.

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 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

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 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 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 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 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 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 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
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);

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.

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 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 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 try { InetAddress ia = InetAddress.getByName("199.1.32.90");
ServerSocket ss = new ServerSocket(80, 50, ia); } catch (IOException e) { System.err.println(e);

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

A server socket can’t be reopened after it’s closed
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
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.

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 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
try { ServerSocket ss = new ServerSocket(port); while (true) { 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
More commonly, a server needs to both read a client request and write a response.

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
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
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) {}

Every pass through this loop, a new thread gets created
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 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
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 Methods Finally, there's the usual toString() method:
public String toString()

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 User Datagram Protocol, UDP for short, provides unguaranteed, connectionless transmission of data across an IP network. By contrast, TCP sockets provide reliable, connection-oriented transmission of data. Both TCP and UDP split data into packets called datagrams. However TCP includes extra headers in the datagram to enable retransmission of lost packets and reassembly of packets into the correct order if they arrive out of order. UDP does not provide this. If a UDP packet is lost, it's lost. It will not be retransmitted. Similarly, packets appear in the receiving program in the order they were received, not necessarily in the order they were sent. Given these disadvantages you may well wonder why anyone would prefer UDP to TCP. The answer is speed. UDP can be up to three times faster than TCP; and there are many applications for which reliable transmission of data is not nearly as important as speed. For example lost or out of order packets may appear as static in an audio or video feed, but the overall picture or sound could still be intelligible. Telephone vs. snail mail analogy. Protocols that use UDP include NFS, FSP, and TFTP.

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
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
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 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
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 example, String s = "My first UDP Packet";
byte[] b = s.getBytes(); DatagramPacket dp = new DatagramPacket(b, b.length);

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.
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.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 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 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
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.

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 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
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 Java Network Programming Java I/O
O’Reilly & Associates, 1997 ISBN Java I/O O’Reilly & Associates, 1999 ISBN Web Client Programming with Java

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