Module 1 networking basics-2

CompRef_2010 / Java The Complete Reference, Ninth Edition /Schildt / 007180 855-8
22
CHAPTER
727
Networking
As all readers know, Java is practically a synonym for Internet programming. There are a
number of reasons for this, not the least of which is its ability to generate secure, cross-
platform, portable code. However, one of the most important reasons that Java is the premier
language for network programming are the classes defined in the java.net package. They
provide an easy-to-use means by which programmers of all skill levels can access network
resources.
This chapter explores the java.net package. It is important to emphasize that networking
is a very large and at times complicated topic. It is not possible for this book to discuss all
of the capabilities contained in java.net. Instead, this chapter focuses on several of its core
classes and interfaces.
Networking Basics
Before we begin, it will be useful to review some key networking concepts and terms. At the
core of Java’s networking support is the concept of a socket. A socket identifies an endpoint
in a network. The socket paradigm was part of the 4.2BSD Berkeley UNIX release in the
early 1980s. Because of this, the term Berkeley socket is also used. Sockets are at the foundation
of modern networking because a socket allows a single computer to serve many different
clients at once, as well as to serve many different types of information. This is accomplished
through the use of a port, which is a numbered socket on a particular machine. A server
process is said to "listen" to a port until a client connects to it. A server is allowed to accept
multiple clients connected to the same port number, although each session is unique. To
manage multiple client connections, a server process must be multithreaded or have some
other means of multiplexing the simultaneous I/O.
Socket communication takes place via a protocol. Internet Protocol (IP) is a low-level
routing protocol that breaks data into small packets and sends them to an address across a
network, which does not guarantee to deliver said packets to the destination. Transmission
Control Protocol (TCP) is a higher-level protocol that manages to robustly string together
these packets, sorting and retransmitting them as necessary to reliably transmit data. A
third protocol, User Datagram Protocol (UDP), sits next to TCP and can be used directly to
support fast, connectionless, unreliable transport of packets.
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728 PART II The Java Library
Once a connection has been established, a higher-level protocol ensues, which is
dependent on which port you are using. TCP/IP reserves the lower 1,024 ports for specific
protocols. Many of these will seem familiar to you if you have spent any time surfing the
Internet. Port number 21 is for FTP; 23 is for Telnet; 25 is for e-mail; 43 is for whois; 80 is
for HTTP; 119 is for netnews—and the list goes on. It is up to each protocol to determine
how a client should interact with the port.
For example, HTTP is the protocol that web browsers and servers use to transfer
hypertext pages and images. It is a quite simple protocol for a basic page-browsing web
server. Here’s how it works. When a client requests a file from an HTTP server, an action
known as a hit, it simply sends the name of the file in a special format to a predefined port
and reads back the contents of the file. The server also responds with a status code to tell
the client whether or not the request can be fulfilled and why.
A key component of the Internet is the address. Every computer on the Internet has
one. An Internet address is a number that uniquely identifies each computer on the Net.
Originally, all Internet addresses consisted of 32-bit values, organized as four 8-bit values.
This address type was specified by IPv4 (Internet Protocol, version 4). However, a new
addressing scheme, called IPv6 (Internet Protocol, version 6) has come into play. IPv6 uses
a 128-bit value to represent an address, organized into eight 16-bit chunks. Although there
are several reasons for and advantages to IPv6, the main one is that it supports a much
larger address space than does IPv4. Fortunately, when using Java, you won’t normally need
to worry about whether IPv4 or IPv6 addresses are used because Java handles the details
for you.
Just as the numbers of an IP address describe a network hierarchy, the name of an
Internet address, called its domain name, describes a machine’s location in a name space.
For example, www.HerbSchildt.com is in the COM top-level domain (reserved for U.S.
commercial sites); it is called HerbSchildt, and www identifies the server for web requests.
An Internet domain name is mapped to an IP address by the Domain Naming Service (DNS).
This enables users to work with domain names, but the Internet operates on IP addresses.
