Understanding processes in Java [linux]

Question

I'm using Ubuntu 16.04.

To execute some logic I need to start a process in Java as

String[] commandLine;
String[] environment;
//...
Process p = Runtime.getRuntime().exec(commandLine, environment);
InputStream processInputStream = p.getInputStream(); //<---- ?

But since JVM and the process are different ones I need to understand how they actually communicate. And through what (channels, sockets tcp/udp, pipes, or something else).

How does they actually transfer data?

"since JVM and the process are different ones I need to understand how they actually communicate" this is too broad. What are you expecting ? What do you think will be different based on the OS ? — AxelH
– AxelH, Commented May 29, 2017 at 13:35
@AxelH What do u mean too broad? If a process asks some data from another process there need to be some inter-process communication. So which one? Socket? If so, which socket (TCP/UDP)? — St.Antario
– St.Antario, Commented May 29, 2017 at 13:36
This is indeed an excellent question; I don't believe many people understand the implications of that question yet... I myself took some time understanding it really. But in the meanwhile, have you had a look at ProcessBuilder's .inheritIO() method? — fge
– fge, Commented May 29, 2017 at 13:38
Might I suggest to edit the question? I do have edit privileges myself but I prefer the OP do it. What about: "what are std{in,out,err} when you create a new process?" — fge
– fge, Commented May 29, 2017 at 13:47
Are you asking about Process.getInputStream() specifically? That method usually corresponds to the operating system construct that is exposed by the platform's C runtime as the stdout of the child process. On Linux or any POSIX-compliant system it will be a pipe. — Daniel Pryden
– Daniel Pryden, Commented May 29, 2017 at 14:14

Rik van de Laar · Accepted Answer · 2017-05-29 13:55:39Z

2

Judging from the javadoc it seems to be using pipes by default.

answered May 29, 2017 at 13:55

Rik van de Laar

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fge · Accepted Answer · 2017-05-29 13:57:00Z

OK, here is a short test, I also have Ubuntu, although it's a 16.10, I think these will behave the same. The program I wrote:

public final class Test
{
    public static void main(final String... args)
        throws IOException
    {
        final ProcessBuilder pb = new ProcessBuilder("yes");
        final Process p = pb.start();

        try (
            final InputStream in = p.getInputStream();
        ) {
            while (true)
                in.read();
        }
    }
}

Using pstree -uplan, I found that that the PID of the yes process was some number n and when I did:

ls -l /proc/n/fd

I got:

lr-x------ 1 fge fge 64 May 29 15:52 0 -> pipe:[1482898]
l-wx------ 1 fge fge 64 May 29 15:52 1 -> pipe:[1482899]
l-wx------ 1 fge fge 64 May 29 15:52 2 -> pipe:[1482900]

which makes me say that the I/O exchange is done using anonymous pipes.

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