jline2 print "^J" when it should print "\n" on a telnet console

Question

I am using JLine 2 (v2.13) in conjunction with a Socket IO to create an admin console for my app which can be accessed via an ordinary telnet client. However I found it prints out strange symbols when it should print out a line break:

act.app.restart  restart app^Jact.job.list  List jobs

The correct output should be:

act.app.restart  restart app
act.job.list     List jobs

I traced into the code and found the following line of code that caused the trouble:

Another weird thing is when I press the up arrow to get the history, the console prints out ^[[A and nothing else happens.

Does anyone have any idea?

Update: added relevant source code

1. The code accepting the incoming telnet connection:

2. The code that creates the ConsoleReader and dispatches the command:

Update 2

Sorry I've disappeared for a while. Just come back as my project gives me a breath of fresh air. So here is what happened: I've created a PrintWriter and use it as a workaround for the ^J issue:

However there are many other issues when jline2 is used along with telnet session:

1. Type <TAB> displays tab instead of activate complete list. However after I hit enter key it gives me a complete list plus an error message:
2. Type <UP> arrow, displays ^[[A, hit enter key it will execute my last command. However I lost the command line editing

The issue is still there and I believe there should be a way to solve it just need some guide...

Answer 1

Edit: My bad, the method I showed is private. Still, it's not a bug -it's code doing what it should be doing. The two public methods that use rawPrint (I put them at the end) use it only for masked output and apparently for printing completions, so ConsoleReader probably doesn't support your use case -it might be a design bug, but I think its meant to be that way. The arrow key problems are mentioned in the docs in passing as a terminal configuration issue.

ConsoleReader.java is here: https://github.com/jline/jline2/blob/master/src/main/java/jline/console/ConsoleReader.java

I think it's the last version, etc.

What you are doing is creating a ConsoleReader the calling println() on it. LF is shown as ^J. This is not a bug in the sense that the method is doing what it's supposed to do:

/*3478*/ public void println(final CharSequence s) throws IOException {
    print(s);
    println();
}

which eventually calls

/*3445*/private int print(final CharSequence buff, int start, int end, int cursorPos) throws IOException {
        checkNotNull(buff);
        for (int i = start; i < end; i++) {
            char c = buff.charAt(i);
            if (c == '\t') {
                int nb = nextTabStop(cursorPos);
                cursorPos += nb;
                while (nb-- > 0) {
                    out.write(' ');
                }
            } else if (c < 32) {
                out.write('^');
                out.write((char) (c + '@')); //LF -> ^J

This function does what you want but is private:

/*3510*/ private void rawPrintln(final String s) throws IOException {
    rawPrint(s); //
    println();
}

/*3499*/ final void rawPrint(final String str) throws IOException {
    out.write(str);
    cursorOk = false;
}

It can be called through the public methods putString(final CharSequence str) at line 895 and printColumns(final Collection items) at line 3715. putString only calls it when using masked output so it's useless to you, and printColumns seems to be meant for completions.

Maybe you are supposed to print lines separately and let ConsoleReader add newlines to them? Technically, LF is a control code and it makes sense to forbid ConsoleReader to print control codes as is. Just split your input in lines, print them one by one.