The Networking Classes and Interfaces
Java supports TCP/IP both by extending the already established stream I/O interface
introduced in Chapter 20 and by adding the features required to build I/O objects across
the network. Java supports both the TCP and UDP protocol families. TCP is used for reliable
stream-based I/O across the network. UDP supports a simpler, hence faster, point-to-point
datagram-oriented model. The classes contained in the java.net package are shown here:
Authenticator InetAddress SocketAddress
CacheRequest InetSocketAddress SocketImpl
CacheResponse InterfaceAddress SocketPermission
ContentHandler JarURLConnection StandardSocketOption
CookieHandler MulticastSocket URI
CookieManager NetPermission URL
DatagramPacket NetworkInterface URLClassLoader

Chapter 22 Networking 729
PartII
DatagramSocket PasswordAuthentication URLConnection
DatagramSocketImpl Proxy URLDecoder
HttpCookie ProxySelector URLEncoder
HttpURLConnection ResponseCache URLPermission (Added by JDK 8.)
IDN SecureCacheResponse URLStreamHandler
Inet4Address ServerSocket
Inet6Address Socket
The java.net package’s interfaces are listed here:
ContentHandlerFactory FileNameMap SocketOptions
CookiePolicy ProtocolFamily URLStreamHandlerFactory
CookieStore SocketImplFactory
DatagramSocketImplFactory SocketOption
In the sections that follow, we will examine the main networking classes and show
several examples that apply to them. Once you understand these core networking classes,
you will be able to easily explore the others on your own.
InetAddress
The InetAddress class is used to encapsulate both the numerical IP address and the domain
name for that address. You interact with this class by using the name of an IP host, which is
more convenient and understandable than its IP address. The InetAddress class hides the
number inside. InetAddress can handle both IPv4 and IPv6 addresses.
Factory Methods
The InetAddress class has no visible constructors. To create an InetAddress object, you have
to use one of the available factory methods. Factory methods are merely a convention whereby
static methods in a class return an instance of that class. This is done in lieu of overloading a
constructor with various parameter lists when having unique method names makes the results
much clearer. Three commonly used InetAddress factory methods are shown here:
static InetAddress getLocalHost( )
throws UnknownHostException
static InetAddress getByName(String hostName)
static InetAddress[ ] getAllByName(String hostName)
The getLocalHost( ) method simply returns the InetAddress object that represents the local
host. The getByName( ) method returns an InetAddress for a host name passed to it. If these
methods are unable to resolve the host name, they throw an UnknownHostException.
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On the Internet, it is common for a single name to be used to represent several
machines. In the world of web servers, this is one way to provide some degree of scaling. The
getAllByName( ) factory method returns an array of InetAddresses that represent all of the
addresses that a particular name resolves to. It will also throw an UnknownHostException if
it can’t resolve the name to at least one address.
InetAddress also includes the factory method getByAddress( ), which takes an IP
address and returns an InetAddress object. Either an IPv4 or an IPv6 address can be used.
The following example prints the addresses and names of the local machine and two
Internet web sites:
// Demonstrate InetAddress.
import java.net.*;
class InetAddressTest
{
public static void main(String args[]) throws UnknownHostException {
InetAddress Address = InetAddress.getLocalHost();
System.out.println(Address);
Address = InetAddress.getByName("www.HerbSchildt.com");
System.out.println(Address);
InetAddress SW[] = InetAddress.getAllByName("www.nba.com");
for (int i=0; i<SW.length; i++)
System.out.println(SW[i]);
}
}
Here is the output produced by this program. (Of course, the output you see may be
slightly different.)
default/166.203.115.212
www.HerbSchildt.com/216.92.65.4
www.nba.com/216.66.31.161
www.nba.com/216.66.31.179
Instance Methods
The InetAddress class has several other methods, which can be used on the objects returned
by the methods just discussed. Here are some of the more commonly used methods:
boolean equals(Object other) Returns true if this object has the same Internet address
as other.
byte[ ] getAddress( ) Returns a byte array that represents the object’s IP address
in network byte order.
String getHostAddress( ) Returns a string that represents the host address
associated with the InetAddress object.
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PartII
String getHostName( ) Returns a string that represents the host name associated
with the InetAddress object.
boolean isMulticastAddress( ) Returns true if this address is a multicast address.
Otherwise, it returns false.
String toString( ) Returns a string that lists the host name and the IP
address for convenience.
Internet addresses are looked up in a series of hierarchically cached servers. That
means that your local computer might know a particular name-to-IP-address mapping
automatically, such as for itself and nearby servers. For other names, it may ask a local DNS
server for IP address information. If that server doesn’t have a particular address, it can go
to a remote site and ask for it. This can continue all the way up to the root server. This
process might take a long time, so it is wise to structure your code so that you cache IP
address information locally rather than look it up repeatedly.
