Running a java program using URLClassLoader

Question

I have been developing text editor for my final project which can compile and run java prorams.
this is the code that i have written for running the class file that is created after compilation---
import java.net.*;
import java.net.URLClassLoader;
import java.net.URL;
import java.io.File;
import java.lang.reflect.*;
public class runMethod extends ClassLoader{
     public void run(String dir, String f){
          File file = new File(dir);
          File[] files = file.listFiles();
     URL[] urls = new URL[files.length];
          try{
               URL url = file.toURL();
          }catch(MalformedURLException e) {
     System.out.println("MalformedURLException: " + e.getMessage());
               ClassLoader loader = new URLClassLoader(urls);
          try{
                    Class clazz = Class.forName(f, true,loader);
                    Method method = clazz.getDeclaredMethod("main", new Class[] { String[].class });
                    method.invoke(null, new Object[] {});
               } catch (Exception ex) {
               System.out.println(ex.getMessage());
But the output it gives me is not the running program but "null". please help me in figuring out what is the problem and why is it not running the class file and giving the output.

Answer 1

Here a complete example.
Suppose that your workspace is C:/java/projects/, that your project is called myproject and the build directory is called classes (myproject/classes/)
create the class MyTargetClass in a package called test1 :
package test1;
public class MyTargetClass {
     public static void main(String[] args) {
          System.out.println("Hello from "+MyTargetClass.class.getName());
}Create the class RunMethod in a package called test2 :
package test2;
import java.net.*;
import java.io.File;
import java.lang.reflect.*;
public class RunMethod {
     public void run(String dir, String f) {
          URL[] urls = null;
          try {
               // Convert the file object to a URL
               File myDir = new File(dir);
               URL url = myDir.toURL();
               urls = new URL[] { url };
          } catch (MalformedURLException e) {
               e.printStackTrace();
          try {
               // Create a new class loader with the directory
               ClassLoader cl = new URLClassLoader(urls);
               // Load in the class
               Class clazz = cl.loadClass(f);
               // Create a new instance of the new class
               //MyTargetClass= (MyTargetClass) clazz.newInstance();
               Method main = clazz.getDeclaredMethod("main", new Class[] { String[].class });
               main.invoke(null, new Object[] { null });
          } catch (Exception e) {
               e.printStackTrace();
     public static void main(String[] args) {
          new RunMethod().run("C:/java/projects/myproject/classes/", "test1.MyTargetClass");
}Compile and run the RunMethod class.
Hope That Helps