Class loader in java

Question

How can I scan a particular jar file instead of scanning complete java class path while using a class loader?

e.g.: I have Jar1, Jar2, Jar3, Jar4 in class path but I want to scan only Jar4 while using class loader to scan for a particular class.

Answer 1

if you are not splitting in modules or using technologies that are split components in different classloaders like OSGI you cannot do it without some sort of structure or a new instance of a classloader.

let's say you use the classloader from the context you are in:

        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        String path = packageName.replace('.', '/');
        Enumeration resources = classLoader.getResources(path);
        List dirs = new ArrayList();
        while (resources.hasMoreElements()) {
            URL resource = resources.nextElement();
            dirs.add(new File(resource.getFile()));
        }
        ArrayList classes = new ArrayList();
        for (File directory : dirs) {
            classes.addAll(findClasses(directory, packageName));
        }
        return classes.toArray(new Class[classes.size()]);

However, this is too complex and sometimes you do not have such structure available. If you get a new classloader you can tell it to scan the parent classloaders last for instance and do something like:

ParentLastURLClassLoader classLoader = new ParentLastURLClassLoader(
        Arrays.asList(new File("a.jar").toURI().toURL()));
Class<?> carClass = classLoader.loadClass("com.acme.Car");
Vehicle someCar = (Vehicle) carClass.newInstance();

In this example, one can note something important: a class is identified with the classloader as factor. So one could not have:

com.acme.Car carClass = classLoader.loadClass("com.acme.Car");

As the first com.acme.Car and the second are loaded from different classloader and therefore are different classes.

This example is from the currently accepted answer at: How do I create a parent-last / child-first ClassLoader in Java, or How to override an old Xerces version that was already loaded in the parent CL?

/**
 * A parent-last classloader that will try the child classloader first and then the parent.
 * This takes a fair bit of doing because java really prefers parent-first.
 * 
 * For those not familiar with class loading trickery, be wary
 */
private static class ParentLastURLClassLoader extends ClassLoader 
{
    private ChildURLClassLoader childClassLoader;

    /**
     * This class allows me to call findClass on a classloader
     */
    private static class FindClassClassLoader extends ClassLoader
    {
        public FindClassClassLoader(ClassLoader parent)
        {
            super(parent);
        }

        @Override
        public Class<?> findClass(String name) throws ClassNotFoundException
        {
            return super.findClass(name);
        }
    }

    /**
     * This class delegates (child then parent) for the findClass method for a URLClassLoader.
     * We need this because findClass is protected in URLClassLoader
     */
    private static class ChildURLClassLoader extends URLClassLoader
    {
        private FindClassClassLoader realParent;

        public ChildURLClassLoader( URL[] urls, FindClassClassLoader realParent )
        {
            super(urls, null);

            this.realParent = realParent;
        }

        @Override
        public Class<?> findClass(String name) throws ClassNotFoundException
        {
            try
            {
                // first try to use the URLClassLoader findClass
                return super.findClass(name);
            }
            catch( ClassNotFoundException e )
            {
                // if that fails, we ask our real parent classloader to load the class (we give up)
                return realParent.loadClass(name);
            }
        }
    }

    public ParentLastURLClassLoader(List<URL> classpath)
    {
        super(Thread.currentThread().getContextClassLoader());

        URL[] urls = classpath.toArray(new URL[classpath.size()]);

        childClassLoader = new ChildURLClassLoader( urls, new FindClassClassLoader(this.getParent()) );
    }

    @Override
    protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
    {
        try
        {
            // first we try to find a class inside the child classloader
            return childClassLoader.findClass(name);
        }
        catch( ClassNotFoundException e )
        {
            // didn't find it, try the parent
            return super.loadClass(name, resolve);
        }
    }
}