// introduction to server sockets in Java, by Narendra Singhal and
// Norm Matloff; paired with Clnt.java

// usage:  java Svr

// accepts a command w or ps from the client, runs the command on the
// local machine, and then sends the command's output back to the
// client, which displays it there

// note:  at the bottom level, there are still the standard system
// calls, e.g. socket(), connect(), bind(), etc.; the difference is that
// in Java they are made by the underlying JVM, rather than by the
// programmer him/herself

import java.lang.*;
import java.io.*;
import java.net.*;

public class Svr  {

   private static final int Port = 3000;

   private static ServerSocket SvrSock = null;

   public static void main(String[] Args) {

      // have special class, ServerSocket, for server sockets; 
      // constructor's actions include includes "bind" operation
      try  {
         SvrSock = new ServerSocket(Port); 
      }
      catch(Exception E) {
         System.out.println( "couldn't create server socket; " + E);
         System.exit(1);    
      }
      System.out.println("server is up ");

      while (true) ProcessACall();
   }

   public static void ProcessACall()

   {  System.out.println("waiting for client connection");
      try  { 

         // when accept() gets a "phone call" from the client, it
         // sets up an ordinary socket (see comments on Socket class 
         // in Clnt.java)
         Socket TmpSock = SvrSock.accept();
         System.out.println("connection from client");
         InputStream Input = TmpSock.getInputStream();
         OutputStream Output = TmpSock.getOutputStream();

         // read command from the client 
         int ClntByte = Input.read();
         String Cmd;
         if (ClntByte == 'w') Cmd = "w";
         else Cmd = "ps";

         try  {
            // set up a non-Java process running Cmd on the
            // native machine (i.e. not on the JVM); throws IOException
            Process Prcs = Runtime.getRuntime().exec(Cmd);

            // get the output from the command
            InputStream In = Prcs.getInputStream();
            byte CmdOut[];
            CmdOut = new byte[1000];
            int CmdOutLength = In.read(CmdOut);

            // send that output and a "homemade" EOF to the client
            CmdOut[CmdOutLength] = '\n';
            CmdOut[CmdOutLength+1] = 'e';
            CmdOut[CmdOutLength+2] = 'n';
            CmdOut[CmdOutLength+3] = 'd';
            CmdOut[CmdOutLength+4] = '\n';
            Output.write(CmdOut);
          }
          catch(IOException E)  {
             System.out.println("" + E);
          }
      }
      catch (Exception E)  {
         System.out.println("connection to current client broken");
      }
   }

}