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ClassLoad an Enum type

How would one go about instantiating an Enum type via a ClassLoader or similar mechanism? (I'm trying to keep everything under the same context classloader for a standalone server application).
I have something like:
ClassLoader loader = new CustomClassLoader(parent, libDir);
Thread.currentThread().setContextClassLoader(loader);
// trouble area
Class<?> containerClass = loader.loadClass("com.somepackage.app.Name$SERVER");
I had wrongly thought simply loading the Enum would be enough to kick it off (it's private constructor contains startup method calls and what-not).
Doing what I have above does not result in any exceptions, but the JVM just terminates after the last line and the server does not start up.
Obviously doing:
containerClass.newInstance();
Results in an exception being thrown.

To expand on my comment, I think the cleanest you'll get is something like this:
public static <T extends Enum<T>> T loadEnum(ClassLoader loader, String classBinaryName, String instanceName) throws ClassNotFoundException {
#SuppressWarnings("unchecked")
Class<T> eClass = (Class<T>)loader.loadClass(classBinaryName);
return Enum.valueOf(eClass, instanceName);
}
There is really no way to avoid the unchecked cast from Class<?> to a proper enum type. But at least the #SuppressWarnings is limited in scope.
Edit:
Upon further checking, there is actually a simpler way of achieving what you need, without needing to know the name of an instance and without warnings:
Class<?> containerClass = loader.loadClass("com.somepackage.app.Name");
containerClass.getEnumConstants()

Loading an enum doesn't cause it to initialize. You have to reference it through either a field reference or a method reference. So even a simple statement like Name name = Name.SERVER; or Name.SERVER.name(); would do the trick.
See section 5.5 Initialization in chapter 5. Loading, Linking, and Initializing of the Java Virtual Machine Specification.

Java : method that takes in argument any attribute of any class

I need to create a method that takes in argument any attribute of any class. But i dont want it to be of type String, to avoid refactoring problems while renaming an attribute and to get the errors in Markers Tab of eclipse, and not while running my application.
Having a class Person :
public class Person {
private String name;
// other attributes...
// getters and setters...
}
Now the needed method :
void getAnAttributeOfAClass( <which_type_or_class_here?> attr_as_arg){
// Now I need to get the name of attribute that would be of class Strin...
}
Is there a function or a method, by which we can specify an attribute?
For example :
Person.class.name
Would it be of class Property ?
EDIT
More exactly (#Smallhacker answer helped me), I need to verify at compile time if the argument is really an attribute of the specified class.
Person.class.name // no compile time error
Person.class.nameXXX // compile time error

The closest to what you want is Reflection API's Field or JavaBeans Introspector API's PropertyDescriptor.
But usually things like that are not needed in Java projects because there are libraries which handle these concerns.
You could pass a Class object along with a String name, then let your method use Introspector internally to read that property.

Not sure I understand you well, but there is a class java.lang.reflect.Field, that has a method getName() that would give your the name of the field.
In your example, to get field name, you would do: Person.class.getDeclaredField("name").
EDIT: to get the value of a field in an object, you would do: field.get(obj);
OK, let's say You have the following variables:
Person person = ...; // initialized with some Person
Field nameField = Person.class.getDeclaredField("name");
Now to get the name of person, you would do:
String personName = (String)nameField.get(person);
Actually, this would throw an exception because name is a private field. You can however bypass the protection by doing:
nameField.setAccessible(true);

Unfortunately, Java lacks an ability to reference member variables in a way that can be analyzed at compile time.
There may be some kind of library to simplify this somewhat, but it wouldn't provide a full solution due to limitations in the language itself.

Maybe java generics can help you with this.
You can do something like:
class YourClass<E> {
void getAnAttributeOfAClass(E attr_as_arg){
// some code
}
}
someVariable = new YourClass<Person>();
someVariable.getAnAtributeOfAClass(someObject); //this will not compile if someObject is not an instance of Person
But I still don't know what you want to do exactly inside the method.

How to use Picocontainer Web?

