Java, problems with custom classloader

Question

i'm writing a custom class loader to load some of my classes (not all).

Class loader are very simple:

public Class loadClass(String className, boolean resolve) throws ClassNotFoundException {
    Class cls=findLoadedClass(className);
    if(cls!=null) {
        return cls;
    }
    // Search first for encrypted classes
    cls=decryptClass(className);
    if(cls==null) {
        // Then try with default system classloader
        cls=super.loadClass(className, resolve);
    }
    return cls;
}

And this is how i use it:

// In my Launcher class
public static void main(String[] args) {
    MyClassLoader loader=new MyClassLoader();
    try {
        final Class main=loader.loadClass("com.MyAppMain");
        Method toInvoke=main.getMethod("main", args.getClass());
        toInvoke.invoke(main, new Object[]{args});
    }
    catch(Exception ex) {
    }
}

All seem to be fine in my small test project, but when i use this loader in my big project(client-server application that use spring+hibernate and IoC) doesn't work. I have not a particolar exception in my classloader, but for example, new Socket instance throw a "java.net.ConnectException: Connection refused" without a real reason...

Other problems is my main form does not become visible... and other strange problems like this.

So, the question, are these problems caused by my classloader that load in different way a different kind of classes?

Edit 1

My project use spring, so i use @Autowired or sometimes

springApplicationContext.getBean(clazz);

to inject a bean.

The problem is spring cannot find my beans if these classes are encrypted(so they need to be loaded by my classloader). There is a workaround for this mistake? Thanks.

Edit 2

I have set my classloader in spring ClassPathXmlApplicationContext and now i notice that spring uses my classloader to load beans class, but despite this it throws an org.springframework.beans.factory.NoSuchBeanDefinitionException becouse it cannot find beans... what can i do? Thanks

If your classes are loading ... you don't have a problem with your classloader. Sockets failing to connect have nothing to do with that. — Brian Roach
– Brian Roach, Commented Nov 1, 2011 at 15:45
I think that your problem is not directly caused by the custom class loading. Try to separate problem to smaller ones and debug them. Put breakpoint into java.net.Socket and see where it tries to connect. I believe that you do not get parameters correctly. — AlexR
– AlexR, Commented Nov 1, 2011 at 15:52
Thank you guys, i think problem is Spring and IoC with my classloader. I edit first question for more details. — blow
– blow, Commented Nov 1, 2011 at 16:58

Igor Nikolaev · Accepted Answer · 2011-11-08 05:43:54Z

1

I'm not very good at class loaders, but from you code it's only possible to assume, that in case your class loader can't find a class, it will redirect to system class loader. It may work fine when you run application standalone, like in your sample, but if it's a web application that is run on application server it will fail.

Application servers usually create a big hierarchy of class loaders and has a separate class loader used to load your application classes. In this case, system class loader knows nothing about your Spring related classes and thus can't load them.

You must keep in mind, that same class may be loaded by several class loaders and if you try to compare same class from different class loaders it will fail.

In your case I would set parent class loader in MyClassLoader constructor. As a parent class loader you can use MyClassLoader.class.getClassLoader() I think.

public class MyClassLoader extends ClassLoader
{
    public MyClassLoader()
    {
       super(MyClassLoader.class.getClassLoader());
    }

    // other code
}

Hope this may help :)

answered Nov 8, 2011 at 5:43

Igor Nikolaev

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1 Comment

Vishwas Upadhyay Over a year ago

upvoting this answer as I also had the same problem. After setting the parent class loader as the current class' class loader in my custom class loader object. it started picking beans from the parent application context.

Andreas Veithen · Accepted Answer · 2011-11-01 17:15:11Z

0

I see two things that may be worth investigating:

The code makes the class loader parent-last. There is always a risk that this causes subtle class loading issues.
Maybe it is just the thread context class loader that is not set properly.

answered Nov 1, 2011 at 17:15

Andreas Veithen

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2 Comments

Andreas Veithen Over a year ago

If your problem is caused by one of these, then make your class loader parent-first and/or set the context class loader.

blow Over a year ago

Thanks again, i try to set my classloader for Thread.currentThread, but it doesn't works, so i try to set classloader for spring ClassPathXmlApplicationContext too, but it doesn't works again... so i can't figure where is the problem.

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