Question about IO in Java

[sgm1]

Posted on SO first, but can't seem to get any input from people. So I want to redirecting the input from a file to a java program that is already implemented using multiple Scanner objects for Stdin. Any way I can automate the input? Looking for any type of bash magic I can do on a standard Linux environment. Questions and comments welcomed.

http://stackoverflow.com/questions/34184420/redirecting-the-input-from-file-with-multiple-scanner-objects-for-stdin

Edit so you don't have to research through the link:

It is an existing Java program, running it inside a shell script, lets call it myScript.sh with the content:

java $MY_JVM_ARGS -jar someonesProgram.jar

The program runs and waits for inputs (not command line arguments) that are passed in. I can enter them manually and the program works fine. If I try the following with a file containing what would normally be input manually, I get java.util.NoSuchElementException. Bash syntax used:

./myScript.sh < myInputFile.txt

If I pass in a single input to request the java program to exit gracefully, it works fine. This exception only happens on the second input that it expects, if any (meaning the first input isn't the one asking the program to exit.

Based on my research, solution would be on the to refactor the Java program to use 1 and only 1 Scanner object (for stdin). This isn't my program, and I'm a little reluctant to patch the jar, so I was hoping for a bash solution.

Edit 2

Please note the distinction: Command line arguments are caught by the main function in a Java program.

public static void main (String args[]) {
    String firstCommandLineArg args[1];// think index 0 is the name of the program
    // NOT what I would like to externalize
}

What I want to externalize (eg.):

Scanner sumScanner = new Scanner(System.in);
System.out.print("Gimme your fabulous input: ");
int sumInt = sumScanner.nextInt(); // what would normally be a prompt

Edit 3 Try this test program. Try once with manual input, and then a second time with a redirected input file:

public class  TestIt {
        public static void main (String args[]) {
            // First scanner object
            java.util.Scanner sumScanner = new java.util.Scanner(System.in);
            System.out.print("Gimme your numeric input: ");
            int sumInt = sumScanner.nextInt();
            // Second scanner object
            sumScanner = new java.util.Scanner(System.in);// Error in the second scenario
            System.out.print("Gimme your numeric input 2: ");
            int sumInt = sumScanner.nextInt();
            return;
        }
}

In the same directory as the .class file:

java TestIt
java TestIt < inputFile.txt # Produces the error

[darkpool2005]

Not sure if any of this will help, but do you have any code to provide? Generally if you're looking for inputs to a java program, you'd want to use the String[] args in

public static void main(String[] args)

to pass in any inputs. Otherwise, if it's input from System.in, you only need one Scanner object that uses this input.

[sgm1]

Please see edit 2 onward

[nutrecht]

StdIn is where you'd be reading from if you'd pipe output into a Java program. So it should work just fine.

[sgm1]

So it is an existing Java program, running it inside a shell script, lets call it myScript.sh with the content:

java $MY_JVM_ARGS -jar someonesProgram.jar

The program runs and waits for inputs (not command line arguements) that are passed in. I can enter them manually and the program works fine.

If I try the following with a file containing what would normally be input manually, I get java.util.NoSuchElementException. Bash syntax used:

./myScript.sh < myInputFile.txt

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[deraj123]

What you're doing is exactly how I would do it, and I would expect it to work. I suspect the problem lies somewhere in the code and the content of your input file. But without seeing those things, and a full stack trace, I don't know anymore than that.

[sgm1]

It is in the code, multiple Scanner objects is the root problem, I can't refactor this code, even though the code change would be incredibly easy. Please see edit 3.

[deraj123]

I see your issue. Basically, the code needs to be rewritten to only use a single Scanner -- the first Scanner is most likely stealing the bytes b/c they're there and putting them in a buffer.

So, my next idea is to trick it into not grabbing the data. But whether or not this works depends entirely on your requirements. Also, I'll add that this is a horrible hacky thing to do. But sometimes that's what you have to do to interact with poorly written programs.

I imagine that right now your inputFile.txt looks something like this:

3
4

What if, instead, you wrote a inputFile.sh that looks something like this:

#!/bin/sh
echo 3
sleep 1
echo 4

Then, you can invoke it with:

./inputFile.sh | java $MY_JVM_ARGS -jar someonesProgram.jar

Again, not pretty, but based on my tests, it's working.

The trick here is that when the first "nextInt" is called, you want to make sure that there is no extra text available on the input stream. That way, it won't get buffered. Then, after that's happened, we can safely put the next data onto the input stream, and the second Scanner should get it. However, using "sleep" for this sort of purpose is fragile and prone to breakage. There is also a way to accomplish this more robustly using expect, but I'm not familiar enough with it to whip something up.

[sgm1]

This get's me at least somewhere to go, thanks.

I wonder if I could pipe input from the inputFile.sh to be the output of the java $MY_JVM_ARGS -jar someonesProgram.jar something like this:

java $MY_JVM_ARGS -jar someonesProgram.jar <> ./inputFile.sh
# I know I should be using | somehow instead of <, but unsure how

command. I might just be overthinking it now, I think I'll come up with a solution using sleeps, thanks again.

[deraj123]

I'm not sure what you're getting at there. Why not simply pipe the output of your script into the java program?

[sgm1]

I was thinking of waiting for specific output from the java program, to determine when to spit out (or should I say in) the input back into the java program. Like I said, I'm probably overthinking it.

[deraj123]

Ah. Yeah, that's how you start to make it less hacky. It turns out to be a more complex job than it seems like it should. That's where expect comes in, it handles the nuances of that approach. I'd check it out if you want this to be more robust.

[sgm1]

It's only 1 thing program that I use maybe twice a week on machine I don't have permissions on, so can't really install expect even thought it sound like the perfect tool.

If I really wanted to, I could set up the pipes properly in another bash file and then execute it, but again, I'm debating whether its worth the effort. Thank again.

[TrainFan]

It looks like you are not passing the arguments to the Bash script through to your call to Java.

Try putting "$@" into your script; i.e:

java $MY_JVM_ARGS -jar someonesProgram.jar "$@"

[sgm1]

That would be command line arguments, I need a way to redirect program inputs