Java Process Class: Why command "java -version" does not produce outputstream while command "ls -l" does?

Question

Below program:

public class ProcessExample {

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

        Process process1 = new ProcessBuilder("/bin/ls", "-l").directory(Path.of("/home/yapkm01").toFile()).start();
        System.out.println("ls command:");
        try (var in = new Scanner(process1.getInputStream())) {
            while (in.hasNextLine()) {
                System.out.println(in.nextLine());
            }
        }

        Process process2 = new ProcessBuilder("/bin/java", "-version").start();
        System.out.println("java command:");
        try (var in = new Scanner(process2.getInputStream())) {
            while (in.hasNextLine()) {
                System.out.println(in.nextLine());
            }
        }

    }

}

Output:

ls command:
total 424
drwxr-xr-x  2 yapkm01 yapkm01   4096 Dec 27  2021 Desktop
drwxr-xr-x  2 yapkm01 yapkm01   4096 Apr 30 01:09 Documents
drwxr-xr-x  2 yapkm01 yapkm01   4096 Jul  1 12:06 Downloads
-rw-r--r--  1 yapkm01 yapkm01   8980 Aug  3  2018 examples.desktop
java command:

Notice there is not output for process2 which is java -version. Of course when do i it manually i get below.

yapkm01-/home/yapkm01):-$ java -version
openjdk version "11.0.16" 2022-07-19
OpenJDK Runtime Environment (build 11.0.16+8-post-Ubuntu-0ubuntu122.04)
OpenJDK 64-Bit Server VM (build 11.0.16+8-post-Ubuntu-0ubuntu122.04, mixed mode, sharing)
(yapkm01-/home/yapkm01):-$

Question: Why there is no output for java -version?

Answer 1

$ java --help | grep version
    -version      print product version to the error stream and exit
    --version     print product version to the output stream and exit
    -showversion  print product version to the error stream and continue
    --show-version
                  print product version to the output stream and continue

As you can see, java -version prints to stderr so obviously you won't see the output in stdout. You need to use java --version or capture stderr

Answer 2

I think this question is really about learning about running subprocesses, based on the name 'ProcessExample'.

However, if what you really want is the version of Java your program is running (which is of course not necessarily the same version that an arbitrary "java" command line will use), then

String version = System.getProperty("java.version")

Answer 3

"/bin/java" is not the correct PATH to java. Try which java to get the correct path, and use that instead. Also, I suggest you use inheritIO with your ProcessBuilder . Change

Process process2 = new ProcessBuilder("/bin/java", "-version").start();

to

Process process2 = new ProcessBuilder("CORRECT_PATH/java",
        "-version").inheritIO().start();