java.lang.ClassCastException: com.sun.proxy.$Proxy

I am running a test under Arquillian OSGi with this code:

An annotation:

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Size {
    int min() default 0;

    int max() default 2147483647;
}

This class

public class FooEntity {
    @Size(min = 0, max = 100)
    public int number;

    public int getNumber() {
        return number;
    }

    public void setNumber(int number) {
        this.number = number;
    }
}

And a validation method somewhere else:

@Override
public List<String> validateEntity(Object entity) throws IllegalAccessException {
    List<String> list = new ArrayList<>();
    for(Field field  : entity.getClass().getDeclaredFields()) {
        for (Annotation annotation : field.getDeclaredAnnotations()) {
            if (annotation.annotationType().getName().equals(Size.class.getName())) {
                Size size = (Size) annotation; // --> Throws 'ClassCastException' exception!!
                int value = field.getInt(entity);
                if (value < size.min() || value > size.max())
                    list.add("Error Message: Invalid size.");
            }
        }
    }
    return list;
}

When I debug this part of the code, I place this same line in the 'watch' window of the IDE and it shows the code is correct, I mean, it returns annotation as Size type, but during run-time the code just breaks. Does anybody knows why is this cast not working? And why is this annotation working under a Proxy class?

Here is the StackTrace of the exception:

