We are in the progress of introducing PostSharp in one of our projects. It's been working great so far! There is one thing though that we haven't managed to solve: how to fire an advice conditionally.
Details:
- we have an attribute StopWatchAttribute which makes it possible to record the time needed to run methods
- this attribute accepts an enumeration "LoggingLevel" which is set in the config file with values like 0, 1, 2 etc
- this parameter is read in a base class called BaseService during runtime: new BaseService().CurrentLoggingSettings
- we tried to set up the attribute constructor like StopWatchAttribute(new BaseService().CurrentLoggingLevel) but we get a compile error: an attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type.
--> summary: we would like the advice to be called conditionally and the condition depends on the parameter in the constructor of the attribute.
Is this possible to do?
Thanks for your help,
Andras
You cannot give variables to attributes, PostSharp or not. Since you're already reading the values from the config, just set your aspect to do the same on the Initialize() method. Override it in the aspect class and then save the value to a local field. You can use that field throughout the aspect. This compiles the value into the aspect essentially hard coding it.
Or, you can pull the value from the config from your advice method (OnMethodStart, etc) so that you can change it in the config at runtime. This is a more 'flexible' way to do it as it doesn't hard code anything.
Remember, your variables are being set at Runtime. PostSharp is a post-compile framework which means it does it's work long before your variables are even known to JIT.
Related
I am generating a class in ByteBuddy.
As part of one method implementation, I would like to set a (let's just say) public instance field in another object to the return value of a MethodCall invocation. (Keeping the example public means that access checks etc. are irrelevant.)
I thought I could use MethodCall#setsField(FieldDescription) to do this.
But from my prior question related to this I learned that MethodCall#setsField(FieldDescription) is intended to work only on fields of the instrumented type, and, looking at it now, I'm not entirely sure why or how I thought it was ever going to work.
So: is there a way for a ByteBuddy-generated method implementation to set an instance field of another object to the return value of a method invocation?
If it matters, the "instrumented method" (in ByteBuddy's terminology) accepts the object whose field I want to set as an argument. Naïvely I'd expect to be able to do something like:
MethodCall.invoke(someMethod).setsField(somePublicField).onArgument(2);
There may be problems here that I am not seeing but I was slightly surprised not to see this DSL option. (It may not exist for perfectly good reasons; I just don't know what they would be.)
This is not possible as of Byte Buddy 1.10.18, the mechanism was originally created to support getters/setters when defining beans, for example. That said, it would not be difficult to add; I think it would even be easiest to allow any custom byte code to be dispatched as a consumer of the method call.
I will look into how this can be done, but as a new feature, this will take some time before I find the empty space to do so. The change is tracked on GitHub.
I am using MethodCall.setsField() to try to set an instance field on another instance.
My generated class that is doing the field-setting, GC, is trying to set the value of an instance field in an instance of something it has created (CI). So the field's declaring type is CI; my field-setting code resides in GC (which is in the same package as CI but otherwise unrelated to it).
The ByteBuddy checks seem to indicate that although GC and CI are in the same package, GC must be assignable to CI in order to set this field! That greatly surprised me, but I am not a bytecode expert, and I might very well be overlooking something obvious. Could someone kindly explain why this check is necessary?
The method call sets the field implicitly on the this instance on which the method is invoked. For this to be possible, a non-static field must be declared by a super type of the type on which the method is invoked.
If you think this is too strict, please file an issue with an example of the code you are trying to generate, including the code to generate it which is currently failing. Maybe I am not thinking straight about this and if there's a restriction to be lifted, I would surely do it.
I am reviewing some code and I realized I don't remember the correct terminology for something. I believe if I had the following code
pnlOne.Visible = False
Would the "visible" part be considered a method, function, or what? I am learning VB alongside JavaScript, and in JS it would be a method. Is it the same for vb?
