I have the following code working in a SpringBoot application, and it does what's I'm expecting.
TypePool typePool = TypePool.Default.ofClassPath();
ByteBuddyAgent.install();
new ByteBuddy()
.rebase(typePool.describe("com.foo.Bar").resolve(), ClassFileLocator.ForClassLoader.ofClassPath())
.implement(typePool.describe("com.foo.SomeInterface").resolve())
.make()
.load(ClassLoader.getSystemClassLoader());
Its makes is so that the class com.foo.Bar implements the interface com.foo.SomeInterface (which has a default implementation)
I would like to . use the above code by referring to the class as Bar.class, not using the string representation of the name. But if I do that I get the following exception.
java.lang.UnsupportedOperationException: class redefinition failed: attempted to change superclass or interfaces
I believe due to the fact that it cause the class to be loaded, prior to the redefinition. I'm just now learning to use ByteBuddy.
I want to avoid some reflection at runtime, by adding the interface and an implementation using ByteBuddy. I've some other code that checks for this interface.
This is impossible, not because of Byte Buddy but no tool is allowed to do this on a regular VM. (There is the so-called dynamic code evolution VM which is capable of that).
If you want to avoid the problem, use redefine rather then rebase. Whenever you instrument a method, you do now however replace the original.
If this is not acceptable, have a look at the Advice class which you can use by the .visit-API to wrap logic around your original code without replacing it.
Related
I have one VB6 ActiveX DLL that exposes a class INewReport. I added some new methods to this class and I was able to rebuild it and keep binary compatibility.
I have a second DLL that exposes a class clsNewReport, which implements the first class using:
Implements RSInterfaces.INewReport
Since I added new methods to INewReport, I had to also add those new methods to clsNewReport.
However, when I try to compile the second DLL, I get the binary-compatibility error "...class implemented an interface in the version-compatible component, but not in the current project".
I'm not sure what is happening here. Since I'm only adding to the class, why can't I maintain binary compatibility with the second DLL? Is there any way around this?
I think this is a correct explanation of what is happening, and some potential workarounds.
I made up a test case which reproduced the problem in the description and then dumped the IDL using OLEView from the old & new DLL which contained the interface.
Here is a diff of the old (left) and new IDL from INewReport:
Important differences:
The UUID of interface _INewReport has changed
A typedef called INewReport___v0 has been added which refers to the original UUID of the interface
(I assume that this is also what is happening to the code referred to in the question.)
So now in the client project the bincomp DLL refers to the original interface UUID; but that UUID only matches against a different name (INewReport___v0 instead of INewReport) than it did originally. I think this is the reason VB6 thinks there is a bincomp mismatch.
How to fix this problem? I've not been able to do anything in VB6 that would allow you to use the updated interface DLL with the client code without having to break bincomp of the client code.
A (bad) option could be to just change the client DLL to use project compatibility... but that may or may not be acceptable in your circumstances. It could cause whatever uses the client DLL to break unless all the consumers were also recompiled. (And this could potentially cause a cascade of broken bincomp).
A better but more complex option would be to define the interface in IDL itself, use the MIDL compiler to generate a typelib (TLB file), and reference that directly. Then you would have full control over the interface naming, etc. You could use the IDL generated from OLEView as a starting point for doing this.
This second option assumes that the interface class is really truly an interface only and has no functional code in it.
Here's how I setup a case to reproduce this:
Step 1. Original interface definition - class called INewReport set to binary compatible:
Sub ProcA()
End Sub
Sub ProcB()
End Sub
Step 2. Create a test client DLL which implements INewReport, also set to binary compatible:
Implements INewReport
Sub INewReport_ProcA()
End Sub
Sub INewReport_ProcB()
End Sub
Step 3: Add ProcC to INewReport and recompile (which also registers the newly built DLL):
(above code, plus:)
Sub ProcC()
End Sub
Step 4: Try to run or compile the test client DLL - instantly get the OP's error. No need to change any references or anything at all.
I was able to recreate your problem, using something similar to DaveInCaz's code. I tried a number of things to fix it, probably repeating things you've already tried. I came up with a possible hypothesis as to why this is happening. It doesn't fix the problem, but it may throw some additional light on it.
Quoting from This doc page:
To ensure compatibility, Visual Basic places certain restrictions on changes you make to default interfaces. Visual Basic allows you to add new classes, and to enhance the default interface of any existing class by adding properties and methods. Removing classes, properties, or methods, or changing the arguments of existing properties or methods, will cause Visual Basic to issue incompatibility warnings.
