I'm exploring if Byte Buddy is the right tool for me to use. I was looking at this answer which talks about how with Byte Buddy, one can take a class Foo loaded with class loader A, rename it to Bar and redefine it on another class loader B. That's incredible.
My question is:
Is it possible to have an object which was created from Foo on class loader A and convert to Bar's object which is coming from class loader B? The solution which I have in mind right now is to basically serialize the object from Foo and then edit the serialized bytes to rename the name of the class. It would be nice if I can do it safely with Byte Buddy. Another solution would be to use Transloader.
Is java-agent or any special instrumentation required for doing all the things that was described in the above SO answer
Thank you so much.
You can always load a class in another class loader without using a Java agent. As for translating the class, replacing the symbol of one class name to anther should not be a problem but you need to watch out for serial version UIDs where there name of the class might be a part of. Byte Buddy is a code generation library and does not add any extra help for such operations.
Related
I'm working in Kotlin, and I have a library that I can't modify (without maintaining my own fork of this huge and rapidly changing library).
There's a class in that library I need to instantiate, and it requires an instance of another class that does some stuff for it. I need to change how that second class works, so it feeds different information to the first class. But the second class is not open, and the first class asks for it by its full, non-open type.
How do I force my way into the non-open class and extend it anyway, against the desires of the library authors? Alternately, how do I cheat the type system to pass my own class off as an instance of the class the library is demanding?
Do I need to do some fiddling around at the JAR/build system level to replace the library's class files with my own versions? Can I use reflection to somehow impersonate the non-open class? Is there some other way to do it?
I saw that the default LocationStrategy is STRONG, which keeps a strong reference to the class loader when creating a ClassFileLocator. Does this mean Byte Buddy can prevent a class loader from being garbage collected (eg when undeploying a webapp from a servlet container) or is there another mechanism to evacuate those?
Also in this regard- the documentation about the WEAK strategy says a ClassFileLocator will "stop working" after the corresponding class loader is garbage collected. What are the implications? How would a locator for a garbage-collected class loader be used?
You are right about your assertion. With a strong type locator, all TypeDescriptions will reference the class loader as dependent types are resolved lazily. This means for example that if you look up a type's field type, that type will only be loaded if you are using it for the first time what might never happen.
Typically, those type descriptions are not stored over the life time of a class being loaded. Since the class loader will never be garbage collected during a loading of one of its classes, referencing the class loader strongly does not render any issue. However, once you want to cache type descriptions in between multiple class loadings (what can make a lot of sence since some applications load thousands of classes using the same class loader), this might become a problem if a class loader would be garbage collected while the cache is still referencing the type description with the underlying class loader.
In this case, reusing the type descriptions will be problematic since no lazily referenced classes can be resolved after the class loader was garbage collected. Note that a type description might be resolved using a specific class loader while the class is defined by a parent of that class loader which is why this might be a problem.
Typically, if you maintain a cache of type descriptions per class loader, this should however not be a problem.
What does doing the following mean, I've been seeing it around online a lot and don't understand why the word object is written
class Solution(object):
Thanks in advance.
It's called 'Inheritance'.
You can read more in section 9.5 here.
https://docs.python.org/3/tutorial/classes.html
class likes a blueprint. as an example you can consider a registration form, this form is a class and if you fill up a copy of this form it will be an object.in a class we have some variables are called attribute or field and we got some functions that each of them is called method. if you allocate some space of your memory to an unit which has value for all attributes of your class, this is your object.
I'm currently trying to learn how to use GObject and there's a point I absolutely don't understand: What's the difference between the class and the instance structure (like "MamanBarClass" and "MamanBar") resp. how do I use them? At the moment I'd put all my object attributes into a private structure (like "MamanBarPrivate"), register it with "g_type_class_add_private" and define properties/getters/setters to access them. But when I leave the class structure empty I get the following error at "g_type_register_static_simple":
specified class size for type `MamanBar' is smaller than `GTypeClass' size
And why are all object methods defined in the class structure (like "GtKWidgetClass")? Probably I'm just screwing up everything, but I only worked with Delphi for OOP yet (I know, nothing to be proud about :D)
Regards
I'm currently trying to learn how to use GObject and there's a point I absolutely don't understand: What's the difference between the class and the instance structure (like "MamanBarClass" and "MamanBar") resp. how do I use them?
The class structure is only created once, and is not instance-specific. It's where you put things which are not instance-specific, such as pointers for virtual methods (which is the most common use for the class struct).
At the moment I'd put all my object attributes into a private structure (like "MamanBarPrivate"), register it with "g_type_class_add_private" and define properties/getters/setters to access them.
Good. That's the right thing to do.
But when I leave the class structure empty I get the following error at "g_type_register_static_simple":
You should never leave the class structure empty. It should always contain the class structure for the type you're inheriting from. For example, if you're trying to create a GObject, the class structure should look like this (at a minimum):
struct _MamanBarClass {
GObjectClass parent_class;
};
Even if you're not inheriting from GObject, you still need the base class for GType:
struct _FooClass {
GTypeClass parent_class;
};
This is how simple inheritance is done in C.
And why are all object methods defined in the class structure (like "GtKWidgetClass")? Probably I'm just screwing up everything, but I only worked with Delphi for OOP yet (I know, nothing to be proud about :D)
Those are virtual public methods. As for why they're defined in the class structure instead of the instance structure, it's because the implementations are the same for every instance.
So we have a java class with two ArrayLists of generics. It looks like
public class Blah
{
public ArrayList<ConcreteClass> a;
public ArrayList<BaseClass> b;
}
by using [ArrayElementType('ConcreteClass')] in the actionscript class, we are able to get all the "a"s converted fine. However with "b", since the actual class coming across the line is a heterogeneous mix of classes like BaseClassImplementation1, BaseClassImplementation2 etc, it gets typed as an object. Is there a way to convert it to the specific concrete class assuming that a strongly typed AS version of the java class exists on the client side
thanks for your help!
Regis
To ensure that all of your DTO classes are marshalled across AS and Java, you need to define each remote class as a "remote class" in AS by using the "RemoteClass" attribute pointing to the java class definition like this [RemoteClass(alias="com.myco.class")].
BlazeDS will perform introspection on the class as it is being serialized/de-serialized and convert it appropriately (see doc below). It doesn't matter how the classes are packed or nested in an array, as long as it can be introspected it should work.
If you need special serialization for a class you can create your own serialization proxys (called beanproxy) by extending "AbastractProxy" and loading them into blazeds using the PropertyProxyRegistry register method on startup.
You will find most of this in the Blaze developers guide http://livedocs.adobe.com/blazeds/1/blazeds_devguide/.
Creating your own beanproxy class look here: //livedocs.adobe.com/blazeds/1/javadoc/flex/messaging/io/BeanProxy.html