Running a jar file from java code

Question

Hi!
Im trying to run a jar file from my code.
I've tried Classloader, but that doesnt work because it doesnt find the images (also embedded in the 2nd jar file).
WHat I would like to do is actually RUN the 2nd jar file from the first jar file. There must be a way to do this right?
any ideas?

Answer 1

ok, I found some wonderful code (see below) that will try to start the jar. But it doesn't. What it does is produce the following error when my application runs...
So it's not finding the images in the jar file that I am trying to run? Strange. I checked the URL that sending, but it seems ok....
I think I will check the url again to make sure......
any ideas?
Uncaught error fetching image:
java.lang.NullPointerException
     at sun.awt.image.URLImageSource.getConnection(Unknown Source)
     at sun.awt.image.URLImageSource.getDecoder(Unknown Source)
     at sun.awt.image.InputStreamImageSource.doFetch(Unknown Source)
     at sun.awt.image.ImageFetcher.fetchloop(Unknown Source)
     at sun.awt.image.ImageFetcher.run(Unknown Source)
the code....
/* From http://java.sun.com/docs/books/tutorial/index.html */
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.net.JarURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.jar.Attributes;
* Runs a jar application from any url. Usage is 'java JarRunner url [args..]'
* where url is the url of the jar file and args is optional arguments to be
* passed to the application's main method.
public class JarRunner {
  public static void main(String[] args) {
    URL url = null;
    try {
      url = new URL(args[0]);//"VideoTagger.jar");
    } catch (MalformedURLException e) {
      System.out.println("Invalid URL: ");
    // Create the class loader for the application jar file
    JarClassLoader cl = new JarClassLoader(url);
    // Get the application's main class name
    String name = null;
    try {
      name = cl.getMainClassName();
    } catch (IOException e) {
      System.err.println("I/O error while loading JAR file:");
      e.printStackTrace();
      System.exit(1);
    if (name == null) {
      fatal("Specified jar file does not contain a 'Main-Class'"
          + " manifest attribute");
    // Get arguments for the application
    String[] newArgs = new String[args.length - 1];
    System.arraycopy(args, 1, newArgs, 0, newArgs.length);
    // Invoke application's main class
    try {
      cl.invokeClass(name, newArgs);
    } catch (ClassNotFoundException e) {
      fatal("Class not found: " + name);
    } catch (NoSuchMethodException e) {
      fatal("Class does not define a 'main' method: " + name);
    } catch (InvocationTargetException e) {
      e.getTargetException().printStackTrace();
      System.exit(1);
  private static void fatal(String s) {
    System.err.println(s);
    System.exit(1);
* A class loader for loading jar files, both local and remote.
class JarClassLoader extends URLClassLoader {
  private URL url;
   * Creates a new JarClassLoader for the specified url.
   * @param url
   *            the url of the jar file
  public JarClassLoader(URL url) {
    super(new URL[] { url });
    this.url = url;
   * Returns the name of the jar file main class, or null if no "Main-Class"
   * manifest attributes was defined.
  public String getMainClassName() throws IOException {
    URL u = new URL("jar", "", url + "!/");
    JarURLConnection uc = (JarURLConnection) u.openConnection();
    Attributes attr = uc.getMainAttributes();
    return attr != null ? attr.getValue(Attributes.Name.MAIN_CLASS) : null;
   * Invokes the application in this jar file given the name of the main class
   * and an array of arguments. The class must define a static method "main"
   * which takes an array of String arguemtns and is of return type "void".
   * @param name
   *            the name of the main class
   * @param args
   *            the arguments for the application
   * @exception ClassNotFoundException
   *                if the specified class could not be found
   * @exception NoSuchMethodException
   *                if the specified class does not contain a "main" method
   * @exception InvocationTargetException
   *                if the application raised an exception
  public void invokeClass(String name, String[] args)
      throws ClassNotFoundException, NoSuchMethodException,
      InvocationTargetException {
    Class c = loadClass(name);
    Method m = c.getMethod("main", new Class[] { args.getClass() });
    m.setAccessible(true);
    int mods = m.getModifiers();
    if (m.getReturnType() != void.class || !Modifier.isStatic(mods)
        || !Modifier.isPublic(mods)) {
      throw new NoSuchMethodException("main");
    try {
      m.invoke(null, new Object[] { args });
    } catch (IllegalAccessException e) {
      // This should not happen, as we have disabled access checks
}