/*
 * Copyright (c) 2000 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 2nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book (recommended),
 * visit http://www.davidflanagan.com/javaexamples2.
 */
package com.davidflanagan.examples.net;
import java.io.*;
import java.net.*;

/**
 * This class implements a simple single-threaded proxy server.
 **/
public class SimpleProxyServer {
    /** The main method parses arguments and passes them to runServer */
    public static void main(String[] args) throws IOException {
        try {
            // Check the number of arguments
            if (args.length != 3) 
                throw new IllegalArgumentException("Wrong number of args.");
	    
            // Get the command-line arguments: the host and port we are proxy
            // for and the local port that we listen for connections on.
            String host = args[0];
            int remoteport = Integer.parseInt(args[1]);
            int localport = Integer.parseInt(args[2]);
            // Print a start-up message
            System.out.println("Starting proxy for " + host + ":" + 
			       remoteport + " on port " + localport);
            // And start running the server
            runServer(host, remoteport, localport);   // never returns
        } 
        catch (Exception e) {
            System.err.println(e);
            System.err.println("Usage: java SimpleProxyServer " +
			       "<host> <remoteport> <localport>");
        }
    }
    
    /**
     * This method runs a single-threaded proxy server for 
     * host:remoteport on the specified local port.  It never returns.
     **/
    public static void runServer(String host, int remoteport, int localport) 
	throws IOException {
        // Create a ServerSocket to listen for connections with
        ServerSocket ss = new ServerSocket(localport);
	
        // Create buffers for client-to-server and server-to-client transfer.
        // We make one final so it can be used in an anonymous class below.
        // Note the assumptions about the volume of traffic in each direction.
        final byte[] request = new byte[1024];
        byte[] reply = new byte[4096];
        
        // This is a server that never returns, so enter an infinite loop.
        while(true) {
            // Variables to hold the sockets to the client and to the server.
            Socket client = null, server = null;
            try {
                // Wait for a connection on the local port
                client = ss.accept();
                
                // Get client streams.  Make them final so they can
                // be used in the anonymous thread below.
                final InputStream from_client = client.getInputStream();
                final OutputStream to_client = client.getOutputStream();

                // Make a connection to the real server.
                // If we cannot connect to the server, send an error to the 
                // client, disconnect, and continue waiting for connections.
                try { server = new Socket(host, remoteport); } 
                catch (IOException e) {
                    PrintWriter out = new PrintWriter(to_client);
                    out.print("Proxy server cannot connect to " + host + ":"+
			      remoteport + ":\n" + e + "\n");
                    out.flush();
                    client.close();
                    continue;
                }
                
                // Get server streams.
                final InputStream from_server = server.getInputStream();
                final OutputStream to_server = server.getOutputStream();

                // Make a thread to read the client's requests and pass them
                // to the server.  We have to use a separate thread because
                // requests and responses may be asynchronous.
                Thread t = new Thread() {
                    public void run() {
                        int bytes_read;
                        try {
                            while((bytes_read=from_client.read(request))!=-1) {
                                to_server.write(request, 0, bytes_read);
                                to_server.flush();
                            }
                        }
                        catch (IOException e) {}

                        // the client closed the connection to us, so close our
                        // connection to the server.  This will also cause the 
                        // server-to-client loop in the main thread exit.
                        try {to_server.close();} catch (IOException e) {}
                    }
                };

                // Start the client-to-server request thread running
                t.start();  

                // Meanwhile, in the main thread, read the server's responses
                // and pass them back to the client.  This will be done in
                // parallel with the client-to-server request thread above.
                int bytes_read;
                try {
                    while((bytes_read = from_server.read(reply)) != -1) {
                        to_client.write(reply, 0, bytes_read);
                        to_client.flush();
                    }
                }
                catch(IOException e) {}

                // The server closed its connection to us, so we close our 
                // connection to our client.
		// This will make the other thread exit.
                to_client.close();
            }
            catch (IOException e) { System.err.println(e); }
            finally {  // Close the sockets no matter what happens.
                try { 
                    if (server != null) server.close(); 
                    if (client != null) client.close(); 
                }
                catch(IOException e) {}
            }
        }
    }
}