Another quote:
The ActiveX rule you must follow to ensure compatibility with multiple interfaces is simple: once an interface is in use, it can never be changed. The interface ID of a standard interface is fixed by the type library that defines the interface.
So, here's a hypothesis. The first quote mentions the default interface, which suggests that it may not be possible to alter custom interfaces in any way. That's suggested by the second quote as well. You're able to alter the interface class, because you are essentially altering its default interface. However, when you attempt to alter the implementing class in kind, to reflect the changes in your interface, your implementation reference is pointing to the older version of the interface, which no longer exists. Of course, the error message doesn't hint at this at all, because it appears to be based on the idea that you didn't attempt to implement the interface.
I haven't been able to prove this, but looking at DaveInCaz's answer, the fact that the UUID has changed seems to bear this idea out.
There's a solid chance I'm misusing classes here which is why I need your help.
I've started developing with Java EE and one of the problems I am facing is I have a process which I have organised in a class, call it: "SendEmail.java".
Now let's say I have two other classes called "Thunderalert.java" and "FloodAlert.java" which will use all the methods that SendEmails.java has within it.
So I want to know the best way of using the SendEmails methods from each of the other classes.
Should I be creating an instance of SendEmails and accessing each method individually and error checking along the way (what if an exception is thrown?).. It's methods are just procedural code, so it's not really an 'object' as such
Shall I just be using the one method that runs all the other internal ones from within SendMail
Should this SendMail be redesigned as a helper class-type design?
I'm still quite new at Java EE so I'm not sure if there are any options available which I am missing
I think you should have one public method inside SendEmail class.
Btw, I would consider changing its name. I think having method send() when class is called SendEmail is not the best way (not to mention about names like call(), invoke() etc).
This is great article about this problem (The Kingdom of Nouns) in java.
What about something like: new Email(recipient, body).send()?
Or if you want to do it in a service style, I'd call it for example MailService
I want to be able to define in my web.config the type of connexion my object will use to get data (variable) (from an xml or from a databases).
I though about using a Strategie Pattern, but I'm somewhat stuck by the need to write somewhere the name of the class, which I do not want.
Any suggestions?
Additionnal info
I have the interface IContext.
It's implemented in ContextXML and ContextDB.
I have the class Context which has a IContext member (called _context).
The Context class reads (through ContextConfiguration) app.config.
I want _context to be able to be a ContextXML or a ContextDB... or a ContextJSon or any other new class that would implements IContext.
Have you thought about creating a ContextManager class and employing "configuration by convention"?
What I would do, is add a member getName to your IContext interface - this just returns a nice human-readable string for each implementation - as simple as "ContextXML" for your ContextXML class.
When your ContextManager (probably a Singleton, BTW) starts up, it scans a known directory for IContext implementations, instantiating them by reflection (or some other mechanism, I'm not familiar with VB.Net but I'm sure there's a way), and placing them in a collection.
Now when you are building up Context objects, you can ask your ContextManager for a suitable IContext - either explicitly [e.g. getIContextByName("ContextDB")] or with a simpler method that just returns whatever has been configured by some other mechanism - i.e. a suite of methods something like this:
getPossibleIContextImplementationNames()
setCurrentIContextImplementation({name})
getCurrentIContext()
Just as an aside, are you stuck with that naming? Because having a Context object that uses an IContext seems a little unusual. If your IContext implementations are actually used to retrieve data from somewhere, why not call the interface IDAO or IDataAccessor?
This question already has answers here:
What is reflection and why is it useful?
(23 answers)
Closed 6 years ago.
I was just curious, why should we use reflection in the first place?
// Without reflection
Foo foo = new Foo();
foo.hello();
// With reflection
Class cls = Class.forName("Foo");
Object foo = cls.newInstance();
Method method = cls.getMethod("hello", null);
method.invoke(foo, null);
We can simply create an object and call the class's method, but why do the same using forName, newInstance and getMthod functions?
To make everything dynamic?
Simply put: because sometimes you don't know either the "Foo" or "hello" parts at compile time.
The vast majority of the time you do know this, so it's not worth using reflection. Just occasionally, however, you don't - and at that point, reflection is all you can turn to.
As an example, protocol buffers allows you to generate code which either contains full statically-typed code for reading and writing messages, or it generates just enough so that the rest can be done by reflection: in the reflection case, the load/save code has to get and set properties via reflection - it knows the names of the properties involved due to the message descriptor. This is much (much) slower but results in considerably less code being generated.
