dynamic class reload

- Use different child classloader to reload the class and create new instance
- Apply proxy pattern to delegate the invocation to new instance, so that changes to the dynamic class become transparent to its caller.

    // The dir contains the compiled classes.
    File classesDir = new File("/temp/dynacode_classes/");

    // The parent classloader
    ClassLoader parentLoader = Postman.class.getClassLoader();


    // Load class "sample.PostmanImpl" with our own classloader.
    URLClassLoader loader1 = new URLClassLoader(
            new URL[] { classesDir.toURL() }, parentLoader);
    Class cls1 = loader1.loadClass("sample.PostmanImpl");
    Postman postman1 = (Postman) cls1.newInstance();

    /*
     * Invoke on postman1 ...
     * Then PostmanImpl.java is modified and recompiled.
     */

    // Reload class "sample.PostmanImpl" with a new classloader.
    URLClassLoader loader2 = new URLClassLoader(
            new URL[] { classesDir.toURL() }, parentLoader);
    Class cls2 = loader2.loadClass("sample.PostmanImpl");
    Postman postman2 = (Postman) cls2.newInstance();

    /*
     * Work with postman2 from now on ...
     * Don't worry about loader1, cls1, and postman1
     * they will be garbage collected automatically.
     */

The Java reflection API includes a handy utility for creating proxies. The class java.lang.reflect.Proxy provides static methods that let you create proxy instances for any Java interface.

InvocationHandler handler = new DynaCodeInvocationHandler(...);
    Postman proxy = (Postman) Proxy.newProxyInstance(
                Postman.class.getClassLoader(),
                new Class[] { Postman.class },
                handler);

public class DynaCodeInvocationHandler implements InvocationHandler {


public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        
        // Get an instance of the up-to-date dynamic class
        Object dynacode = getUpToDateInstance();
        
        // Forward the invocation
        return method.invoke(dynacode, args);
    }
}

Reference link