Load a class from a jar

Question

I´m trying to load a class from a jar, I´m using a classLoader.

I have this parts of code for prepare the classLoader:

private void loadClass(){

    try{
        JarFile jarFile = new JarFile( Path);
        Enumeration e = jarFile.entries();

        URL[] urls = { new URL("jar:file:" + Path +"!/") };
        classLoader = URLClassLoader.newInstance(urls);


    } catch (MalformedURLException ex) {
        // TODO Auto-generated catch block
        ex.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}

Now I load a class, and I try to get a new instance

....           
           loadClass();

           Class device = classLoader.loadClass( "org.myPackage.MyClass");

           MyMotherClass Device = ( MyMotherClass) device.newInstance();
...

MyClass extends of MyMotherClass, and when I do classLoader.loadClass( "org.myPackage.MyClass"), the MyMotherClass it is in the classLoader. At the moment, all right.

now, in device.newInstance(), I get a exception. The problem is the other classes that are used by MyClass, are not in the classpath.

What can i do?

I have a another method that load all the needed classes in the classLoader, but does not work when I get the new instance. I can not change MyClass and the others

Answer 1

Here's some code I use to load a jar dynamically at run-time. I exploit reflection to circumvent the fact that you ain't really supposed to do this (that is, modify the class path after the JVM has started). Just change my.proprietary.exception to something sensible.

    /*
     * Adds the supplied library to java.class.path.
     * This is benign if the library is already loaded.
     */
    public static synchronized void loadLibrary(java.io.File jar) throws my.proprietary.exception
    {
        try {
            /*We are using reflection here to circumvent encapsulation; addURL is not public*/
            java.net.URLClassLoader loader = (java.net.URLClassLoader)ClassLoader.getSystemClassLoader();
            java.net.URL url = jar.toURI().toURL();
            /*Disallow if already loaded*/
            for (java.net.URL it : java.util.Arrays.asList(loader.getURLs())){
                if (it.equals(url)){
                    return;
                }
            }
            java.lang.reflect.Method method = java.net.URLClassLoader.class.getDeclaredMethod("addURL", new Class[]{java.net.URL.class});
            method.setAccessible(true); /*promote the method to public access*/
            method.invoke(loader, new Object[]{url});
        } catch (final NoSuchMethodException | 
            java.lang.IllegalAccessException | 
            java.net.MalformedURLException | 
            java.lang.reflect.InvocationTargetException e){
            throw new my.proprietary.exception(e.getMessage());
        }
    }