java.lang.ClassCastException: com.sun.proxy.$Proxy47 cannot be cast to persistence.ext.annotation.Size
    at persistence.ext.internal.PersistenceImpl.validateEntity(PersistenceImpl.java:144)
    at integration.PersistenceExtTest.ValidateSizeAnnotation(PersistenceExtTest.java:84)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
    at org.jboss.arquillian.junit.Arquillian$8$1.invoke(Arquillian.java:370)
    at org.jboss.arquillian.container.test.impl.execution.LocalTestExecuter.execute(LocalTestExecuter.java:60)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.invokeObservers(EventContextImpl.java:99)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:81)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:145)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:116)
    at org.jboss.arquillian.core.impl.EventImpl.fire(EventImpl.java:67)
    at org.jboss.arquillian.container.test.impl.execution.ContainerTestExecuter.execute(ContainerTestExecuter.java:38)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.invokeObservers(EventContextImpl.java:99)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:81)
    at org.jboss.arquillian.test.impl.TestContextHandler.createClassContext(TestContextHandler.java:92)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createTestContext(TestContextHandler.java:130)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createSuiteContext(TestContextHandler.java:73)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:145)
    at org.jboss.arquillian.test.impl.EventTestRunnerAdaptor.test(EventTestRunnerAdaptor.java:136)
    at org.jboss.arquillian.junit.Arquillian$8.evaluate(Arquillian.java:363)
    at org.jboss.arquillian.junit.Arquillian$4.evaluate(Arquillian.java:245)
    at org.jboss.arquillian.junit.Arquillian.multiExecute(Arquillian.java:422)
    at org.jboss.arquillian.junit.Arquillian.access$200(Arquillian.java:54)
    at org.jboss.arquillian.junit.Arquillian$5.evaluate(Arquillian.java:259)
    at org.jboss.arquillian.junit.Arquillian$7$1.invoke(Arquillian.java:315)
    at org.jboss.arquillian.container.test.impl.execution.BeforeLifecycleEventExecuter.on(BeforeLifecycleEventExecuter.java:35)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.invokeObservers(EventContextImpl.java:99)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:81)
    at org.jboss.arquillian.test.impl.TestContextHandler.createClassContext(TestContextHandler.java:92)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createTestContext(TestContextHandler.java:130)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createSuiteContext(TestContextHandler.java:73)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:145)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:116)
    at org.jboss.arquillian.test.impl.EventTestRunnerAdaptor.fireCustomLifecycle(EventTestRunnerAdaptor.java:159)
    at org.jboss.arquillian.junit.Arquillian$7.evaluate(Arquillian.java:311)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.jboss.arquillian.junit.Arquillian$2.evaluate(Arquillian.java:204)
    at org.jboss.arquillian.junit.Arquillian.multiExecute(Arquillian.java:422)
    at org.jboss.arquillian.junit.Arquillian.access$200(Arquillian.java:54)
    at org.jboss.arquillian.junit.Arquillian$3.evaluate(Arquillian.java:218)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at org.jboss.arquillian.junit.Arquillian.run(Arquillian.java:166)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:160)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:138)
    at org.jboss.arquillian.junit.container.JUnitTestRunner.execute(JUnitTestRunner.java:66)
    at org.jboss.arquillian.osgi.JUnitBundleTestRunner.execute(JUnitBundleTestRunner.java:51)
    at org.jboss.arquillian.protocol.jmx.JMXTestRunner.doRunTestMethod(JMXTestRunner.java:180)
    at org.jboss.arquillian.protocol.jmx.JMXTestRunner.runTestMethodInternal(JMXTestRunner.java:162)
    at org.jboss.arquillian.protocol.jmx.JMXTestRunner.runTestMethod(JMXTestRunner.java:120)
    at org.jboss.arquillian.osgi.ArquillianBundleActivator$2.runTestMethod(ArquillianBundleActivator.java:111)
    at org.jboss.arquillian.protocol.jmx.JMXTestRunner.runTestMethod(JMXTestRunner.java:137)
    at org.jboss.arquillian.osgi.ArquillianBundleActivator$2.runTestMethod(ArquillianBundleActivator.java:124)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:112)
    at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:46)
    at com.sun.jmx.mbeanserver.MBeanIntrospector.invokeM(MBeanIntrospector.java:237)
    at com.sun.jmx.mbeanserver.PerInterface.invoke(PerInterface.java:138)
    at com.sun.jmx.mbeanserver.MBeanSupport.invoke(MBeanSupport.java:252)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801)
    at javax.management.MBeanServerInvocationHandler.invoke(MBeanServerInvocationHandler.java:298)
    at com.sun.proxy.$Proxy27.runTestMethod(Unknown Source)
    at org.jboss.arquillian.protocol.jmx.JMXMethodExecutor.invoke(JMXMethodExecutor.java:80)
    at org.jboss.arquillian.container.test.impl.execution.RemoteTestExecuter.execute(RemoteTestExecuter.java:109)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.invokeObservers(EventContextImpl.java:99)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:81)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:145)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:116)
    at org.jboss.arquillian.core.impl.EventImpl.fire(EventImpl.java:67)
    at org.jboss.arquillian.container.test.impl.execution.ClientTestExecuter.execute(ClientTestExecuter.java:57)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.invokeObservers(EventContextImpl.java:99)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:81)
    at org.jboss.arquillian.container.test.impl.client.ContainerEventController.createContext(ContainerEventController.java:142)
    at org.jboss.arquillian.container.test.impl.client.ContainerEventController.createTestContext(ContainerEventController.java:129)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createClassContext(TestContextHandler.java:92)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createTestContext(TestContextHandler.java:130)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createSuiteContext(TestContextHandler.java:73)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:145)
    at org.jboss.arquillian.test.impl.EventTestRunnerAdaptor.test(EventTestRunnerAdaptor.java:136)
    at org.jboss.arquillian.junit.Arquillian$8.evaluate(Arquillian.java:363)
    at org.jboss.arquillian.junit.Arquillian$4.evaluate(Arquillian.java:245)
    at org.jboss.arquillian.junit.Arquillian.multiExecute(Arquillian.java:422)
    at org.jboss.arquillian.junit.Arquillian.access$200(Arquillian.java:54)
    at org.jboss.arquillian.junit.Arquillian$5.evaluate(Arquillian.java:259)
    at org.jboss.arquillian.junit.Arquillian$7.evaluate(Arquillian.java:321)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.jboss.arquillian.junit.Arquillian$2.evaluate(Arquillian.java:204)
    at org.jboss.arquillian.junit.Arquillian.multiExecute(Arquillian.java:422)
    at org.jboss.arquillian.junit.Arquillian.access$200(Arquillian.java:54)
    at org.jboss.arquillian.junit.Arquillian$3.evaluate(Arquillian.java:218)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.jboss.arquillian.junit.Arquillian.run(Arquillian.java:166)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:74)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:211)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:67)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

Why is this cast not working?

I suspect, that the real reason for your problems is that you have (somehow) managed to get copies of the Size interface loaded; i.e. in different classloaders. The class names appear to be the same, but if you check the classloaders (getClass().getClassLoader()), I suspect that you will find they are different.