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学习java的经典书籍

Note that these could be any streams, they just happen to be connected to the
network. 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
the client sends the line consisting of &#8220;END&#8221; the program breaks
out of the loop and closes the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here&#8217;s
the client:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: JabberClient.java</font>
<font color="#009900">// Very simple client that just sends</font>
<font color="#009900">// lines to the server and reads lines</font>
<font color="#009900">// that the server sends.</font>
<font color="#0000ff">import</font> java.net.*;
<font color="#0000ff">import</font> java.io.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> JabberClient {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) 
      <font color="#0000ff">throws</font> IOException {
    <font color="#009900">// Passing null to getByName() produces the</font>
    <font color="#009900">// special "Local Loopback" IP address, for</font>
    <font color="#009900">// testing on one machine w/o a network:</font>
    InetAddress addr = 
      InetAddress.getByName(<font color="#0000ff">null</font>);
    <font color="#009900">// Alternatively, you can use </font>
    <font color="#009900">// the address or name:</font>
    <font color="#009900">// InetAddress addr = </font>
    <font color="#009900">//    InetAddress.getByName("127.0.0.1");</font>
    <font color="#009900">// InetAddress addr = </font>
    <font color="#009900">//    InetAddress.getByName("localhost");</font>
    System.out.println("addr = " + addr);
    Socket socket = 
      <font color="#0000ff">new</font> Socket(addr, JabberServer.PORT);
    <font color="#009900">// Guard everything in a try-finally to make</font>
    <font color="#009900">// sure that the socket is closed:</font>
    <font color="#0000ff">try</font> {
      System.out.println("socket = " + socket);
      BufferedReader in =
        <font color="#0000ff">new</font> BufferedReader(
          <font color="#0000ff">new</font> InputStreamReader(
            socket.getInputStream()));
      <font color="#009900">// Output is automatically flushed</font>
      <font color="#009900">// by PrintWriter:</font>
      PrintWriter out =
        <font color="#0000ff">new</font> PrintWriter(
          <font color="#0000ff">new</font> BufferedWriter(
            <font color="#0000ff">new</font> OutputStreamWriter(
              socket.getOutputStream())),<font color="#0000ff">true</font>);
      <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 10; i ++) {
        out.println("howdy " + i);
        String str = in.readLine();
        System.out.println(str);
      }
      out.println("END");
    } <font color="#0000ff">finally</font> {
      System.out.println("closing...");
      socket.close();
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
you can see all three ways to produce the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InetAddress</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of the local loopback IP address: using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>localhost</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
or the explicit reserved address 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>127.0.0.1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Of course, if you want to connect to a machine across a network you substitute
that machine&#8217;s IP address. When the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InetAddress
addr
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is printed (via the automatic call to its 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method) the result is:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">localhost/127.0.0.1</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">By
handing 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getByName(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it defaulted to finding the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>localhost</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and that produced the special address 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>127.0.0.1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that the <A NAME="Index2655"></A><A NAME="Index2656"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is created with both the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InetAddress</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and the port number. To understand what it means when you print out one of these 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">objects,
remember that an Internet connection is determined uniquely by these four
pieces of data: 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>clientHost</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>clientPortNumber</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>serverHost</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>serverPortNumber</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
When the server comes up, it takes up its assigned port (8080) on the localhost
(127.0.0.1). When the client comes up, it is allocated to the next available
port on its machine, 1077 in this case, which also happens to be on the same
machine (127.0.0.1) as the server. Now, in order for data to move between the
client and server, each side has to know where to send it. Therefore, during
the process of connecting to the &#8220;known&#8221; server, the client sends a
&#8220;return address&#8221; so the server knows where to send its data. This
is what you see in the example output for the server side:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">Socket[addr=127.0.0.1,port=1077,localport=8080]</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
means that the server just accepted a connection from 127.0.0.1 on port 1077
while listening on its local port (8080). On the client side:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">Socket[addr=localhost/127.0.0.1,PORT=8080,localport=1077]</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">which
means that the client made a connection to 127.0.0.1 on port 8080 using the
local port 1077.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You&#8217;ll
notice that every time you start up the client anew, the local port number is
incremented. It starts at 1025 (one past the reserved block of ports) and keeps
going up until you reboot the machine, at which point it starts at 1025 again.
(On UNIX machines, once the upper limit of the socket range is reached, the
numbers will wrap around to the lowest available number again.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Once
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object has been created, the process of turning it into a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BufferedReader</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PrintWriter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is the same as in the server (again, in both cases you start with a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">).
Here, the client initiates the conversation by sending the string
&#8220;howdy&#8221; followed by a number. Note that the buffer must again be
flushed (which happens automatically via the second argument to the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PrintWriter
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">constructor).
If the buffer isn&#8217;t flushed, the whole conversation will hang because the
initial &#8220;howdy&#8221; will never get sent (the buffer isn&#8217;t full
enough to cause the send to happen automatically). Each line that is sent back
from the server is written to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>System.out</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to verify that everything is working correctly. To terminate the conversation,
the agreed-upon &#8220;END&#8221; is sent. If the client simply hangs up, then
the server throws an exception.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that the same care is taken here to ensure that the network resources
represented by the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are properly cleaned up, using a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try-finally</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
block.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Sockets
produce a <A NAME="Index2657"></A>&#8220;dedicated&#8221;
connection that persists until it is explicitly disconnected. (The dedicated
connection can still be disconnected un-explicitly if one side, or an
intermediary link, of the connection crashes.) This means the two parties are
locked in communication and the connection is constantly open. This seems like
a logical approach to networking, but it puts an extra load on the network.
Later in the chapter you&#8217;ll see a different approach to networking, in
which the connections are only temporary.
</FONT><a name="_Toc375545497"></a><a name="_Toc408018770"></a><P></DIV>

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