How do I invoke a class from another jar file?

Question

I am currently trying to load a class or run it by using reflection from java to invoke the main class from the jar file.

Whenever I try to run it I will get the following error:

`java.lang.ClassNotFoundException: MyClass
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at com.test.Test.main(Test.java:43)`

The program will also find the jar saying Found MyClass an I also initialize the main class to be the class that it found. Problem is that it doesn't find the class at all even though it's technically found and set.

I already tried the method with invoke to load main class inside of the project which worked fine.

`private static JarFile jarFile;

@SuppressWarnings("unused")
public static void main(String[] args) throws IOException
{
    /** Input of jar file */
    jarFile = new JarFile("MyJarFile.jar");
    Enumeration<JarEntry> e = jarFile.entries();
    URL[] urls = { new URL("jar:file:" + "MyJarFile.jar" +"!/") };
    URLClassLoader cl = URLClassLoader.newInstance(urls);

    Class<?> mainClass = null;

    while (e.hasMoreElements()) 
    {
        JarEntry je = e.nextElement();
        if(je.isDirectory() || !je.getName().endsWith(".class")){
            continue;
        }

        String className = je.getName().substring(0, je.getName().length()-6);
        className = className.replace('/', '.');
        try
        {
            Class<?> c = cl.loadClass(className);
            if (className.equals("MyClass"))
            {
                mainClass = c;
                System.out.println("Found MyClass");
                try
                {
                    /** The method I use to invoke the main class */
                    Class<?> cls = Class.forName(className);
                    Method method = cls.getMethod("main", String[].class);
                    String[] params = new String[]{};
                    method .invoke(null, (Object) params);
                    jarFile.close();
                }
                catch (Exception ex)
                {
                    ex.printStackTrace();
                }
            }
            System.out.println("Loaded: " + className);
        }

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

    if (mainClass == null)
    {
        return;
    }
}
`

I basically expect the program to run the main method inside of the jar and start it.

Answer 1

I'm really not sure why you're trying to load the class using Class.forName when you've already loaded it from the correct classloader using ClassLoader.loadClass .

Remove line

Class<?> cls = Class.forName(className);

(which tries to load the class from the classloader in which the line itself runs, which is not the classloader than can load MyClass )

And change the line after it to:

Method method = c.getMethod("main", String[].class);