Execute external Java code and get output

Question

I want to execute a Java CLI-program from within another Java program and get the output of the CLI-program. I've tried two different implementations (using runtime.exec() and ProcessBuilder) and they don't quite work.

Here's the peculiar part; the implementations work (catch the output) for when executing commands such as pwd but for some reason they do not catch the output of a Hello World java program executed with java Hello.

Execution code:

public static void executeCommand(String command)
{
    System.out.println("Command: \"" + command + "\"");

    Runtime runtime = Runtime.getRuntime();
    try
    {
        Process process = runtime.exec(command);

        BufferedReader stdInput = new BufferedReader(new
                InputStreamReader(process.getInputStream()));

        BufferedReader stdError = new BufferedReader(new
                InputStreamReader(process.getErrorStream()));

        // read the output from the command
        System.out.println("Standard output of the command:\n");
        String s = null;
        while ((s = stdInput.readLine()) != null) {
            System.out.println(s);
        }

        // read any errors from the attempted command
        System.out.println("Standard error of the command (if any):\n");
        while ((s = stdError.readLine()) != null) {
            System.out.println(s);
        }
    } catch (Exception e)
    {
        e.printStackTrace();
    }
}

Example output

Command: "cd /Users/axelkennedal/Desktop && java Hello"
Standard output of the command:

Standard error of the command (if any):

Command: "pwd"
Standard output of the command:

/Users/axelkennedal/Dropbox/Programmering/Java/JavaFX/Kode
Standard error of the command (if any):

I have verified that Hello does indeed print "Hello world" to the CLI when running Hello directly from the CLI instead of via executeCommand().

Hello world

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hello world!");
    }
}

Does Runtime.exec pass the command through a shell? I don't think so, but your command needs to be interpreted by the shell to handle &&. — blm
– blm, Commented Oct 31, 2015 at 17:38
Yes. It's been a long time since I've used Runtime.exec, but I'm pretty sure it doesn't do any command parsing or invoke a shell first. Being pwd is just the command with no arguments it works, but your first command has arguments and needs to be interpreted by the shell. — blm
– blm, Commented Oct 31, 2015 at 17:43
Thanks for the insight, I do believe seeing somewhere that ProcessBuilder on the other hand can handle things like && — Kennedal
– Kennedal, Commented Oct 31, 2015 at 17:45
I don't see that offhand in the ProcessBuilder javadocs, but I might be missing it. In this particular case, Runtime.exec(new String[]{"java", "Hello"}, null, "/Users/axelkennedal/Desktop") would probably work. — blm
– blm, Commented Oct 31, 2015 at 17:49

WillShackleford · Accepted Answer · 2015-10-31 18:21:23Z

1

This "cd /Users/axelkennedal/Desktop && java Hello" is not one command but two commands separated by the &&. In general it means do the first command and if the first command succeeds do the second command. You can't pass this as a single command but you can implement the logic yourself:

eg to execute "cmd1 && cmd2"

if (Runtime.getRuntime().exec("cmd1").waitFor() == 0) {
    Runtime.getRuntime().exec("cmd2").waitFor();
}

However, because in this case cmd1 is to change directories there is a better way, which is to use the directory function of ProcessBuilder instead of the first command.

Process p = new ProcessBuilder("java","hello")
        .directory(new File("/Users/axelkennedal/Desktop"))
        .start();

answered Oct 31, 2015 at 18:21

WillShackleford

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1 Comment

Kennedal Over a year ago

I just found that in the documentation, which is great :D Thank you!

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