作者：Greg Travis
<让我们通过一个Java程序的例子，学习建立一个简单的TCP/IP服务器。大部分代码都放在一个类中，这样就很轻易入门->
本文介绍了快速实现一个简单的TCP/IP服务器的技巧。核心代码都在单个类Server.java中，所以很轻易建立和使用。
Server.java的结构很简单。它监听一个端口，对于每一个进来的连接，生成处理该连接的线程。实际处理该连接的子程序并没有实现，所以你必须自己扩展Server.java，写个子程序来实现。
(这样，从技术上讲，这就不是“仅用一个类的服务器”，因为你还需要用另外一个类，来完成某件事情。假如你真的坚持打算只用一个类，只需把子类中的代码放到Server.java中。)
下面的EchoServer.java的代码体现了该程序的简单性。EchoServer 完成如下的简单功能：把客户发来的信息再发回去。
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The One Class Server
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EchoServer.java
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import java.io.*;
import java.io.*;
public class EchoServer extends Server
{
// A single argument: the port on which to listen
public EchoServer( int port ) {
super( port );
}
// This function is called for each new connection; it
// implements whatever functionality is needed from the server.
// In this case, it simply sends back to the client all data
// that the client sends it. If it receives a ´q´, the
// whole server is shut down.
public void process( InputStream in, OutputStream out ) {
try {
while (true) {
int c = in.read();
if (c==´q´) {
close();
} else {
out.write( c );
}
}
} catch( IOException ie ) { System.out.println( ie ); }
}
// Command-line: "java EchoServer <port>"
static public void main( String args[] ) throws Exception {
int port = new Integer( args[0] ).intValue();
new EchoServer( port );
}
}
Server类的实现是很有趣的，因为它对于所有线程使用了一个Server类的实例。
Listener 线程
创建的第一个线程是“listener”线程。该线程中的代码对一个端口进行监听，等待进入的连接。一旦一个连接进入时，一个新的“connection”线程被创建，用于处理该连接。
Listener线程必须把用某种方法把新连接的Socket对象传送给新线程。因为Java在创建线程时，不具有传递参数的功能，所以使用了另外一个技术：哈希表。
一个哈希表对象可以把线程对象映射为Socket对象。listener 线程创建了一个新的线程，然后把该新线程和新的socket放到哈希表中。当新线程开始执行时，它从哈希表基于自己的线程对象中，通过调用Thread.currentThread()，读入socket。
对于Listener线程来说，因为它不需要其他线程“传递”什么信息，此时的哈希表中就没有Socket对象，这样，程序就可以判定这是一个listener 线程而不是一个connection线程。
下面为Listener线程的代码。现在的这个实现很简单，因为它省略了一些处理。尤其是发生例外时的处理，我没有编写这些代码。因为这需要做更多的工作，而且依靠于具体的应用。
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The One Class Server
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The Listener Thread
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import java.io.*;
import java.net.*;
import java.util.*;
abstract public class Server implements Runnable
{
// The port the server will listen on
private int port;
// Used to "pass" a Socket to the new thread that will process it
private Hashtable handoff = new Hashtable();
// The first thread -- we store it here so we can kill it
// first when closing.
private Thread listener;
// A list of the Threads that have been started
private Vector threads = new Vector();
// A list of the Sockets that have connected to us
private Vector sockets = new Vector();
// The listen socket
private ServerSocket ss;
public Server( int port ) {
this.port = port;
// Start the listener thread. Because we haven´t passed a Socket
// object to this thread in the handoff table, it will know
// that it is to be the listener thread.
listener = new Thread( this );
listener.start();
}
synchronized public void close() {
// First, make sure there aren´t any incoming connections
listener.stop();
// Now, close all the sockets
for (Enumeration e = sockets.elements(); e.hasMoreElements();){
Socket s = (Socket)e.nextElement();
try {
s.close();
} catch( IOException ie ) { System.out.println( ie );}
}
// And stop all the threads
for (Enumeration e = threads.elements(); e.hasMoreElements();){
Thread t = (Thread)e.nextElement();
// But let´s not stop *ourselves* yet!
if (t != Thread.currentThread())
t.stop();
}
System.out.println( "Shutting down!" );
// Now we can stop ourselves.
Thread.currentThread().stop();
}
// This routine does the actual work of the server. It´s not
// implemented, so you have to extend this class to actually get
// something done.
abstract public void process( InputStream in, OutputStream out );
// This routine processes all the connections. All the threads
// started by this class run this same routine of the same instance
// of Server.
public void run() {
// Get the Socket that is being "passed" to us by the listener
// thread. If there is no Socket here for us, then we ARE the
// listener thread, or at least we are about to be.
Socket s = (Socket)handoff.get( Thread.currentThread() );
if (s==null) {
// Aha -- we are the very first thread, the listener thread.
// Start listening.
try {
// Set up the listen socket.
ss = new ServerSocket( port );
System.out.println( "Listening on "+port );
while (true) {
// Accept a new connection
s = ss.accept();
synchronized( this ) {
System.out.println( "Connection from "+s.getInetAddress() );
// Make a new thread to handle this connection
Thread t = new Thread( this );
// Store the thread and socket in the lists
sockets.addElement( s );
threads.addElement( t );
// The Socket object is "passed" to the new thread by
// getting stuffed here. When the new thread is started,
// it will pull the Socket object out of here based on
// its own Thread object.
handoff.put( t, s );
// All set! Start the thread!
t.start();
}
}
} catch( IOException ie ) {}
} else {
// We are a processing socket.
try {
InputStream in = s.getInputStream();
OutputStream out = s.getOutputStream();
// Call the actually-do-something routine in the subclass of
// this object, so that something can actually get done.
process( in, out );
} catch( IOException ie ) { System.out.println( ie ); }
}
}
}
此处，我们也没有对socket，线程列表以及对handoff哈希表进行整理，大家可以修改这个类，以便能够完善这些工作，我把它们的代码编写工作作为练习，留给读者。
