ch9.htm

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has a destination and return address on the outside and contains
the data to be sent on the inside. A socket in this mode does
not need to connect to a destination socket; it simply sends the
datagram. The UDP protocol promises only to make a best-effort
delivery attempt. Connectionless operation is fast and efficient,
but not guaranteed.
<P>
Connection-oriented operation uses the Transport Control Protocol
(TCP). A socket in this mode needs to connect to the destination
before sending data. Once connected, the sockets are accessed
using a streams interface: open-read-write-close. Everything sent
by one socket is received by the other end of the connection in
exactly the same order it was sent. Connection-oriented operation
is less efficient than connectionless, but it's guaranteed.
<P>
Sun Microsystems has always been a proponent of internetworking,
so it isn't surprising to find rich support for sockets in the
Java class hierarchy. In fact, the Java classes have significantly
reduced the skill needed to create a sockets program. Each transmission
mode is implemented in a separate set of Java classes. The connection-oriented
classes will be discussed first.
<H3><A NAME="JavaConnectionOrientedClasses">Java Connection-Oriented
Classes</A></H3>
<P>
The connection-oriented classes within Java have both a client
and a server representative. The client half tends to be the simplest
to set up, so it will be covered first.
<P>
Listing 9.1 shows a simple client application. It requests an
HTML document from a server and displays the response to the console.
<HR>
<BLOCKQUOTE>
<B>Listing 9.1. A simple socket client.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.io.*;<BR>
import java.net.*;<BR>
<BR>
/**<BR>
 * An application that opens a connection to a Web server and
reads<BR>
 * a single Web page from the connection.<BR>
 * NOTE: &quot;merlin&quot; is the name of my local machine.<BR>
 */<BR>
public class SimpleWebClient {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public static void main(String args[])
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Open a client socket connection<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Socket
clientSocket1 = new Socket(&quot;merlin&quot;, 80);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;Client1:
&quot; + clientSocket1);<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Get a Web page<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;getPage(clientSocket1);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch (UnknownHostException
uhe)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;UnknownHostException:
&quot; + uhe);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch (IOException
ioe)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.err.println(&quot;IOException:
&quot; + ioe);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;/**<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Request a Web page using the passed
client socket.<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Display the reply and close the client
socket.<BR>
&nbsp;&nbsp;&nbsp;&nbsp; */<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public static void getPage(Socket clientSocket)
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Acquire the input and output streams<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;DataOutputStream
outbound = new DataOutputStream(<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;clientSocket.getOutputStream()
);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;DataInputStream
inbound = new DataInputStream(<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;clientSocket.getInputStream()
);<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Write the HTTP request to the server<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;outbound.writeBytes(&quot;GET
/ HTTP/1.0\r\n\r\n&quot;);<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Read the response<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String
responseLine;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;while
((responseLine = inbound.readLine()) != null)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Display each line to the console<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(responseLine);
<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
This code checks for EOF.&nbsp;&nbsp;There is a bug in the<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
socket close code under Win 95.&nbsp;&nbsp;readLine() will<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
not return null when the client socket is closed<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
by the server.<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
( responseLine.indexOf(&quot;&lt;/HTML&gt;&quot;) != -1 )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;break;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Clean up<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;outbound.close();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;inbound.close();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;clientSocket.close();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch (IOException
ioe)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;IOException:
&quot; + ioe);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD><B>Note</B></TD></TR>
<TR VALIGN=TOP><TD>
<BLOCKQUOTE>
The examples in this chapter are coded as applications so as to avoid security restrictions. Run the code from the command line <TT>java ClassName</TT>.
