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学习java的经典书籍

      <font color="#0000ff">try</font> {
        System.out.println(
          "Connection accepted: "+ 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">while</font> (<font color="#0000ff">true</font>) {  
          String str = in.readLine();
          <font color="#0000ff">if</font> (str.equals("END")) <font color="#0000ff">break</font>;
          System.out.println("Echoing: " + str);
          out.println(str);
        }
      <font color="#009900">// Always close the two sockets...</font>
      } <font color="#0000ff">finally</font> {
        System.out.println("closing...");
        socket.close();
      }
    } <font color="#0000ff">finally</font> {
      s.close();
    }
  } 
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
just needs a port number, not an IP address (since it&#8217;s running on 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>this</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
machine!). When you call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>blocks</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
until some client tries to connect to it. That is, it&#8217;s there waiting for
a connection but other processes can run (see Chapter 14). When a connection is
made, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
returns 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">
object representing that connection. 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
responsibility for cleaning up the sockets is crafted carefully here. If the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
constructor fails, the program just quits (notice we must assume that the
constructor for 
</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 leave any open network sockets lying around if it fails). For
this case, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>throws</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>IOException</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
so a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
block is not necessary. If the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
constructor is successful then all other method calls must be guarded in 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 to ensure that, no matter how the block is left, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is properly closed.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
same logic is used for the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
returned by 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
fails, then we must assume that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t exist or hold any resources, so it doesn&#8217;t need to be
cleaned up. If it&#8217;s successful, however, the following statements must be
in 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 so that if they fail the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Socket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will still be cleaned up. Care is required here because sockets use important
non-memory resources, so you must be diligent in order to clean them up (since
there is no destructor in Java to do it for you).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Both
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ServerSocket</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>accept(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are printed 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">.
This means that their 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods are automatically called. These produce:
</FONT><P></DIV>

<font color="#990000"><PRE>ServerSocket[addr=0.0.0.0,PORT=0,localport=8080]
Socket[addr=127.0.0.1,PORT=1077,localport=8080]</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Shortly,
you&#8217;ll see how these fit together with what the client is doing.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
next part of the program looks just like opening files for reading and writing
except that the 
</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">
are created from 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. Both the 
</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
are converted to Java 1.1<A NAME="Index2639"></A>
<A NAME="Index2640"></A><A NAME="Index2641"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Reader
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">and
<A NAME="Index2642"></A><A NAME="Index2643"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Writer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects using the &#8220;converter&#8221; classes <A NAME="Index2644"></A><A NAME="Index2645"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStreamReader</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index2646"></A><A NAME="Index2647"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStreamWriter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
respectively. You could also have used the Java 1.0<A NAME="Index2648"></A>
<A NAME="Index2649"></A><A NAME="Index2650"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index2651"></A><A NAME="Index2652"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
classes directly, but with output there&#8217;s a distinct advantage to using
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Writer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
approach. This appears with <A NAME="Index2653"></A><A NAME="Index2654"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PrintWriter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which has an overloaded constructor that takes a second argument, a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">flag
that indicates whether to automatically flush the output at the end of each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>println(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(but 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>not
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>print(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
statement. Every time you write to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>out</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
its buffer must be flushed so the information goes out over the network.
Flushing is important for this particular example because the client and server
each wait for a line from the other party before proceeding. If flushing
doesn&#8217;t occur, the information will not be put onto the network until the
buffer is full, which causes lots of problems in this example.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
writing network programs you need to be careful about using automatic flushing.
Every time you flush the buffer a packet must be created and sent. In this
case, that&#8217;s exactly what we want, since if the packet containing the
line isn&#8217;t sent then the handshaking back and forth between server and
client will stop. Put another way, the end of a line is the end of a message.
But in many cases messages aren&#8217;t delimited by lines so it&#8217;s much
more efficient to not use auto flushing and instead let the built-in buffering
decide when to build and send a packet. This way, larger packets can be sent
and the process will be faster.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that, like virtually all streams you open, these are buffered. There&#8217;s an
exercise at the end of the chapter to show you what happens if you don&#8217;t
buffer the streams (things get slow).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
infinite 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>while</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
loop reads lines from the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
BufferedReader in 
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">and
writes information 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">
and 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">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>out</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.