Inet4Address and Inet6Address
Java includes support for both IPv4 and IPv6 addresses. Because of this, two subclasses of
InetAddress were created: Inet4Address and Inet6Address. Inet4Address represents a
traditional-style IPv4 address. Inet6Address encapsulates a newer IPv6 address. Because they
are subclasses of InetAddress, an InetAddress reference can refer to either. This is one way
that Java was able to add IPv6 functionality without breaking existing code or adding many
more classes. For the most part, you can simply use InetAddress when working with IP
addresses because it can accommodate both styles.
TCP/IP Client Sockets
TCP/IP sockets are used to implement reliable, bidirectional, persistent, point-to-point,
stream-based connections between hosts on the Internet. A socket can be used to connect
Java’s I/O system to other programs that may reside either on the local machine or on any
other machine on the Internet.
NOTE As a general rule, applets may only establish socket connections back to the host from which the
applet was downloaded.This restriction exists because it would be dangerous for applets loaded
through a firewall to have access to any arbitrary machine.
There are two kinds of TCP sockets in Java. One is for servers, and the other is for clients.
The ServerSocket class is designed to be a "listener," which waits for clients to connect
before doing anything. Thus, ServerSocket is for servers. The Socket class is for clients. It is
designed to connect to server sockets and initiate protocol exchanges. Because client
sockets are the most commonly used by Java applications, they are examined here.
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The creation of a Socket object implicitly establishes a connection between the client
and server. There are no methods or constructors that explicitly expose the details of
establishing that connection. Here are two constructors used to create client sockets:
Socket(String hostName, int port)
throws UnknownHostException,
IOException
Creates a socket connected to the named host
and port.
Socket(InetAddress ipAddress, int port)
throws IOException
Creates a socket using a preexisting InetAddress
object and a port.
Socket defines several instance methods. For example, a Socket can be examined at any
time for the address and port information associated with it, by use of the following methods:
InetAddress getInetAddress( ) Returns the InetAddress associated with the Socket
object. It returns null if the socket is not connected.
int getPort( ) Returns the remote port to which the invoking
Socket object is connected. It returns 0 if the socket
is not connected.
int getLocalPort( ) Returns the local port to which the invoking Socket
object is bound. It returns –1 if the socket is not
bound.
You can gain access to the input and output streams associated with a Socket by use of
the getInputStream( ) and getOuptutStream( ) methods, as shown here. Each can throw an
IOException if the socket has been invalidated by a loss of connection. These streams are
used exactly like the I/O streams described in Chapter 20 to send and receive data.
InputStream getInputStream( )
throws IOException
Returns the InputStream associated with the
invoking socket.
OutputStream getOutputStream( )
throws IOException
Returns the OutputStream associated with the
invoking socket.
Several other methods are available, including connect( ), which allows you to specify a
new connection; isConnected( ), which returns true if the socket is connected to a server;
isBound( ), which returns true if the socket is bound to an address; and isClosed( ), which
returns true if the socket is closed. To close a socket, call close( ). Closing a socket also
closes the I/O streams associated with the socket. Beginning with JDK 7, Socket also
implements AutoCloseable, which means that you can use a try-with-resources block
to manage a socket.
The following program provides a simple Socket example. It opens a connection to a
"whois" port (port 43) on the InterNIC server, sends the command-line argument down the
socket, and then prints the data that is returned. InterNIC will try to look up the argument
as a registered Internet domain name, and then send back the IP address and contact
information for that site.
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PartII
// Demonstrate Sockets.
import java.net.*;
import java.io.*;
class Whois {
public static void main(String args[]) throws Exception {
int c;
// Create a socket connected to internic.net, port 43.
Socket s = new Socket("whois.internic.net", 43);
// Obtain input and output streams.
InputStream in = s.getInputStream();
OutputStream out = s.getOutputStream();
// Construct a request string.
String str = (args.length == 0 ? "MHProfessional.com" : args[0]) + "n";
// Convert to bytes.
byte buf[] = str.getBytes();
// Send request.
out.write(buf);
// Read and display response.
while ((c = in.read()) != -1) {
System.out.print((char) c);
}
s.close();
}
}
If, for example, you obtained information about MHProfessional.com, you’d get something
similar to the following:
Whois Server Version 2.0
Domain names in the .com and .net domains can now be registered
with many different competing registrars. Go to http://www.internic.net
for detailed information.
