How to replace classes in a running application in java ?

There are 2 ways:

1. To overwrite the class loader you're using by first using the existing classloader to bootstrap your application and specifically for the class that you need to have dynamic update, you have to use the overwritten classloader.
2. To use OSGi framework. Depending on the scale of your application, OSGi framework may not be a good choice as it requires you to follow it's coding convention.

Hope it helps

Here is a working test. Every 5 secs Test.main() reloads test.Test1.class from the file system and calls Test1.hello()

package test;

public class Test1 {
    public void hello() {
        System.out.println("Hello !");
    }
}

public class Test {

    static class TestClassLoader extends ClassLoader {
        @Override
        public Class<?> loadClass(String name) throws ClassNotFoundException {
            if (name.equals("test.Test1")) {
                try {
                    InputStream is = Test.class.getClassLoader().getResourceAsStream("test/Test1.class");
                    byte[] buf = new byte[10000];
                    int len = is.read(buf);
                    return defineClass(name, buf, 0, len);
                } catch (IOException e) {
                    throw new ClassNotFoundException("", e);
                }
            }
            return getParent().loadClass(name);
        }
    }

    public static void main(String[] args) throws Exception {
        for (;;) {
            Class cls = new TestClassLoader().loadClass("test.Test1");
            Object obj = cls.newInstance();
            cls.getMethod("hello").invoke(obj);
            Thread.sleep(5000);
        }
    }
}

Run it. Then change and recompile Test1

System.out.println("Hello !!!");

while Test is running. You will see Test1.hello output changed

...
Hello !
Hello !
Hello !!!
Hello !!!

This is how eg Tomcat reloads webapps. It has a separate ClassLoader for each webapp and loads a new version in a new ClassLoader. The old one is GCed just like any Java object as well as the old classes.

Note that we loaded Test1 with TestClassLoader and invoked its first method with reflection. But all Test1 dependencies will be implicitly loaded with Test1 class loader, that is all the Test1 application will be loaded by JVM into TestClassLoader.