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📁 学习java的经典书籍
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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<a href="tij_c.html">Contents</a> | <a href="tij0163.html">Prev</a> | <a href="tij0165.html">Next</a>
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<H2 ALIGN=LEFT>
Sockets</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>socket</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is the software abstraction used to represent the &#8220;terminals&#8221; of a
connection between two machines. For a given connection, there&#8217;s a socket
on each machine, and you can imagine a hypothetical &#8220;cable&#8221; running
between the two machines with each end of the &#8220;cable&#8221; plugged into
a socket. Of course, the physical hardware and cabling between machines is
completely unknown. The whole point of the abstraction is that we don&#8217;t
have to know more than is necessary.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
Java, you create a socket to make the connection to the other machine, then you
get an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(or, with the appropriate converters, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Reader</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Writer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">from
the socket in order to be able to treat the connection as an IO stream object.
There are two stream-based socket classes: a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that a server uses to &#8220;listen&#8221; for incoming connections and a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that a client uses in order to initiate a connection. Once a client makes a
socket connection, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
returns (via the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
<A NAME="Index2633"></A><A NAME="Index2634"></A>method)
a corresponding server side 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
through which direct communications will take place. From then on, you have a
true 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
connection and you treat both ends the same way because they 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>are</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the same. At this point, you use the methods <A NAME="Index2635"></A><A NAME="Index2636"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getInputStream(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index2637"></A><A NAME="Index2638"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getOutputStream(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to produce the corresponding 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects from each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
These must be wrapped inside buffers and formatting classes just like any other
stream object described in Chapter 10.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
use of the term 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
would seem to be another example of a confusing name scheme in the Java
libraries. You might think 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
would be better named &#8220;ServerConnector&#8221; or something without the
word &#8220;Socket&#8221; in it. You might also think that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
should both be inherited from some common base class. Indeed, the two classes
do have several methods in common but not enough to give them a common base
class. Instead, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
job is to wait until some other machine connects to it, then to return an actual 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is why 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
seems to be a bit misnamed, since its job isn&#8217;t really to be a socket but
instead to make a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object when someone else connects to it.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">However,
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
does create a physical &#8220;server&#8221; or listening socket on the host
machine. This socket listens for incoming connections and then returns an
&#8220;established&#8221; socket (with the local and remote endpoints defined)
via the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. The confusing part is that both of these sockets (listening and
established) are associated with the same server socket. The listening socket
can accept only new connection requests and not data packets. So while 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t make much sense programmatically, it does &#8220;physically.&#8221;
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
you create a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you give it only a port number. You don&#8217;t have to give it an IP address
because it&#8217;s already on the machine it represents. When you create a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
however, you must give both the IP address and the port number where
you&#8217;re trying to connect. (On the other hand, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that comes back from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket.accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
already contains all this information.)
</FONT><a name="_Toc375545496"></a><a name="_Toc408018769"></a><P></DIV>
<A NAME="Heading517"></A><H3 ALIGN=LEFT>
A
simple server and client
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
example makes the simplest use of servers and clients using sockets. All the
server does is wait for a connection, then uses the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
produced by that connection to create an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
After that, everything it reads from the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
it echoes to the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
until it receives the line END, at which time it closes the connection.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
client makes the connection to the server, then creates an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Lines of text are sent through the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The client also creates an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to hear what the server is saying (which, in this case, is just the words
echoed back).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Both
the server and client use the same port number and the client uses the local
loopback address to connect to the server on the same machine so you
don&#8217;t have to test it over a network. (For some configurations, you might
need to be 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>connected</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to a network for the programs to work, even if you aren&#8217;t communicating
over that network.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here
is the server:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: JabberServer.java</font>
<font color="#009900">// Very simple server that just</font>
<font color="#009900">// echoes whatever the client sends.</font>
<font color="#0000ff">import</font> java.io.*;
<font color="#0000ff">import</font> java.net.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> JabberServer {  
  <font color="#009900">// Choose a port outside of the range 1-1024:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">final</font> <font color="#0000ff">int</font> PORT = 8080;
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) 
      <font color="#0000ff">throws</font> IOException {
    ServerSocket s = <font color="#0000ff">new</font> ServerSocket(PORT);
    System.out.println("Started: " + s);
    <font color="#0000ff">try</font> {
      <font color="#009900">// Blocks until a connection occurs:</font>
      Socket socket = s.accept();
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