Domain Name: MHPROFESSIONAL.COM
Registrar: CSC CORPORATE DOMAINS, INC.
Whois Server: whois.corporatedomains.com
Referral URL: http://www.cscglobal.com
Name Server: NS1.MHEDU.COM
Name Server: NS2.MHEDU.COM
.
.
.
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Here is how the program works. First, a Socket is constructed that specifies the host
name "whois.internic.net" and the port number 43. Internic.net is the InterNIC web site
that handles whois requests. Port 43 is the whois port. Next, both input and output streams
are opened on the socket. Then, a string is constructed that contains the name of the web
site you want to obtain information about. In this case, if no web site is specified on the
command line, then "MHProfessional.com" is used. The string is converted into a byte
array and then sent out of the socket. The response is read by inputting from the socket,
and the results are displayed. Finally, the socket is closed, which also closes the I/O streams.
In the preceding example, the socket was closed manually by calling close( ). If you are
using JDK 7 or later, then you can use a try-with-resources block to automatically close the
socket. For example, here is another way to write the main( ) method of the previous
program:
// Use try-with-resources to close a socket.
int c;
// Create a socket connected to internic.net, port 43. Manage this
// socket with a try-with-resources block.
try ( Socket s = new Socket("whois.internic.net", 43) ) {
// Obtain input and output streams.
InputStream in = s.getInputStream();
OutputStream out = s.getOutputStream();
// Construct a request string.
String str = (args.length == 0 ? "MHProfessional.com" : args[0]) + "n";
// Convert to bytes.
byte buf[] = str.getBytes();
// Send request.
out.write(buf);
// Read and display response.
while ((c = in.read()) != -1) {
}
}
// The socket is now closed.
}
In this version, the socket is automatically closed when the try block ends.
So the examples will work with earlier versions of Java and to clearly illustrate when a
network resource can be closed, subsequent examples will continue to call close( )
explicitly. However, in your own code, you should consider using automatic resource
management since it offers a more streamlined approach. One other point: In this version,
exceptions are still thrown out of main( ), but they could be handled by adding catch
clauses to the end of the try-with-resources block.
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PartII
NOTE For simplicity, the examples in this chapter simply throw all exceptions out of main( ).This allows
the logic of the network code to be clearly illustrated. However, in real-world code, you will normally
need to handle the exceptions in an appropriate way.
URL
The preceding example was rather obscure because the modern Internet is not about the
older protocols such as whois, finger, and FTP. It is about WWW, the World Wide Web. The
Web is a loose collection of higher-level protocols and file formats, all unified in a web
browser. One of the most important aspects of the Web is that Tim Berners-Lee devised a
scalable way to locate all of the resources of the Net. Once you can reliably name anything
and everything, it becomes a very powerful paradigm. The Uniform Resource Locator
(URL) does exactly that.
The URL provides a reasonably intelligible form to uniquely identify or address
information on the Internet. URLs are ubiquitous; every browser uses them to identify
information on the Web. Within Java’s network class library, the URL class provides a
simple, concise API to access information across the Internet using URLs.
All URLs share the same basic format, although some variation is allowed. Here are two
examples: http://www.MHProfessional.com/ and http://www.MHProfessional.com:80/
index.htm. A URL specification is based on four components. The first is the protocol to
use, separated from the rest of the locator by a colon (:). Common protocols are HTTP,
FTP, gopher, and file, although these days almost everything is being done via HTTP (in
fact, most browsers will proceed correctly if you leave off the "http://" from your URL
specification). The second component is the host name or IP address of the host to use;
this is delimited on the left by double slashes (//) and on the right by a slash (/) or
optionally a colon (:). The third component, the port number, is an optional parameter,
delimited on the left from the host name by a colon (:) and on the right by a slash (/). (It
defaults to port 80, the predefined HTTP port; thus, ":80" is redundant.) The fourth part is
the actual file path. Most HTTP servers will append a file named index.html or index.htm
to URLs that refer directly to a directory resource. Thus, http://www.MHProfessional.com/
is the same as http://www.MHProfessional.com/index.htm.
Java’s URL class has several constructors; each can throw a MalformedURLException.