I'm trying to use Picocontainer Web (picocontainer-web-core-2.5.1.jar).
I have configured everything and I checked out that everything works just fine, until trying to retrieve anything from container... :p
I though I should use static method getRequestComponentForThread(Class type) from PicoServletContainerFilter class, which looks like this:
public static Object getRequestComponentForThread(Class type) {
MutablePicoContainer requestContainer = ServletFilter.currentRequestContainer.get();
MutablePicoContainer container = new DefaultPicoContainer(requestContainer);
container.addComponent(type);
return container.getComponent(type);
}
But as you can see, in that method, new instance of DefaultPicoContainer is created and type which I'm trying to retrieve is being registered.
if type is a Class - new instance is created and returned, instead of cached one from parent container...
if type is a Interface - runtime exception ("'ExampleInterface' is not instantiable") is being thrown, at 3rd line (addComponent).
And my question is: How to use this library? I was pretty sure that I understand it, but implementation of this one method blows my mind...

Actually you should not use getComponent unless there's a special case.
App/Session/Request containers are created for you when you add pico context listener to the web.xml.
Just configure components for each scope and picocontainer will inject stuff automatically and instantiate components when needed. Also use Startable lifecycle interface.

I figured out one acceptable solution - writing own version of org.picocontainer.web.PicoServletContainerFilter.ServletFilter - and adding one method:
public class MyComponentContainer extends PicoServletContainerFilter {
/*
code from original class PicoServletContainerFilter.ServletFilter
[...]
*/
public static <T> T getComponent(Class<T> clazz) {
return (T) currentRequestContainer.get().getComponent(clazz);
}
}
I'm not sure if it's the best to do, but it work's fine for me. However, if you know better solution I'd be grateful for information :)

Play-Framework: Causing a 'IllegalArgumentException occured

The error I get is 'IllegalArgumentException occured' Can not set java.util.ArrayList field mi.types.ListOfObjects.objects to java.util.LinkedList
I'm doing the following...
ListOfObjects objects = li.getUsersObjects();
which works perfectly fine normally, however when I do the exact same call with the exact same code inside of Play it doesn't. I'm calling it in my Security Controller inside the authenticate() function. There's a case where the users objects are gotten from an external server. That's when this call is made. It works fine without Play and I'm pretty sure worked before I put it in the Security Controller. Why would putting it here cause such a problem?
EDIT
ListOfObjects is a custom object which contains some values.. It's located inside li which is a library I'm importing. It looks something like this...
public class ListOfObjects {
private ArrayList<Object> objects;
public ArrayList<Object> getObjects(){
return objects;
}
}
getUserObjects simply returns the objects for that user. It grabs them from a server and then uses gson to parse them. However I'm getting the above error when I attempt to.
EDIT2
No matter what I do my type for getUserObjects seems to always be returning as a LinkedList even though it's an ArrayList object. I've tried calling getObjects() directly and there's no change.
EDIT 3
getUserObjects is defined as...
Gson gson = new Gson();
ListOfObjects objects = gson.fromJson(r.getBody(), ListOfObjects.class);
StackTrace...
where r.getBody() is the JSON response from the server
Execution exception (In /app/controllers/Security.java around line 57)
IllegalArgumentException occured : Can not set java.util.ArrayList field lm.types.ListOfObjects.objects to java.util.LinkedList
play.exceptions.JavaExecutionException: Can not set java.util.ArrayList field lm.types.ListOfObjects.objects to java.util.LinkedList
at play.mvc.ActionInvoker.invoke(ActionInvoker.java:227)
at Invocation.HTTP Request(Play!)
Caused by: java.lang.IllegalArgumentException: Can not set java.util.ArrayList field lm.types.ListOfObjects.objects to java.util.LinkedList
at com.google.gson.FieldAttributes.set(FieldAttributes.java:188)
at com.google.gson.JsonObjectDeserializationVisitor.visitFieldUsingCustomHandler(JsonObjectDeserializationVisitor.java:118)
at com.google.gson.ObjectNavigator.navigateClassFields(ObjectNavigator.java:158)
at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:131)
at com.google.gson.JsonDeserializationContextDefault.fromJsonObject(JsonDeserializationContextDefault.java:73)
at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:51)
at com.google.gson.Gson.fromJson(Gson.java:568)
at com.google.gson.Gson.fromJson(Gson.java:515)
at com.google.gson.Gson.fromJson(Gson.java:484)
at com.google.gson.Gson.fromJson(Gson.java:434)
at com.google.gson.Gson.fromJson(Gson.java:406)
at lm.lib.LMethods.getUsersObjects(LMethods.java:165)
at controllers.Security.createNewUser(Security.java:57)
at controllers.Security.authenticate(Security.java:36)
at play.utils.Java.invokeStaticOrParent(Java.java:159)
at controllers.Secure$Security.invoke(Secure.java:193)
at controllers.Secure$Security.access$0(Secure.java:184)
at controllers.Secure.authenticate(Secure.java:64)
at play.mvc.ActionInvoker.invokeWithContinuation(ActionInvoker.java:540)
at play.mvc.ActionInvoker.invoke(ActionInvoker.java:498)
at play.mvc.ActionInvoker.invokeControllerMethod(ActionInvoker.java:474)
at play.mvc.ActionInvoker.invokeControllerMethod(ActionInvoker.java:469)
at play.mvc.ActionInvoker.invoke(ActionInvoker.java:157)
... 1 more
Debugger failed to attach: recv failed during handshake: Connection reset by peer

