Pipe input to Java program in Bash

Question

I have a java program:

public class ProcessMain {
    public static final void main(String[] args) throws Exception {        

        Scanner keyboard = new Scanner(System.in);
        boolean exit = false;
            do
            {   if(keyboard.hasNext()){
                    String input = keyboard.next();
                    System.out.println(input);
                    if( "abort".equals(input)){
                    ABORT();
                    exit = true;
                    }
                }else{
                    System.out.println("Nothing");
                    try {
                        Thread.sleep(5000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }while (!exit);
        }

    private static void ABORT(){
        System.out.println("ABORT!!!!");
    }
}

In Linux, a script:

rm testfifo
mkfifo testfifo
cat > testfifo &
echo $!


java -cp "Test.jar" com.example.ProcessMain < testfifo

Terminal A runs the script, "Nothing" can be printed every 5 seconds. And then Terminal B execute echo "abort" >testfifo, but the program cannot display ABORT, it still displays Nothing every 5 seconds.

Please help!!

Do you want to read also other input from the fifo? If not what do you want to achieve? Maybe there is a better way to do. — SubOptimal
– SubOptimal, Commented May 10, 2016 at 8:27
@SubOptimal Here is what I want to achieve: The program should be a long running process. After a while, I have to input "abort" to stop the process. Thanks. — William LAM
– William LAM, Commented May 10, 2016 at 8:29

SubOptimal · Accepted Answer · 2016-05-10 09:36:59Z

2

If you only need an external trigger to stop current processing. You might create a semaphore file and stop as soon it is created by another process.

See the following snippet.

// it will check for the file in the current directory
File semaphore = new File("abort.semaphore");
semaphore.deleteOnExit();
System.out.println("run until exist: " + semaphore);
while (!semaphore.exists()) {
    System.out.println("Nothing");
    try {
        Thread.sleep(5000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}
System.out.println("ABORT!!!!");

As long the file abort.semaphore does not exist, the program will print to the console and wait five seconds.

edit On Linux you might use a signal handler and send an SIGABRT to the running process.

the following snippet uses an internal proprietary API

import sun.misc.Signal;
import sun.misc.SignalHandler;

public class SigAbrt {

  private static volatile boolean abort = false;

  public static void main(String[] args) throws Exception {

    Signal.handle(new Signal("ABRT"), new SignalHandler () {
      public void handle(Signal sig) {
        System.out.println("got a SIGABRT");
        abort = true;
      }
    });

    for(int i=0; i<100; i++) {
      Thread.sleep(1000);
      System.out.print('.');
      if (abort) {
          System.out.println("ABORT");
          break;
      }
    }
  }
}

run it

session one

java SigAbrt

session two

// first find the PID of SigAbrt
jps

example output of session two

2323  Jps
4242  SigAbrt

now send a SIGABRT to the SigAbrt process

kill -s SIGABRT 4242

example output of session one

...........got a SIGABRT
.ABORT

edited May 10, 2016 at 9:36

answered May 10, 2016 at 8:38

SubOptimal

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2 Comments

William LAM Over a year ago

This can solve my problem! Is there any disadvantage on monitoring a file instead of linux shell pipe?

SubOptimal Over a year ago

@WilliamLAM Is there any disadvantage on monitoring a file depending on the frequency you check, the speed of the file system where the file is checked there might be a performance impact. Depending on the former and if you are able to, try to create it on an in-memory filesystem (e.g. a directory which is mount with tmpfs, this might show you some examples mount -t tmpfs). I add for Linux an other solution. Your process could also listen on a socket or ... There are other possibilities aswell. The easiest is the semaphore one. You need to make the decission.

Mahendra · Accepted Answer · 2016-05-10 06:58:51Z

0

Program not printing on console might be because of your testfifo file is empty.

Try this:

printf "Hello\nMy\nFriend\nabort" > testfifo

java -cp "Test.jar" com.example.ProcessMain < testfifo

It will work.

answered May 10, 2016 at 6:58

Mahendra

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1 Comment

William LAM Over a year ago

It works at the start of the program. But I would like to abort the program after it runs a while. Please help.

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