One commonly used form specifies the URL with a string that is identical to what you see
displayed in a browser:
URL(String urlSpecifier) throws MalformedURLException
The next two forms of the constructor allow you to break up the URL into its
component parts:
URL(String protocolName, String hostName, int port, String path )
throws MalformedURLException
URL(String protocolName, String hostName, String path)
throws MalformedURLException

Another frequently used constructor allows you to use an existing URL as a reference
context and then create a new URL from that context. Although this sounds a little
contorted, it’s really quite easy and useful.
URL(URL urlObj, String urlSpecifier) throws MalformedURLException
The following example creates a URL to a page on HerbSchildt.com and then examines
its properties:
// Demonstrate URL.
import java.net.*;
class URLDemo {
public static void main(String args[]) throws MalformedURLException {
URL hp = new URL(http://www.HerbSchildt.com/WhatsNew");
System.out.println("Protocol: " + hp.getProtocol());
System.out.println("Port: " + hp.getPort());
System.out.println("Host: " + hp.getHost());
System.out.println("File: " + hp.getFile());
System.out.println("Ext:" + hp.toExternalForm());
}
}
When you run this, you will get the following output:
Protocol: http
Port: -1
Host: www.HerbSchildt.com
File: /WhatsNew
Ext:http://www.HerbSchildt.com/WhatsNew
Notice that the port is –1; this means that a port was not explicitly set. Given a URL object,
you can retrieve the data associated with it. To access the actual bits or content information
of a URL, create a URLConnection object from it, using its openConnection( ) method,
like this:
urlc = url.openConnection()
openConnection( ) has the following general form:
URLConnection openConnection( ) throws IOException
It returns a URLConnection object associated with the invoking URL object. Notice that it
may throw an IOException.
URLConnection
URLConnection is a general-purpose class for accessing the attributes of a remote resource.
Once you make a connection to a remote server, you can use URLConnection to inspect
the properties of the remote object before actually transporting it locally. These attributes
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PartII
are exposed by the HTTP protocol specification and, as such, only make sense for URL
objects that are using the HTTP protocol.
URLConnection defines several methods. Here is a sampling:
int getContentLength( ) Returns the size in bytes of the content
associated with the resource. If the length is
unavailable, –1 is returned.
long getContentLengthLong( ) Returns the size in bytes of the content
associated with the resource. If the length is
unavailable, –1 is returned.
String getContentType( ) Returns the type of content found in the resource.
This is the value of the content-type header field.
Returns null if the content type is not available.
long getDate( ) Returns the time and date of the response
represented in terms of milliseconds since
January 1, 1970 GMT.
long getExpiration( ) Returns the expiration time and date of the
resource represented in terms of milliseconds
since January 1, 1970 GMT. Zero is returned if
the expiration date is unavailable.
String getHeaderField(int idx) Returns the value of the header field at index idx.
(Header field indexes begin at 0.) Returns null if
the value of idx exceeds the number of fields.
String getHeaderField(String fieldName) Returns the value of header field whose name
is specified by fieldName. Returns null if the
specified name is not found.
String getHeaderFieldKey(int idx) Returns the header field key at index idx.
(Header field indexes begin at 0.) Returns null
if the value of idx exceeds the number of fields.
Map<String, List<String>>
getHeaderFields( )
Returns a map that contains all of the header
fields and values.
long getLastModified( ) Returns the time and date, represented in terms
of milliseconds since January 1, 1970 GMT, of
the last modification of the resource. Zero is
returned if the last-modified date is unavailable.
InputStream getInputStream( )
throws IOException
Returns an InputStream that is linked to the
resource. This stream can be used to obtain the
content of the resource.
Notice that URLConnection defines several methods that handle header information. A
header consists of pairs of keys and values represented as strings. By using getHeaderField( ),
you can obtain the value associated with a header key. By calling getHeaderFields( ), you can
obtain a map that contains all of the headers. Several standard header fields are available
directly through methods such as getDate( ) and getContentType( ).