Exception message clearly says that Gson deserializes list as LinkedList, whereas field of your ListOfObjects is declared as ArrayList.
The general rule to avoid this kind of problems is not to use implementation classes in field declarations. Use List instead:
public class ListOfObjects {
private List<Object> objects;
...
}

ClassCastException error when casting back to original class

I have the following code:
public void doJob() {
MyObj s;
for ( Object o : MyObj.all().fetch()) {
s = (MyObj) o; // ClassCastException here
if (!s.fileExists()) {
//Do some stuff
}
}
}
which is throwing this exception:
play.exceptions.JavaExecutionException: models.MyObj cannot be cast to models.MyObj
at play.jobs.Job.call(Job.java:155)
at Invocation.Job(Play!)
Caused by: java.lang.ClassCastException: models.MyObj cannot be cast to models.MyObj
at jobs.OrphanSurveys.doJob(OrphanSurveys.java:18)
at play.jobs.Job.doJobWithResult(Job.java:50)
at play.jobs.Job.call(Job.java:146)
... 1 more
(This method runs inside a Play Job class, if that matters.)
The MyObj.all().fetch() is returning an Iterable of some kind containing all of the MyObj objects in the database. MyObj inherits this method from the Play! Framework's Model class, if that matters. That's why it's returning a list of Objects rather than MyObjs, and I can't change how it works.
So, is there some reason that I can't cast back to MyObj? I can see how there would be some weirdness casting back from an Object, but Java seems to know what the class of the object used to be.
Thanks!

It looks like you have ClassLoader issues. The objects being returned by your fetch() method were loaded in a different ClassLoader than the one being used in the current thread to try and cast.
Try this to confirm. Add the three lines of code to your exising code.
for ( Object o : MyObj.all().fetch()) {
// Check classloaders
System.out.println(o.getClass().getClassLoader());
System.out.println(MyObj.class.getClassLoader());
break;
//
s = (MyObj) o; // ClassCastException here
if (!s.fileExists()) {
//Do some stuff
}
}

I saw a recent post here on StackOverflow that indicated that if two otherwise identical instances of the same class are loaded by different classloaders, you cannot cast between them.
Post in question
Check whether you are not subject to the multiple classloader condition here too.

From your stack trace, apparently, there's some other kinds of entries in your collection.
Use o.getClass().getName() inside your loop to know what is .all().fetch() really returning.
Note: Maybe some model.Survey objects?