Modularization without OSGI in Web project

Question

I have a project that will be fairly large, so it would be very interesting to modularize, I know, there is Osgi, but I found it very complex and had difficulty adapting my application, the impression I had is that it will complicate more than facilitating maintenance,is too bold simply to use a class that is in a separate jar through an interface, so I’m thinking of doing through Classloader, the structure I think is the following

Core Project

package core;

public interface IModulo {
<T> T getString();
}

Example Project Module

package modulo1;

import core.IModulo;

public class Teste implements IModulo{

public <T> T getString() {
    return (T) "Hello";
}
}

Main project, where the modules will be "installed", this main project already has the Core project as a dependency, that is, the Imodulo interface is already in the classpath, if I run through a simple main class, it works, but by Tomcat no, see the code snippet

  import core.IModulo;
   public class Main {
public static void main(String[] args) {

    try {
        String jarDoModulo = "C:\\modulo1.jar";
        File file = new File(jarDoModulo);
        URL url;
        url = file.toURL();

        URL[] urls = new URL[] { url };
        ClassLoader cl = new URLClassLoader(urls);          
        Class<IModulo> cls = (Class<IModulo>) cl.loadClass("modulo1.Teste");
        IModulo modulo1 = cls.newInstance();
        System.out.println(modulo1.getString());

    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (InstantiationException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}

}

The error that appears is

Caused by: java.lang.ClassNotFoundException: core.IModulo
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
... 60 more

Answer 1

From what I understand its requirement is to basically separate implementations of certain interfaces from the core. These implementations are in projects jar apart from your main web project.

The problem you bumped into happens because Tomcat has its own hierarchy of class loaders.

The way your code was written, inside the Tomcat IModulo shall be administered by a WebappClassLoader and the class Teste for his URLClassLoader (which has no instance of IModulo loaded, therefore, fires an exception).

There are two recommended paths here:

1. Put the modulo1.jar in the briefcase WEB-INF/lib of your main project. This way the classes of your module will be available for the application and you will not even need to mess with class loaders. Modularity is still maintained in the sense that it is possible to change the module implementation without changing the core application.

2. Use or write a ClassLoader customized (if you really want to leave your module in one location non-standard). Until the Tomcat 7 you could use one Virtualwebapploader to make your module jar available; from Tomcat 8 you can use the Preresources.