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The following example creates a URLConnection using the openConnection( ) method
of a URL object and then uses it to examine the document’s properties and content:
// Demonstrate URLConnection.
import java.net.*;
import java.io.*;
import java.util.Date;
class UCDemo
{
int c;
URL hp = new URL(http://www.internic.net");
URLConnection hpCon = hp.openConnection();
// get date
long d = hpCon.getDate();
if(d==0)
System.out.println("No date information.");
else
System.out.println("Date: " + new Date(d));
// get content type
System.out.println("Content-Type: " + hpCon.getContentType());
// get expiration date
d = hpCon.getExpiration();
if(d==0)
System.out.println("No expiration information.");
else
System.out.println("Expires: " + new Date(d));
// get last-modified date
d = hpCon.getLastModified();
if(d==0)
System.out.println("No last-modified information.");
else
System.out.println("Last-Modified: " + new Date(d));
// get content length
long len = hpCon.getContentLengthLong();
if(len == -1)
System.out.println("Content length unavailable.");
else
System.out.println("Content-Length: " + len);
if(len != 0) {
System.out.println("=== Content ===");
InputStream input = hpCon.getInputStream();
while (((c = input.read()) != -1)) {
}
input.close();
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PartII
} else {
System.out.println("No content available.");
}
}
}
The program establishes an HTTP connection to www.internic.net over port 80. It then
displays several header values and retrieves the content. You might find it interesting to try
this example, observing the results, and then for comparison purposes try a different web
site of your own choosing.
HttpURLConnection
Java provides a subclass of URLConnection that provides support for HTTP connections.
This class is called HttpURLConnection. You obtain an HttpURLConnection in the same
way just shown, by calling openConnection( ) on a URL object, but you must cast the result
to HttpURLConnection. (Of course, you must make sure that you are actually opening an
HTTP connection.) Once you have obtained a reference to an HttpURLConnection object,
you can use any of the methods inherited from URLConnection. You can also use any of
the several methods defined by HttpURLConnection. Here is a sampling:
static boolean getFollowRedirects( ) Returns true if redirects are automatically followed
and false otherwise. This feature is on by default.
String getRequestMethod( ) Returns a string representing how URL requests
are made. The default is GET. Other options, such
as POST, are available.
int getResponseCode( )
throws IOException
Returns the HTTP response code. –1 is returned
if no response code can be obtained. An
IOException is thrown if the connection fails.
String getResponseMessage( )
throws IOException
Returns the response message associated with
the response code. Returns null if no message
is available. An IOException is thrown if the
connection fails.
static void setFollowRedirects(boolean how) If how is true, then redirects are automatically
followed. If how is false, redirects are not
automatically followed. By default, redirects
are automatically followed.
void setRequestMethod(String how)
throws ProtocolException
Sets the method by which HTTP requests are
made to that specified by how. The default method
is GET, but other options, such as POST, are
available. If how is invalid, a ProtocolException
is thrown.

The following program demonstrates HttpURLConnection. It first establishes a
connection to www.google.com. Then it displays the request method, the response code,
and the response message. Finally, it displays the keys and values in the response header.
// Demonstrate HttpURLConnection.
import java.net.*;
import java.io.*;
import java.util.*;
class HttpURLDemo
{
URL hp = new URL(http://www.google.com");
HttpURLConnection hpCon = (HttpURLConnection) hp.openConnection();
// Display request method.
System.out.println("Request method is " +
hpCon.getRequestMethod());
// Display response code.
System.out.println("Response code is " +
hpCon.getResponseCode());
// Display response message.
System.out.println("Response Message is " +
hpCon.getResponseMessage());
// Get a list of the header fields and a set
// of the header keys.
Map<String, List<String>> hdrMap = hpCon.getHeaderFields();
Set<String> hdrField = hdrMap.keySet();
System.out.println("nHere is the header:");
// Display all header keys and values.
for(String k : hdrField) {
System.out.println("Key: " + k +
" Value: " + hdrMap.get(k));
}
}
}
The output produced by the program is shown here. (Of course, the exact response
returned by www.google.com will vary over time.)
Request method is GET
Response code is 200
Response Message is OK
Here is the header:
Key: Transfer-Encoding Value: [chunked]

PartII
Key: X-Frame-Options Value: [SAMEORIGIN]
Key: null Value: [HTTP/1.1 200 OK]
Key: Server Value: [gws]
Key: Cache-Control Value: [private, max-age=0]
Key: Set-Cookie Value:
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Notice how the header keys and values are displayed. First, a map of the header keys and
values is obtained by calling getHeaderFields( ) (which is inherited from URLConnection).
