CODEWITHSUNDEEP
java
Kanishka Dilshan
Kanishka Dilshan

Reputation: 734

How to use java.lang.Process class to Provide inputs to another process

Just assume there is a program which takes inputs from the standard input.

For example:

cin>>id;

What I want to figure out is how to execute the process and give some input to its standard input. Getting the output of the process is not an issue for me. It works properly. The question is how to feed inputs for such processes using java.lang.Process class.

If there are any other third party libraries like Apache commons please mention them also.

Thanks in advance!

Upvotes: 3

Views: 2729

Answers (2)

aioobe
aioobe

Reputation: 421040

You need to start a separate thread which reads from the output of one process and writes it as input to the other process.

Something like this should do:

class DataForwarder extends Thread {

    OutputStream out;
    InputStream in;

    public DataForwarder(InputStream in, OutputStream out) {
        this.out = out;
        this.in = in;
    }

    @Override
    public void run() {
        byte[] buf = new byte[1024];
        System.out.println("Hej");
        try {
            int n;
            while (-1 != (n = in.read(buf)))
                out.write(buf, 0, n);
            out.close();
        } catch (IOException e) {
            // Handle in some suitable way.
        }
    }
}

Which would be used for prod >> cons as follows:

class Test {
    public static void main(String[] args) throws IOException {

        Process prod = new ProcessBuilder("ls").start();
        Process cons = new ProcessBuilder("cat").start();

        // Start feeding cons with output from prod.
        new DataForwarder(prod.getInputStream(), cons.getOutputStream()).start();
    }
}

Upvotes: 2

Michał Kosmulski
Michał Kosmulski

Reputation: 10020

Use Process.getOutputStream() and write() to it. It's a bit tricky since you use the output stream to input data to the process, but the name reflects the interface that is returned (from your app's point of view it's output because you are writing to it).

Upvotes: 3

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