[net] Telnet program doesn't terminate with write()

[Anjib Mulepati <an...@hotmail.com>]

Hi All,

I couldn't understand why this program doesn't terminate if I send 
command? If I don't send the command it does terminate. I am trying to 
understand the way to issue sequence of command in telnet server.

public class TelnetSample {
     private TelnetClient telnet = new TelnetClient();
       private InputStream in;
       private PrintStream out;
       private char prompt = '.';

       public TelnetSample( String server, String user, String password ) {
        try {
          // Connect to the specified server
          telnet.connect( server, 23 );

          // Get input and output stream references
          in = telnet.getInputStream();
          out = new PrintStream( telnet.getOutputStream() );

          // Advance to a prompt
         readUntil( prompt + " " );
        }
        catch( Exception e ) {
          e.printStackTrace();
        }
       }

        public String readUntil( String pattern ) {
        try {
          char lastChar = pattern.charAt( pattern.length() - 1 );
          StringBuffer sb = new StringBuffer();
          char ch = ( char )in.read();
          while( true ) {
           System.out.print( ch );
           sb.append( ch );
           if( ch == lastChar ) {
             if( sb.toString().endsWith( pattern ) ) {
              return sb.toString();
             }
           }
           ch = ( char )in.read();
          }
        }
        catch( Exception e ) {
          e.printStackTrace();
        }
        return null;
       }

       public void write( String value ) {
        try {
          out.println( value );
          out.flush();
          System.out.println( value );
        }
        catch( Exception e ) {
          e.printStackTrace();
        }
       }

       public String sendCommand( String command ) {
        try {
          write( command );
          return readUntil( prompt + " " );
        }
        catch( Exception e ) {
          e.printStackTrace();
        }
        return null;
       }

       public void disconnect() {
        try {
          telnet.disconnect();
        }
        catch( Exception e ) {
          e.printStackTrace();
        }
       }

       public static void main( String[] args ) {
        try {
          TelnetSample telnet = new TelnetSample( "telehack.com",
                              "username",
                              "password" );
          telnet.sendCommand( "date" );
          telnet.disconnect();
          System.out.println("Disconnected");
        }
        catch( Exception e ) {
          e.printStackTrace();
        }
       }

}


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Re: [net] Telnet program doesn't terminate with write()

[sebb <se...@gmail.com>]

On 13 September 2014 20:58, Anjib Mulepati <an...@hotmail.com> wrote:
> Hi All,

What version of NET are you using?
Java? OS?

> I couldn't understand why this program doesn't terminate if I send command?

What do you mean by not terminating?
Does it print "Disconnected" ?
Or does it hang somewhere previously, if so where?

> If I don't send the command it does terminate. I am trying to understand the
> way to issue sequence of command in telnet server.
>
> public class TelnetSample {
>     private TelnetClient telnet = new TelnetClient();
>       private InputStream in;
>       private PrintStream out;
>       private char prompt = '.';
>
>       public TelnetSample( String server, String user, String password ) {
>        try {
>          // Connect to the specified server
>          telnet.connect( server, 23 );
>
>          // Get input and output stream references
>          in = telnet.getInputStream();
>          out = new PrintStream( telnet.getOutputStream() );
>
>          // Advance to a prompt
>         readUntil( prompt + " " );
>        }
>        catch( Exception e ) {
>          e.printStackTrace();
>        }
>       }
>
>        public String readUntil( String pattern ) {
>        try {
>          char lastChar = pattern.charAt( pattern.length() - 1 );
>          StringBuffer sb = new StringBuffer();
>          char ch = ( char )in.read();

What happens if read reaches EOF?

>          while( true ) {
>           System.out.print( ch );
>           sb.append( ch );
>           if( ch == lastChar ) {
>             if( sb.toString().endsWith( pattern ) ) {
>              return sb.toString();
>             }
>           }
>           ch = ( char )in.read();

Ditto

>          }
>        }
>        catch( Exception e ) {
>          e.printStackTrace();
>        }
>        return null;
>       }
>
>       public void write( String value ) {
>        try {
>          out.println( value );
>          out.flush();
>          System.out.println( value );
>        }
>        catch( Exception e ) {
>          e.printStackTrace();
>        }
>       }
>
>       public String sendCommand( String command ) {
>        try {
>          write( command );
>          return readUntil( prompt + " " );
>        }
>        catch( Exception e ) {
>          e.printStackTrace();
>        }
>        return null;
>       }
>
>       public void disconnect() {
>        try {
>          telnet.disconnect();
>        }
>        catch( Exception e ) {
>          e.printStackTrace();
>        }
>       }
>
>       public static void main( String[] args ) {
>        try {
>          TelnetSample telnet = new TelnetSample( "telehack.com",
>                              "username",
>                              "password" );
>          telnet.sendCommand( "date" );
>          telnet.disconnect();
>          System.out.println("Disconnected");
>        }
>        catch( Exception e ) {
>          e.printStackTrace();
>        }
>       }
>
> }
>
>
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