how to compile & run java program in another java program?

I have a Main.java and Test.java classes that I want to compile and run Main.java in Test.java code. Here is my code

    Process pro1 = Runtime.getRuntime().exec("javac Main.java");
    pro1.waitFor();
    Process pro2 = Runtime.getRuntime().exec("java Main");

    BufferedReader in = new BufferedReader(new InputStreamReade开发者_如何学Pythonr(pro2.getInputStream()));
    String line = null;

    while ((line = in.readLine()) != null) {
        System.out.println(line);
    }

I just print "ok" in Main.java but this code doesn't print anything. What is the problem ?

I have modified the code to include some checks:

public class Laj {

  private static void printLines(String name, InputStream ins) throws Exception {
    String line = null;
    BufferedReader in = new BufferedReader(
        new InputStreamReader(ins));
    while ((line = in.readLine()) != null) {
        System.out.println(name + " " + line);
    }
  }

  private static void runProcess(String command) throws Exception {
    Process pro = Runtime.getRuntime().exec(command);
    printLines(command + " stdout:", pro.getInputStream());
    printLines(command + " stderr:", pro.getErrorStream());
    pro.waitFor();
    System.out.println(command + " exitValue() " + pro.exitValue());
  }

  public static void main(String[] args) {
    try {
      runProcess("javac Main.java");
      runProcess("java Main");
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

Here is the Main.java:

public class Main {
  public static void main(String[] args) {
    System.out.println("ok");
  }
}

When everything is fine, it just works:

alqualos@ubuntu:~/tmp$ java Laj
javac Main.java exitValue() 0
java Main stdout: ok
java Main exitValue() 0

Now, for example, if I have some error in Main.java:

alqualos@ubuntu:~/tmp$ java Laj
javac Main.java stderr: Main.java:3: package Systems does not exist
javac Main.java stderr:     Systems.out.println("ok");
javac Main.java stderr:            ^
javac Main.java stderr: 1 error
javac Main.java exitValue() 1
java Main stdout: ok
java Main exitValue() 0

It still prints "ok" because the previously compiled Main.class is still there, but at least you can see what exactly is happening when your processes are running.

You also need to

pro2.waitFor();

because executing that process will take some time and you can't take the exitValue() before the process has finished.

I have added the condition in Laj class main function to check for compilation process has completed successfully or not..

public class Laj {

  private static void printLines(String name, InputStream ins) throws Exception {
    String line = null;
    BufferedReader in = new BufferedReader(
        new InputStreamReader(ins));
    while ((line = in.readLine()) != null) {
        System.out.println(name + " " + line);
    }
  }

  private static int runProcess(String command) throws Exception {
    Process pro = Runtime.getRuntime().exec(command);
    printLines(command + " stdout:", pro.getInputStream());
    printLines(command + " stderr:", pro.getErrorStream());
    pro.waitFor();
   // System.out.println(command + " exitValue() " + pro.exitValue());
    return pro.exitValue();
  }

  public static void main(String[] args) {
    try {
    int k =  runProcess("javac Main.java");
    if (k==0)
    k=runProcess("java Main");

    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}