</BLOCKQUOTE>

</TD></TR>
</TABLE></CENTER>
<P>
Recall that a client socket issues a connect to a listening server
socket. Client sockets are created and connected by using a constructor
from the Socket class. The following line creates a client socket
and connects it to a host:
<BLOCKQUOTE>
<TT>Socket clientSocket = new Socket(&quot;merlin&quot;,
80);</TT>
</BLOCKQUOTE>
<P>
The first parameter is the name of the host you want to connect
to; the second parameter is the port number. A host name specifies
only the destination computer. The port number is required to
complete the transaction and allow an individual application to
receive the call. In this case, 80 was specified, the well-known
port number for the HTTP protocol. Other well-known port numbers
are shown in Table 9.1. Port numbers are not mandated by any governing
body, but are assigned by convention-this is why they are said
to be &quot;well known.&quot;<BR>
<P>
<CENTER><B>Table 9.1. Well-known port numbers.</B></CENTER>
<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=50%>
<TR VALIGN=TOP><TD WIDTH=73><I>Service</I></TD><TD WIDTH=48><CENTER><I>Port</I></CENTER>
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=73>echo</TD><TD WIDTH=48><CENTER>7</CENTER></TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=73>daytime</TD><TD WIDTH=48><CENTER>13</CENTER>
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=73>ftp</TD><TD WIDTH=48><CENTER>21</CENTER></TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=73>telnet</TD><TD WIDTH=48><CENTER>23</CENTER></TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=73>smtp</TD><TD WIDTH=48><CENTER>25</CENTER></TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=73>finger</TD><TD WIDTH=48><CENTER>79</CENTER></TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=73>http</TD><TD WIDTH=48><CENTER>80</CENTER></TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=73>pop3</TD><TD WIDTH=48><CENTER>110</CENTER></TD>
</TR>
</TABLE></CENTER>
<P>
<P>
Because the Socket class is connection oriented, it provides a
streams interface for reads and writes. Classes from the java.io
package should be used to access a connected socket:
<BLOCKQUOTE>
<TT>DataOutputStream outbound = new DataOutputStream(
clientSocket.getOutputStream() );<BR>
DataInputStream inbound = new DataInputStream( clientSocket.getInputStream()
);</TT>
</BLOCKQUOTE>
<P>
Once the streams are created, normal stream operations can be
performed:
<BLOCKQUOTE>
<TT>outbound.writeBytes(&quot;GET / HTTP/1.0\r\n\r\n);
<BR>
String responseLine;<BR>
while ( (responseLine = inbound.readLine()) != null)<BR>
{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(responseLine);<BR>
}</TT>
</BLOCKQUOTE>
<P>
The above code snippet requests a Web page and echoes the response
to the screen. When the program is done using the socket, the
connection needs to be closed:
<BLOCKQUOTE>
<TT>outbound.close();<BR>
inbound.close();<BR>
clientSocket.close();</TT>
</BLOCKQUOTE>
<P>
Notice that the socket streams are closed first. All socket streams
should be closed before the socket is closed. This application
is relatively simple, but all client programs follow the same
basic script:
<OL>
<LI>Create the client socket connection.
<LI>Acquire read and write streams to the socket.
<LI>Use the streams according to the server's protocol.
<LI>Close the streams.
<LI>Close the socket.
</OL>
<P>
Using a server socket is only slightly more complicated, as explained
in the following section.
<H4>Server Sockets</H4>
<P>
Listing 9.2 is a partial listing of a simple server application.
The complete server example can be found on the CD-ROM in SimpleWebServer.java.
<HR>
<BLOCKQUOTE>
<B>Listing 9.2. A simple server application.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>/**<BR>
 * An application that listens for connections and serves a simple
<BR>
 * HTML document.<BR>
 */<BR>
class SimpleWebServer {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public static void main(String args[])
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ServerSocket serverSocket
= null;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Socket clientSocket
= null;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;int connects =
0;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Create the server socket<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;serverSocket
= new ServerSocket(80, 5);<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;while
(connects &lt; 5)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Wait for a connection<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;clientSocket
= serverSocket.accept();<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//Service
the connection<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ServiceClient(clientSocket);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;connects++;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;serverSocket.close();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch (IOException