Java套接字实现网络编程之基础篇

　　在SSClient源代码编译完成后，可以输入java SSClient 来执行这段程序，如果有合适的程序运行在不同的主机上，采用主机名/IP地址为参数的输入方式，比如www.sina.com.cn是运行服务器程序的主机，那么输入方式就是java SSClientwww.sina.com.cn。

　　技巧

　　Socket类包含了许多有用的方法。比如getLocalAddress()将返回一个包含客户程序IP地址的InetAddress子类对象的引用;getLocalPort()将返回客户程序的端口号;getInetAddress()将返回一个包含服务器IP地址的InetAddress子类对象的引用;getPort()将返回服务程序的端口号。

　　ServerSocket类

　　由于SSClient使用了流套接字，所以服务程序也要使用流套接字。这就要创建一个ServerSocket对象，ServerSocket有几个构造函数，最简单的是ServerSocket(int port)，当使用ServerSocket(int port)创建一个ServerSocket对象，port参数传递端口号，这个端口就是服务器监听连接请求的端口，如果在这时出现错误将抛出IOException异常对象，否则将创建ServerSocket对象并开始准备接收连接请求。

　　接下来服务程序进入无限循环之中，无限循环从调用ServerSocket的accept()方法开始，在调用开始后accept()方法将导致调用线程阻塞直到连接建立。在建立连接后accept()返回一个最近创建的Socket对象，该Socket对象绑定了客户程序的IP地址或端口号。

　　由于存在单个服务程序与多个客户程序通讯的可能，所以服务程序响应客户程序不应该花很多时间，否则客户程序在得到服务前有可能花很多时间来等待通讯的建立，然而服务程序和客户程序的会话有可能是很长的(这与电话类似)，因此为加快对客户程序连接请求的响应，典型的方法是服务器主机运行一个后台线程，这个后台线程处理服务程序和客户程序的通讯。

　　为了示范我们在上面谈到的慨念并完成SSClient程序，下面我们创建一个SSServer程序，程序将创建一个ServerSocket对象来监听端口10000的连接请求，如果成功服务程序将等待连接输入，开始一个线程处理连接，并响应来自客户程序的命令。下面就是这段程序的代码:Listing 3: SSServer.java

// SSServer.java
import java.io.*;import java.net.*;
import java.util.*;
class SSServer
{
　public static void main (String [] args) throws IOException
　{　
System.out.println ("Server starting...\n");
// Create a server socket that listens for incoming connection
// requests on port 10000.
ServerSocket server = new ServerSocket (10000);
while (true)
{
　// Listen for incoming connection requests from client
　// programs, establish a connection, and return a Socket
　// object that represents this connection.
　Socket s = server.accept ();
　System.out.println ("Accepting Connection...\n");
　// Start a thread to handle the connection.
　new ServerThread (s).start ();
}
　}
}
class ServerThread extends Thread
{
　private Socket s;
　ServerThread (Socket s)
　{
this.s = s;
　}
　public void run ()
　{
BufferedReader br = null;
PrintWriter pw = null;
try
{
　// Create an input stream reader that chains to the socket's
　// byte-oriented input stream. The input stream reader
　// converts bytes read from the socket to characters. The
　// conversion is based on the platform's default character
　// set.
　InputStreamReader isr;
　isr = new InputStreamReader (s.getInputStream ());
　// Create a buffered reader that chains to the input stream
　// reader. The buffered reader supplies a convenient method
　// for reading entire lines of text.
　br = new BufferedReader (isr);
　// Create a print writer that chains to the socket's byte-
　// oriented output stream. The print writer creates an
　// intermediate output stream writer that converts
　// characters sent to the socket to bytes. The conversion
　// is based on the platform's default character set.
　pw = new PrintWriter (s.getOutputStream (), true);
　// Create a calendar that makes it possible to obtain date
　// and time information.
　Calendar c = Calendar.getInstance ();
　// Because the client program may send multiple commands, a
　// loop is required. Keep looping until the client either
　// explicitly requests termination by sending a command
　// beginning with letters BYE or implicitly requests
　// termination by closing its output stream.
　do
　{
// Obtain the client program's next command.
String cmd = br.readLine ();
// Exit if client program has closed its output stream.
if (cmd == null)
　break;
// Convert command to uppercase, for ease of comparison.
cmd = cmd.toUpperCase ();
// If client program sends BYE command, terminate.
if (cmd.startsWith ("BYE"))
　break;
// If client program sends DATE or TIME command, return
// current date/time to the client program.
if (cmd.startsWith ("DATE") || cmd.startsWith ("TIME"))
　pw.println (c.getTime ().toString ());
// If client program sends DOM (Day Of Month) command,
// return current day of month to the client program.
if (cmd.startsWith ("DOM"))
　pw.println ("" + c.get (Calendar.DAY_OF_MONTH));
// If client program sends DOW (Day Of Week) command,
// return current weekday (as a string) to the client
// program.
if (cmd.startsWith ("DOW"))
　switch (c.get (Calendar.DAY_OF_WEEK))
{
　case Calendar.SUNDAY : pw.println ("SUNDAY");
break;
　case Calendar.MONDAY : pw.println ("MONDAY");
break;
　case Calendar.TUESDAY : pw.println ("TUESDAY");
break;
　case Calendar.WEDNESDAY: pw.println ("WEDNESDAY");
break;
　case Calendar.THURSDAY : pw.println ("THURSDAY");
break;
　case Calendar.FRIDAY : pw.println ("FRIDAY");
break;
　case Calendar.SATURDAY : pw.println ("SATURDAY");
}
// If client program sends DOY (Day of Year) command,
// return current day of year to the client program.
if (cmd.startsWith ("DOY"))
　pw.println ("" + c.get (Calendar.DAY_OF_YEAR));
　// If client program sends PAUSE command, sleep for three
　// seconds.
　
if (cmd.startsWith ("PAUSE"))
try
{
　Thread.sleep (3000);
}
catch (InterruptedException e)
{
}
　　while (true);
　{
　catch (IOException e)
　{
System.out.println (e.toString ());
　}
　finally
　{
System.out.println ("Closing Connection...\n");
try
{
　if (br != null)
br.close ();
if (pw != null)
　pw.close ();
if (s != null)
　s.close ();
}
catch (IOException e)
{
}
　}
}
}

运行这段程序将得到下面的输出: