为什么以及在哪些情况下使用不同的类加载器两次加载 bean？

Question

Recently I visited an interview.

I was asked:

MyClass myClass =(MyClass) applicationContext.getBean("myClass");

throws ClassCastException

but applicationContext.getBean("myClass").getClass() returns MyClass .

I was surprised about the question. I could reply only that classes loaded by different classloaders.

1. How to achieve this ?
2. Why Spring uses different classloaders ?

Answer 1

Yes different classloader will lead to this scenario. This is not that common scenario however some application uses multiple classloaders (most common examples are containers and application servers). You can reproduce it

You can initialize spring application context by specifying URLClassLoader

Have your class located at

/home/jigar.joshi/foo/package/MyClass

and configure application context to use URLClassLoader like this

URL[] classURLs = { new URL("file:///home/jigar.joshi/foo") };
URLClassLoader urlClassLoader = new URLClassLoader(classURLs);
ApplicationContext context = new     ClassPathXmlApplicationContext("spring/applicationContext.xml") {
            protected void initBeanDefinitionReader(XmlBeanDefinitionReader reader) {
                super.initBeanDefinitionReader(reader);
                reader.setValidationMode(XmlBeanDefinitionReader.VALIDATION_NONE);
                reader.setBeanClassLoader(urlClassLoader);
                setClassLoader(urlClassLoader);
            }
        };

        MyClass m = context.getBean("MyClass");