Next, a set of the header keys is retrieved by calling keySet( ) on the map. Then, the key set
is cycled through by using a for-each style for loop. The value associated with each key is
obtained by calling get( ) on the map.
The URI Class
The URI class encapsulates a Uniform Resource Identifier (URI). URIs are similar to URLs. In
fact, URLs constitute a subset of URIs. A URI represents a standard way to identify a resource.
A URL also describes how to access the resource.
Cookies
The java.net package includes classes and interfaces that help manage cookies and can be
used to create a stateful (as opposed to stateless) HTTP session. The classes are CookieHandler,
CookieManager, and HttpCookie. The interfaces are CookiePolicy and CookieStore. The
creation of a stateful HTTP session is beyond the scope of this book.
NOTE For information about using cookies with servlets, see Chapter 38.
TCP/IP Server Sockets
As mentioned earlier, Java has a different socket class that must be used for creating server
applications. The ServerSocket class is used to create servers that listen for either local or
remote client programs to connect to them on published ports. ServerSockets are quite
different from normal Sockets. When you create a ServerSocket, it will register itself with
the system as having an interest in client connections. The constructors for ServerSocket
reflect the port number that you want to accept connections on and, optionally, how long
you want the queue for said port to be. The queue length tells the system how many client
connections it can leave pending before it should simply refuse connections. The default
22-ch22.indd 741 14/02/14 5:11 PM

is 50. The constructors might throw an IOException under adverse conditions. Here are
three of its constructors:
ServerSocket(int port) throws IOException Creates server socket on the specified port
with a queue length of 50.
ServerSocket(int port, int maxQueue)
throws IOException
Creates a server socket on the specified port
with a maximum queue length of maxQueue.
ServerSocket(int port, int maxQueue,
InetAddress localAddress)
throws IOException
Creates a server socket on the specified port
with a maximum queue length of maxQueue.
On a multihomed host, localAddress specifies
the IP address to which this socket binds.
ServerSocket has a method called accept( ), which is a blocking call that will wait for a
client to initiate communications and then return with a normal Socket that is then used
for communication with the client.
Datagrams
TCP/IP-style networking is appropriate for most networking needs. It provides a serialized,
predictable, reliable stream of packet data. This is not without its cost, however. TCP
includes many complicated algorithms for dealing with congestion control on crowded
networks, as well as pessimistic expectations about packet loss. This leads to a somewhat
inefficient way to transport data. Datagrams provide an alternative.
Datagrams are bundles of information passed between machines. They are somewhat
like a hard throw from a well-trained but blindfolded catcher to the third baseman. Once
the datagram has been released to its intended target, there is no assurance that it will
arrive or even that someone will be there to catch it. Likewise, when the datagram is received,
there is no assurance that it hasn’t been damaged in transit or that whoever sent it is still
there to receive a response.
Java implements datagrams on top of the UDP protocol by using two classes: the
DatagramPacket object is the data container, while the DatagramSocket is the mechanism
used to send or receive the DatagramPackets. Each is examined here.
DatagramSocket
DatagramSocket defines four public constructors. They are shown here:
DatagramSocket( ) throws SocketException
DatagramSocket(int port) throws SocketException
DatagramSocket(int port, InetAddress ipAddress) throws SocketException
DatagramSocket(SocketAddress address) throws SocketException
The first creates a DatagramSocket bound to any unused port on the local computer. The
second creates a DatagramSocket bound to the port specified by port. The third constructs
a DatagramSocket bound to the specified port and InetAddress. The fourth constructs a
DatagramSocket bound to the specified SocketAddress. SocketAddress is an abstract
22-ch22.indd 742 14/02/14 5:11 PM

PartII
class that is implemented by the concrete class InetSocketAddress. InetSocketAddress
encapsulates an IP address with a port number. All can throw a SocketException if an
error occurs while creating the socket.
DatagramSocket defines many methods. Two of the most important are send( ) and
receive( ), which are shown here:
void send(DatagramPacket packet) throws IOException
void receive(DatagramPacket packet) throws IOException
The send( ) method sends a packet to the port specified by packet. The receive( ) method
waits for a packet to be received and returns the result.
DatagramSocket also defines the close( )method, which closes the socket. Beginning
with JDK 7, DatagramSocket implements AutoCloseable, which means that a DatagramSocket
can be managed by a try-with-resources block.
Other methods give you access to various attributes associated with a DatagramSocket.
