GradeServer.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.Arrays;
import java.util.stream.Stream;

class ExecHelpers {

  /**
    Takes an input stream, reads the full stream, and returns the result as a
    string.

    In Java 9 and later, new String(out.readAllBytes()) would be a better
    option, but using Java 8 for compatibility with ieng6.
  */
  static String streamToString(InputStream out) throws IOException {
    String result = "";
    while(true) {
      int c = out.read();
      if(c == -1) { break; }
      result += (char)c;
    }
    return result;
  }

  /**
    Takes a command, represented as an array of strings as it would by typed at
    the command line, runs it, and returns its combined stdout and stderr as a
    string.
  */
  static String exec(String[] cmd) throws IOException {
    Process p = new ProcessBuilder()
                    .command(Arrays.asList(cmd))
                    .redirectErrorStream(true)
                    .start();
    InputStream outputOfBash = p.getInputStream();
    return String.format("%s\n", streamToString(outputOfBash));
  }

}

class Handler implements URLHandler {
    public String handleRequest(URI url) throws IOException {
       if (url.getPath().equals("/grade")) {
           String[] parameters = url.getQuery().split("=");
           if (parameters[0].equals("repo")) {
               String[] cmd = {"bash", "grade.sh", parameters[1]};
               String result = ExecHelpers.exec(cmd);
               return result;
           }
           else {
               return "Couldn't find query parameter repo";
           }
       }
       else {
           return "Don't know how to handle that path!";
       }
    }
}

class GradeServer {
    public static void main(String[] args) throws IOException {
        if(args.length == 0){
            System.out.println("Missing port number! Try any number between 1024 to 49151");
            return;
        }

        int port = Integer.parseInt(args[0]);

        Server.start(port, new Handler());
    }
}

class ExecExamples {
  public static void main(String[] args) throws IOException {
    String[] cmd1 = {"ls", "lib"};
    System.out.println(ExecHelpers.exec(cmd1));

    String[] cmd2 = {"pwd"};
    System.out.println(ExecHelpers.exec(cmd2));

    String[] cmd3 = {"touch", "a-new-file.txt"};
    System.out.println(ExecHelpers.exec(cmd3));
  }
}