Calling a command line program in Java

Question

I am making a GUI for a command line program using JavaFX.

If I just pack the program inside the .JAR, can I call it? Or do I need to extract the program into the local machine first before making a process? I ask because since a .JAR is a ZIP file, I suppose you can't just run a file inside the .JAR because it is compressed or something.

• aioobe: The command line program that you want to launch, is it a Java program? Or like a native binary?
• Saturn: @aioobe native binary
• aioobe: Then it's the operating system that launches the program. Unless the operating system in question is able to launch programs from within zip-files (which I doubt) you would have to extract the binary and write it to disk before launching.
• aioobe: this is a good question and I can imagine future visitors struggling with a similar use case so I took the time to write an exhaustive answer.

Answer 1

You'd have to do this in two steps:

1. Extract the binary and write it to disk (because the OS is probably only able to launch the program if it's a file on disk)

2. Launch the program, preferably using ProcessBuilder.

Here's a complete demo:

test.c

#include <stdio.h>
int main() {
 printf("Hello world\n");
 return 0;
}

Test.java

import java.io.*;
import java.nio.file.*;
import java.nio.file.attribute.PosixFilePermission;
import java.util.Collections;
public class Test {
 public static void main(String[] args) throws Exception {
 Path binary = Paths.get("mybinary");
 Files.copy(Test.class.getResourceAsStream("mybinary"), binary);
 System.out.println("Launching external program...");
 // Needed for Linux at least
 Files.setPosixFilePermissions(binary,
 Collections.singleton(PosixFilePermission.OWNER_EXECUTE));
 Process p = new ProcessBuilder("./mybinary").inheritIO().start();
 p.waitFor();
 System.out.println("External program finished.");
 }
}

Demo

$ cc mybinary.c -o mybinary
$ ./mybinary
Hello world
$ javac Test.java
$ jar -cvf Test.jar Test.class mybinary
$ rm mybinary Test.class
$ ls
mybinary.c Test.jar Test.java
$ java -cp Test.jar Test
Launching external program...
Hello world
External program finished.

Further reading:

• Run external program concurrently and communicate with it through stdin / stdout
• How to make an executable jar file?
• Where does Java's ProcessBuilder look to execute commands?
• ProcessBuilder won't run with arguments

Answer 2

You have to extract your program first to let the os execute it. There's no way to 'stream' it to ram and execute from there.