Here is a sampling:
InetAddress getInetAddress( ) If the socket is connected, then the address is returned.
Otherwise, null is returned.
int getLocalPort( ) Returns the number of the local port.
int getPort( ) Returns the number of the port to which the socket is
connected. It returns –1 if the socket is not connected to
a port.
boolean isBound( ) Returns true if the socket is bound to an address. Returns
false otherwise.
boolean isConnected( ) Returns true if the socket is connected to a server. Returns
false otherwise.
void setSoTimeout(int millis)
throws SocketException
Sets the time-out period to the number of milliseconds
passed in millis.
DatagramPacket
DatagramPacket defines several constructors. Four are shown here:
DatagramPacket(byte data [ ], int size)
DatagramPacket(byte data [ ], int offset, int size)
DatagramPacket(byte data [ ], int size, InetAddress ipAddress, int port)
DatagramPacket(byte data [ ], int offset, int size, InetAddress ipAddress, int port)
The first constructor specifies a buffer that will receive data and the size of a packet. It
is used for receiving data over a DatagramSocket. The second form allows you to specify an
offset into the buffer at which data will be stored. The third form specifies a target address
and port, which are used by a DatagramSocket to determine where the data in the packet
will be sent. The fourth form transmits packets beginning at the specified offset into the
data. Think of the first two forms as building an "in box," and the second two forms as
stuffing and addressing an envelope.
22-ch22.indd 743 14/02/14 5:11 PM

DatagramPacket defines several methods, including those shown here, that give access
to the address and port number of a packet, as well as the raw data and its length.
InetAddress getAddress( ) Returns the address of the source (for datagrams
being received) or destination (for datagrams
being sent).
byte[ ] getData( ) Returns the byte array of data contained in the
datagram. Mostly used to retrieve data from
the datagram after it has been received.
int getLength( ) Returns the length of the valid data contained
in the byte array that would be returned from
the getData( ) method. This may not equal the
length of the whole byte array.
int getOffset( ) Returns the starting index of the data.
int getPort( ) Returns the port number.
void setAddress(InetAddress ipAddress) Sets the address to which a packet will be sent.
The address is specified by ipAddress.
void setData(byte[ ] data) Sets the data to data, the offset to zero, and the
length to number of bytes in data.
void setData(byte[ ] data, int idx, int size) Sets the data to data, the offset to idx, and the
length to size.
void setLength(int size) Sets the length of the packet to size.
void setPort(int port) Sets the port to port.
A Datagram Example
The following example implements a very simple networked communications client and
server. Messages are typed into the window at the server and written across the network to
the client side, where they are displayed.
// Demonstrate datagrams.
import java.net.*;
class WriteServer {
public static int serverPort = 998;
public static int clientPort = 999;
public static int buffer_size = 1024;
public static DatagramSocket ds;
public static byte buffer[] = new byte[buffer_size];
public static void TheServer() throws Exception {
int pos=0;
while (true) {
int c = System.in.read();
switch (c) {
case -1:
System.out.println("Server Quits.");
22-ch22.indd 744 14/02/14 5:11 PM

PartII
ds.close();
return;
case 'r':
break;
case 'n':
ds.send(new DatagramPacket(buffer,pos,
InetAddress.getLocalHost(),clientPort));
pos=0;
break;
default:
buffer[pos++] = (byte) c;
}
}
}
public static void TheClient() throws Exception {
while(true) {
DatagramPacket p = new DatagramPacket(buffer, buffer.length);
ds.receive(p);
System.out.println(new String(p.getData(), 0, p.getLength()));
}
}
if(args.length == 1) {
ds = new DatagramSocket(serverPort);
TheServer();
} else {
ds = new DatagramSocket(clientPort);
TheClient();
}
}
}
This sample program is restricted by the DatagramSocket constructor to running between
two ports on the local machine. To use the program, run
java WriteServer
in one window; this will be the client. Then run
java WriteServer 1
This will be the server. Anything that is typed in the server window will be sent to the client
window after a newline is received.
NOTE The use of datagrams may not be allowed on your computer. (For example, a firewall may prevent
their use.) If this is the case, the preceding example cannot be used.Also, the port numbers used in
the program work on the author's system, but may have to be adjusted for your environment.
22-ch22.indd 745 14/02/14 5:11 PM

Module 1 networking basics-2

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