telnet session simulation - troubles

[XiNoID]

Hello, thanks for reading my topic...I'm just pre-beginner in java, i have been studying it for 3 weeks...So..Thats enough for prologue ;)
I have troubles with this script:

import java.net.*;
import java.io.*;
 
public class Aster {
 
    private static String host = "172.20.20.247";
    private static int port = 5038;
    private static String login = "Action: Login";
    private static String usname = "UserName: admin";
    private static String password = "Secret: 12345";    
 
    private static final String CRLF = "\r\n";
 
    private static boolean DEBUG = true;
 
 
    public static void main(String args[]) {
         host = System.getProperty("host", host);
        Integer Port = Integer.getInteger("port");
        if(Port != null) port = Port.intValue();
        login = System.getProperty("login", login);
        usname = System.getProperty("usname", usname);
        password = System.getProperty("password", password);
        String debug = System.getProperty("debug", "false"); 
        if("true".equalsIgnoreCase(debug)) DEBUG = true;
 
        Socket socket = null;
        boolean error = false;
        try {
            socket = new Socket(host, port);
            BufferedReader r = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            BufferedWriter w = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
 
            readLines(r, 3);
            writeLine(w, "login");
            readLines(r, 3);
            writeLine(w, usname);
            readLines(r, 1);
            writeLine(w, password);
            readLines(r, 1);
            writeLine(w, SHUTDOWN);
            readLines(r, 1);
        } catch(IOException ioe) {
            System.out.println("I/O Error: " + ioe);
            error = true;
        } finally {        
            if(socket != null) {
                try {
                    socket.close();
                } catch(IOException ignore) {}
            }
        }
        if(error) System.exit(1);
    }
 
 
    public static void readLines(BufferedReader r, int count) throws IOException {
        for(int i = 0;i < count;i++) {
            String t = r.readLine();
            if(DEBUG) System.out.println(t);
        }
    }
 
 
    public static void writeLine(BufferedWriter w, String out) throws IOException {
        w.write(out + CRLF, 0, (out + CRLF).length());
        w.flush();
    }
}

So..this script connects to needed host..and I see the first line from server ("Welcome")...and after this line happens nothing...Commands are not sended to OutputStreamWriter..where is my mistake?
Thanks for any suggestions and answers....

[chaarmann]

I am nearly sure that you have not done this program by yourself (the programming style is too good for a beginner and the errors are too simple). I think this program is a homework for you and your teacher wants you to figure out the errors yourself, because if you do, he knows you have clearly understood how it works.

So i will not simply tell you the errors here (which would be contraproductive to your teachers intention and bad for yourself), but help you in understanding it.

First, use a debugger and execute the program step by step. Watch every variable value change. if you use eclipse, you already have a debugger inside.
If you don't have a debugger, then change the program so that it writes out the values for every variable after each line. For example if there is a line

 w.write(out + CRLF, 0, (out + CRLF).length());

in your program,
then insert 2 lines right below

System.out.println("out + CRLF =" + out + CRLF);
System.out.println("(out + CRLF).length()" + (out + CRLF).length());

Then you will automatically detect why it does not "send".

Also ask yourself:
there is written:
readLines(r, 3);
and when it executes it, you get back "Welcome". that means getting the data from server works. But what is the meaning of number 3 here? What if you use another number? Does it always come back from this method and executes the next line?:
writeLine(w, "login");
What is this line doing?

Just figure out the answers of these simple questions by debugging as described above.
If you do, you get the solution.

Happy programming,
Chris

If you give someone water when he is thirsty and asks for it, he always comes back to you. If you are not there, he surely will die. You have set up a relationship that neither he nor you like. But if you teach him how to dig a well instead of giving him water, he will be first angry that you don't give him water right away and he has to do some work, but the thirst will let him work and when he is done he will be glad to be independent and not need to bother you ever again and will surely survive.

[Nepomuk]

            ...
            readLines(r, 3);
            writeLine(w, "login");
            readLines(r, 3);
            writeLine(w, usname);
            readLines(r, 1);
            writeLine(w, password);
            readLines(r, 1);
            writeLine(w, SHUTDOWN);
            readLines(r, 1);
            ...
    }
 
 
    public static void readLines(BufferedReader r, int count) throws IOException {
        for(int i = 0;i < count;i++) {
            String t = r.readLine();
            if(DEBUG) System.out.println(t);
        }
    }
    ...
}

So..this script connects to needed host..and I see the first line from server ("Welcome")...and after this line happens nothing...Commands are not sended to OutputStreamWriter..where is my mistake?
Thanks for any suggestions and answers....

Hi XiNoID! Welcome to bytes.com!

It's great to have you here!

When you post, please always keep to the Posting Guidelines and when you post code, please post it in [code] ... [/code] tags (as, I see, you have).

About your question:
First of all, this is not a script, it's an application. Have a look at this wikipedia article to find out, what the difference is.

Now, see that call readLines(r, 3)? What is it supposed to do? And what should the output be?

See, the BufferedReader extends the Reader class. The Reader class has a method Reader.read() which is used by the BufferedReader. And in the description of Reader.read() it says:

This method will block until a character is available, an I/O error occurs, or the end of the stream is reached.

So, it's waiting for too further lines and won't continue, until it's received them or the Stream is closed. Can you figure it out from here?


Apart from that, I'll just wish you the best and hope you enjoy being part of bytes.com!

Greetings,
Nepomuk

[XiNoID]

Thanks for your replies! Chaarmann, you are right, that is not my code, it is modified example that I've found somewhere ..But it is not my homework :) I always wanted to begin serious programming (I had some little expirence before.,) And my choice stopped on Java :) I'm reading the book, that was printed in 2001 year i think i should buy newer one :)...Thanks for your advice, it will be very useful for me...(Sadly I didnt know

readLines(r, 3); 

what digit "3" means here..I will figure it out today.) Nepomuk, thank you too! i figured that

readLines(r, 3); 

"3" here means quantity of lines...and each line is an array that is placed in buffer... Everything works now :) I think i will go and buy new book it will make my study much easier i hope :) Anyway thanks everybody!

[Nepomuk]

Everything works now :) I think i will go and buy new book it will make my study much easier i hope :) Anyway thanks everybody!

I'm glad it works now. But if you're so new to programming, you may want to start with something simpler.

Check the Java Articles Index in our Java Articles Section - the Java tutorial by sun is really great and you may not even have to buy a new book. Also, there are some good books available for free as PDF files (e.g. Thinking in Java, 3rd Edition by Bruce Eckel) which you can use as further references.

Greetings,
Nepomuk