How do I switch user with su command in Unix box programatically with Jsch libary?

Question

I have a very basic Java program that is using Jsch library to automate execution of some shell scripts located in my Unix box.

In order to automate execution of these shell scripts following steps need to be followed:

• Login into Unix box with user john
• Switch to another user simba
• Provide credentials of this new user using OutputStream bypassing the input prompt at cli and flush it using os.flush()
• Check that the user switch happened using whoami command

I wrote following program to achieve this functionality but as you can see in the console output the user switch did NOT happen and the second whoami command gives me the same username (ie: john)

Another question that comes to my mind is what should I do for commands that do not have any output from cli such as su -, logout, bash etc. It doesn't make sense to wait for the output in that infinite while(true) loop when nothing is going to be returned.

Please guide.

Current Console Output

Connecting SSH to my-unix-box.net - Please wait for few seconds... 
Connected!

Executing command: whoami
john
Executing command: su - simba
Setting suPasswd now....
Executing command: whoami
john  //WRONG: SHOULD BE "simba" 

Disconnected channel and session

Process finished with exit code 0

SSHConn.java

public class SSHConn {

    static Session session;
    static String[] commands = {"whoami", "su - simba", "whoami"};

    public static void main(String[] args) throws Exception {
        open();
        runCmd(commands);
        close();
    }

    public static void runCmd(String[] commands) throws JSchException, IOException {
        for (String cmd : commands) {
            System.out.println("Executing command: " + cmd);
            Channel channel = session.openChannel("exec");
            ((ChannelExec) channel).setCommand(cmd);
            InputStream in = channel.getInputStream();
            OutputStream out = channel.getOutputStream();
            channel.connect();
            //passing creds only when you switch user
            if (cmd.startsWith("su -")) {
                System.out.println("Setting suPasswd now....");
                out.write((Constants.suPasswd + "\n").getBytes());
                out.flush();
            }
            System.out.println("Flushed suPasswd to cli...");
            //capture output that we receive from cli (note: some commands such as "su -" does not return anything)
            if (!cmd.startsWith("su -")) {
                captureCmdOutput(in, channel);
            }
            channel.setInputStream(null);
            channel.disconnect();
        }
    }

    public static void captureCmdOutput(InputStream in, Channel channel) throws IOException {
        System.out.println("Capturing cmdOutput now...");
        byte[] tmp = new byte[1024];
        while (true) {
            System.out.println("in the while loop...");
            while (in.available() > 0) {
                System.out.println("into the available loop...");
                int i = in.read(tmp, 0, 1024);
                if (i < 0) {
                    break;
                }
                System.out.print(new String(tmp, 0, i));
            }
            if (channel.isClosed()) {
                break;
            }
            try {
                Thread.sleep(1000);
            } catch (Exception ee) {
                System.out.println(ee.getMessage());
            }
        }
        System.out.println("Command output captured...");
    }

    public static void open() throws JSchException {
        JSch jSch = new JSch();
        session = jSch.getSession(Constants.userId, Constants.host, 22);
        Properties config = new Properties();
        config.put("StrictHostKeyChecking", "no");
        session.setConfig(config);
        session.setPassword(Constants.userPasswd);
        System.out.println("Connecting SSH to " + Constants.host + " - Please wait for few seconds... ");
        session.connect();
        System.out.println("Connected!\n");
    }

    public static void close() {
        session.disconnect();
        System.out.println("\nDisconnected channel and session");
    }

}

pom.xml

<dependency>
    <groupId>com.jcraft</groupId>
    <artifactId>jsch</artifactId>
    <version>0.1.51</version>
</dependency>

su Command Usage:

 su [options] [-] [USER [arg]...]

Change the effective user id and group id to that of USER.
A mere - implies -l.   If USER not given, assume root.

Options:
 -m, -p, --preserve-environment  do not reset environment variables
 -g, --group <group>             specify the primary group
 -G, --supp-group <group>        specify a supplemental group

 -, -l, --login                  make the shell a login shell
 -c, --command <command>         pass a single command to the shell with -c
 --session-command <command>     pass a single command to the shell with -c
                                 and do not create a new session
 -f, --fast                      pass -f to the shell (for csh or tcsh)
 -s, --shell <shell>             run shell if /etc/shells allows it

 -h, --help     display this help and exit
 -V, --version  output version information and exit

For more details see su(1).