Simple Java Custom Class Loader Implementation

Simple implementation of Java custom class loader. An example of the class usage is provided in the code snippet. Have in mind if you are using eclipse the application root folder during the implementation process is the project folder and not the bin folder. This may lead to ClassNotFoundException. This implementation can load classes from jar files which are in the class path!

 * @author team

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;

 * Simple custom class loader implementation
public class CustomClassLoader extends ClassLoader {

     * The HashMap where the classes will be cached
    private Map<String, Class<?>> classes = new HashMap<String, Class<?>>();

    public String toString() {
        return CustomClassLoader.class.getName();

    protected Class<?> findClass(String name) throws ClassNotFoundException {

        if (classes.containsKey(name)) {
            return classes.get(name);

        byte[] classData;

        try {
            classData = loadClassData(name);
        } catch (IOException e) {
            throw new ClassNotFoundException("Class [" + name
                    + "] could not be found", e);

        Class<?> c = defineClass(name, classData, 0, classData.length);
        classes.put(name, c);

        return c;

     * Load the class file into byte array
     * @param name
     *            The name of the class e.g. com.codeslices.test.TestClass}
     * @return The class file as byte array
     * @throws IOException
    private byte[] loadClassData(String name) throws IOException {
        BufferedInputStream in = new BufferedInputStream(
                ClassLoader.getSystemResourceAsStream(name.replace(".", "/")
                        + ".class"));
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        int i;

        while ((i = != -1) {

        byte[] classData = out.toByteArray();

        return classData;

     * Simple usage of the CustomClassLoader implementation
     * @param args
     * @throws ClassNotFoundException
     * @throws IllegalAccessException
     * @throws InstantiationException
     * @throws SecurityException
     * @throws NoSuchMethodException
     * @throws InvocationTargetException
     * @throws IllegalArgumentException
    public static void main(String[] args) throws ClassNotFoundException,
            InstantiationException, IllegalAccessException,
            NoSuchMethodException, SecurityException, IllegalArgumentException,
        CustomClassLoader loader = new CustomClassLoader();
        // This class should be in your application class path
        Class<?> c = loader.findClass("net.codeslices.test.TestClass");
        Object o = c.newInstance();
        Method m = c.getMethod("toString");


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2014-04-02 15:03:49
This example is good for standalone. In web application I am setting the classloader and all classes to null before running System.gc(). what should be the path of Java class in web application if the native method is called in a class that is called by JSP. Class ca = cl.findClass("com.codethesis.example.A");
Jennifer C.
2014-02-20 18:33:40
Very useful code snippet. I desperately needed an implementation of custom java classloader in order to handle dll unloading and this worked great!

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