CODEWITHSUNDEEP
javamultithreadingbufferedreaderprocessbuilderbufferedwriter
Jeff B
Jeff B

Reputation: 1060

Multiprocess via processbuilder Communication, freeze at readline() for BufferedReader()

I am trying to allow communication between one program (the program launcher, if you will) and the programs it launches via processbuilder. I have the output working fine, but the input seems to stop when it reaches the readline() method in helloworld (the created process).

Below is helloworld.java:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Scanner;


public class helloworld {
public static void main (String[] args) {
System.out.println ("println(\"Hello World!\")");
System.out.println ("getInput()");
Scanner in = new Scanner(System.in);
BufferedReader br = new BufferedReader(
        new InputStreamReader(System.in));
String input = "";
try {
    // wait until we have data to complete a readLine()
    while (!br.ready()) {
      Thread.sleep(200);
    }
    System.out.println("println(\"Attempting to resolve input\")");
    input = br.readLine();

^This is where program hangs^

    if(input != null){
        System.out.println("println(\"This should appear\")");
    }
    System.out.println("println(\"input recieved " + input + "\")");
} catch (InterruptedException | IOException e) {
    System.out.println("ConsoleInputReadTask() cancelled");
  }

System.out.println("println(\"You said: " + input + "\")");
//System.out.println("println(\"You said: " + in. + "!\")");

in.close();
System.exit(0);
}
}

This is where the output (println) from the other process is recieved:

public void run() {
    try {
        //cfile = files[indexval].getAbsolutePath();
        String[] commands = 
            {
                "java", //Calling a java program
                "-cp" , //Denoting class path
                cfile.substring(0,cfile.lastIndexOf(File.separator) ), //File path
                program}; //Class name

        ProcessBuilder probuilder = new ProcessBuilder( commands );

        //start the process
        Process process = probuilder.start();

        //Read out dir output
        //probuilder.inheritIO(); //Can inherit all IO calls
        InputStream is = process.getInputStream();
        OutputStream os = process.getOutputStream();
        InputStreamReader isr = new InputStreamReader(is);
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(os));
        BufferedReader br = new BufferedReader(isr);
        String line;
        /*System.out.printf("Output of running %s is:\n",
                Arrays.toString(commands));*/
        while ((line = br.readLine()) != null) {
            myController.runCommand(line, "Please enter something!", bw);
            //System.out.println(line);
        }
        br.close();
        os.close();
        } catch (IOException e){
            e.printStackTrace();
        }
    System.out.println("programclosed");
}

And here is the function that it calls:

public synchronized void runCommand(String line, Object... arguments) throws     IOException {
    String[] tokens;
    if(line.contains("(")){
        tokens = line.split("\\(",2);

    switch(tokens[0]){
    case "println": //Println - format println(String strng)
        tokens[1] = tokens[1].substring(1, tokens[1].length() - 2);
        System.out.println(tokens[1]);
        break;
    case "getInput": //Get input - format getInput(String command, String message, BufferedWriter br)
        Scanner reader = new Scanner(System.in);
        System.out.println(arguments.length);
        System.out.println(((String)arguments[0]));
        BufferedWriter in = ((BufferedWriter)arguments[1]);
        in.write(reader.nextLine());
        System.out.println("sending input");
        in.flush();
        reader.close();
        break;
    default:
        System.out.println("Invalid command recieved!");
    }
    } else
        System.out.println("Invalid command recieved!");


}

The output I recieve is:

Hello World!

2 Please enter something!

This is a test input

sending input

Attempting to resolve input

As you can see, I successfully exit the while(!br.ready()) loop, and I stop at br.readLine();

I am aware inheritIO() exist, but for this case I am using the BufferedOuput to send commands which are then parsed and sent to the switch statement, which in turn calls the corresponding function. This is because multiple processes could be launched from the process manager, think of the fun when multiple System.in calls arrive, with nothing to determine which process it is for! In addition, this allows for me to call any type of function, even those not related to println or input.

Upvotes: 1

Views: 652

Answers (1)

radiaph
radiaph

Reputation: 4121

I believe the issue here is a result of the following:

  1. BufferedReader.ready() returns true if there are any characters available to be read. It does not guarantee that there are any carriage returns among them. (docs)
  2. BufferedReader.readLine() looks for a carriage return to complete a line. If one is not found, it blocks.
  3. BufferedWriter.write() does not automatically write a terminating carriage return.

To test whether this is actually the problem, replace this line in runCommand():

in.write(reader.nextLine());

with:

in.write(reader.nextLine() + "\n");

Upvotes: 1

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