使用ProcessBuilder在.java文件中运行shell命令

Question

I'm running into a pesky little error and I'm not sure how to solve it. What I'm trying to do is pretty simple: Run an external .java file using ProcessBuilder. Just like that tittle suggests.

The problem is that the file that runs and creates this ProcessBuilder is in a different directory than the other file I want to run.

What I'm trying to do to get around this is:

String command[] = {"javac",JAVA_FILE_LOCATION};
ProcessBuilder processBuilder = new ProcessBuilder("cd");
Process process0 = processBuilder.start();

to reset the directory and

processBuilder = new ProcessBuilder(command);
Process process = processBuilder.start();

to compile the other file

if( process.getErrorStream().read() != -1 ){
    print("Compilation Errors",process.getErrorStream());
}

and just some error handling here. However it doesn't get past this error handle part. I can't figure out the proper way of doing this. Here's the error message:

my_PC:Processes user$ java -jar process.jar
************* Compilation Errors***********************
avac: file not found: /Users/Desktop/Piano.java
Usage: javac <options> <source files>
use -help for a list of possible options
Exception in thread "main" java.lang.IllegalThreadStateException: process hasn't exited
at java.lang.UNIXProcess.exitValue(UNIXProcess.java:423)
at my.package.address.Main.main(Main.java:28)

Does anyone know what I'm doing wrong ? Also I can post the whole code if needed, but I figured it's not really relevant here.

UPDATED: added my code

package some.random.package;

import java.io.*;

public class Main {
/**
* Provide absolute JAVA file path
*/
private static final String JAVA_FILE_LOCATION = "/Users/Desktop/piano.java";

    public static void main(String args[]) throws IOException
    {
        String command[] = {"javac",JAVA_FILE_LOCATION};
        ProcessBuilder processBuilder = new ProcessBuilder(command).directory(new File("/Users/Desktop/"));
        Process process = processBuilder.start();
        if( process.getErrorStream().read() != -1 ){
            print("Compilation Errors",process.getErrorStream());
        }
        /**
        * check if compile is successful
        * 0 => successful
        */
        if( process.exitValue() == 0 )
        {
            process = new ProcessBuilder(new String[]{"java","piano"}).directory(new File("/Users/Desktop/")).start();
            if( process.getErrorStream().read() != -1 )
                print("Errors ",process.getErrorStream());
            else
                print("Output ",process.getInputStream());

            /**
            * Check if RuntimeException or Errors encounter during execution then print errors on console
            * Otherwise print Output
            */
        }
    }
    private static void print(String status,InputStream input) throws IOException
    {
        BufferedReader in = new BufferedReader(new InputStreamReader(input));
        System.out.println("************* "+status+"***********************");
        String line = null;
        while((line = in.readLine()) != null ){
        System.out.println(line);
      }
    in.close();
    }
}

Answer 1

The cd command you execute with the first ProcessBuilder affects only that child process , it does not change the working directory of the main executing program. When you run the second ProcessBuilder , it will have the working directory as the main program, completely unaffected by the cd executed by the previous ProcessBuilder .

To execute a program from a different directory, use the directory(...) method of ProcessBuilder :

String command[] = {"javac", JAVA_FILE_LOCATION};
ProcessBuilder processBuilder = new ProcessBuilder(command).directory(new File("path/to/dir"));
Process process0 = processBuilder.start();

Btw, the path "/Users/Desktop/piano.java" looks very strange. The path to a user's desktop on a Mac is usually /Users/username/Desktop , so the path in your code implies that your username is "Desktop", which is very unusual.

And, to make it easier to work with the program, I suggest to put the base directory path ( /Users/username/Desktop ) in a variable, rather then hardcoding it at multiple places. That way it will be easier to make changes, no need to edit at multiple places.