Another example would be dependency injection, where the names of the types used for the dependencies are often provided in configuration files: the DI framework then has to use reflection to construct all the components involved, finding constructors and/or properties along the way.
It is used whenever you (=your method/your class) doesn't know at compile time the type should instantiate or the method it should invoke.
Also, many frameworks use reflection to analyze and use your objects. For example:
hibernate/nhibernate (and any object-relational mapper) use reflection to inspect all the properties of your classes so that it is able to update them or use them when executing database operations
you may want to make it configurable which method of a user-defined class is executed by default by your application. The configured value is String, and you can get the target class, get the method that has the configured name, and invoke it, without knowing it at compile time.
parsing annotations is done by reflection
A typical usage is a plug-in mechanism, which supports classes (usually implementations of interfaces) that are unknown at compile time.
You can use reflection for automating any process that could usefully use a list of the object's methods and/or properties. If you've ever spent time writing code that does roughly the same thing on each of an object's fields in turn -- the obvious way of saving and loading data often works like that -- then that's something reflection could do for you automatically.
The most common applications are probably these three:
Serialization (see, e.g., .NET's XmlSerializer)
Generation of widgets for editing objects' properties (e.g., Xcode's Interface Builder, .NET's dialog designer)
Factories that create objects with arbitrary dependencies by examining the classes for constructors and supplying suitable objects on creation (e.g., any dependency injection framework)
Using reflection, you can very easily write configurations that detail methods/fields in text, and the framework using these can read a text description of the field and find the real corresponding field.
e.g. JXPath allows you to navigate objects like this:
//company[#name='Sun']/address
so JXPath will look for a method getCompany() (corresponding to company), a field in that called name etc.
You'll find this in lots of frameworks in Java e.g. JavaBeans, Spring etc.
It's useful for things like serialization and object-relational mapping. You can write a generic function to serialize an object by using reflection to get all of an object's properties. In C++, you'd have to write a separate function for every class.
I have used it in some validation classes before, where I passed a large, complex data structure in the constructor and then ran a zillion (couple hundred really) methods to check the validity of the data. All of my validation methods were private and returned booleans so I made one "validate" method you could call which used reflection to invoke all the private methods in the class than returned booleans.
This made the validate method more concise (didn't need to enumerate each little method) and garuanteed all the methods were being run (e.g. someone writes a new validation rule and forgets to call it in the main method).
After changing to use reflection I didn't notice any meaningful loss in performance, and the code was easier to maintain.
in addition to Jons answer, another usage is to be able to "dip your toe in the water" to test if a given facility is present in the JVM.
Under OS X a java application looks nicer if some Apple-provided classes are called. The easiest way to test if these classes are present, is to test with reflection first
some times you need to create a object of class on fly or from some other place not a java code (e.g jsp). at that time reflection is useful.
I have a Util module in my VB.NET program that has project-wide methods such as logging and property parsing. The general practice where I work seems to be to call these methods directly without prefixing them with Util. When I was new to VB, it took me a while to figure out where these methods/functions were coming from. As I use my own Util methods now, I can't help thinking that it's a lot clearer and more understandable to add Util. before each method call (you know immediately that it's user-defined but not within the current class, and where to find it), and is hardly even longer. What's the general practice when calling procedures/functions of VB modules? Should we prefix them with the module name or not?
Intellisense (and "Goto Definition") should make it trivial to find where things are located, but I always preface the calls with a better namespace, just for clarity of reading. Then it's clear that it's a custom function, and not something built in or local to the class you're working with.
Maybe there's a subtle difference I'm missing, but I tend to use shared classes instead of modules for any code that's common and self-contained - it just seems easier to keep track of for me, and it would also enforce your rule of prefacing it, since you can't just call it from everywhere without giving a namespace to call it from.
I usually put the complete namespace for a shared function, for readibility.
Call MyNameSpace.Utils.MySharedFunction()
Util is such a generic name.
Example from the .Net framework. You have System.Web.HttpUtility.UrlEncode(...). Usually you refer to this as HttpUtility.UrlEncode since you have an import statement at the top.
The name of the class which has the static utility methods should be readable and explainable. That is good practice. If you have good class names they might just as well reside in a Utils namespace, but the class name should not be Utils.
Put all your logging in a Logger class. All your string handing in a StringUtils class etc. And try to keep the class names as specific as possible, and I'd rather have more classes with fewer functions than the other way around.