Tuesday, January 12, 2010

How to Write a Custom Class Loader to Load Classes from a Jar

A custom class loader is needed when the developer needs to load classes from some custom repositories, to implement hot deployment features and to allow unloading of classes.

According to Java2 class loading system, a custom class loader should subclass java.lang.ClassLoader and overrride findClass() method which is responsible for loading the class bytes and returning a defined class.

Following code is a custom class loader which loads classes from jars, inside the directory called "jar".

public class JarClassLoader extends ClassLoader {
    private String jarFile = "jar/test.jar"; //Path to the jar file
    private Hashtable classes = new Hashtable(); //used to cache already defined classes

    public JarClassLoader() {
        super(JarClassLoader.class.getClassLoader()); //calls the parent class loader's constructor

    public Class loadClass(String className) throws ClassNotFoundException {
        return findClass(className);

    public Class findClass(String className) {
        byte classByte[];
        Class result = null;

        result = (Class) classes.get(className); //checks in cached classes
        if (result != null) {
            return result;

        try {
            return findSystemClass(className);
        } catch (Exception e) {

        try {
            JarFile jar = new JarFile(jarFile);
            JarEntry entry = jar.getJarEntry(className + ".class");
            InputStream is = jar.getInputStream(entry);
            ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
            int nextValue = is.read();
            while (-1 != nextValue) {
                nextValue = is.read();

            classByte = byteStream.toByteArray();
            result = defineClass(className, classByte, 0, classByte.length, null);
            classes.put(className, result);
            return result;
        } catch (Exception e) {
            return null;


This code can be improved to
* Load classes from any jar inside the directory
* Dynamically pick up classes if the jar is updated (by keeping a modified time field)
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