In VB.net, that is a "property". Properties in VB.net and C# as essentially glorified methods for getting and setting a value. (They actually compile down to something like get_Visible and set_Visible methods.)
pnlOne is an instance of a class and Visible is its property
Visible could be either ..
a Property; or
a Field (called "Member Variable" in VB)
.. depending on how it is declared. Both Properties and Fields are specializations of "Members"1. See Differences Between Properties and Variables in Visual Basic.
I suspect Visible is a Property in this case, and it will be for all standard Control types .. however, to verify this either way requires knowledge of the Type of the object named by pnlOne.
1
Methods (or "Sub/Function Procedures") are a different kind of Member and it is not appropriate to call either a Property or Field a "Function" or a "Method". (Note: various references inconsistently make a distinction between a Method and a Procedure; in VB.NET they an be thought of as synonyms.)
Nit: the correct term in JavaScript would be property; properties can evaluate to function-objects and can thus also can be considered methods when they do so - usually when this is used meaningfully. In any case, the code would have to be different (e.g. jsObj.set_Visible(true)) if a method was used.
Is there an elegant/convinient way (without creating many "empty" classes or at least they should be not annoying) to have fluent interfcaes that maintain order on compilation level.
Fluent interfaces:
http://en.wikipedia.org/wiki/Fluent_interface
with an idea to permit this compilation
var fluentConfig = new ConfigurationFluent().SetColor("blue")
.SetHeight(1)
.SetLength(2)
.SetDepth(3);
and decline this
var fluentConfig = new ConfigurationFluent().SetLength(2)
.SetColor("blue")
.SetHeight(1)
.SetDepth(3);
Each step in the chain needs to return an interface or class that only includes the methods that are valid to use after the current step. In other words, if SetColor must come first, ConfigurationFluent should only have a SetColor method. SetColor would then return an object that only has a SetHeight method, and so forth.
In reality, the return values could all be the same instance of ConfigurationFluent but cast to different interfaces explicitly implemented by that class.
I've got a set of three ways of doing this in C++ using essentially a compile time FSM to validate the actions. You can find the code on github.
The short answer is no, there is no elegant or convenient way to enforce an order of constructing a class that properly impelemnts the "Fluent Interface" as you've linked.
The longer answer starts with playing devil's advocate. If I had dependent properties (i.e. properties that required other properties to be set first), then I could implement them something like this:
method SetLength(int millimeters)
if color is null throw new ValidationException
length = millimeters
return this
end
(NOTE: the above does not map to any real language, it is just psuedocode)
So now I have exceptions to worry about. If I don't obey the rules, the fluent object will throw an exception. Now let's say I have a declaration like yours:
var config = new Fluent().SetLength(2).SetHeight(1).SetDepth(3).SetColor("blue");
When I catch the ValidationException because length depends on the color being set first, how am I as the user supposed to know what the correct order is? Even if I had each SetX method on a different line, the stacktrace will just give me the line where the config variable was declared in most languages. Furthermore, how am I supposed to keep the rules of this object straight in my head compared to other objects? It is a cocophony of conflicting ideals.
Such precedence checks violate the spirit of the "Fluent Interface" approach. That approach was designed for conveniently configure complex objects. You take the convenience out when you attempt to enforce order.
To properly and elegantly implement the fluent interface there are a couple of guidelines that are best observed to make consumers of your class thank you:
Provide meaningful default values: minimizes need to change values, and minimizes chances of creating an invalid object.
Do not perform configuration validation until explicitly asked to do so. That event can be when we use the configuration to create a new fully configured object, or when the consumer explicitly calls a Validate() method.
In any exceptions thrown, make sure the error message is clear and points out any inconsistencies.
maybe the compiler could check that methods are called in the same order as they are defined.
this could be a new feature for compilers.
Or maybe by means of annotations, something like:
class ConfigurationFluent {
#Called-before SetHeight
SetColor(..) {}
#Called-After SetColor
SetHeight(..) {}
#Called-After SetHeight
SetLength(..){ }
#Called-After SetLength
SetDepth(..) {}
}
You can implement a state machine of valid sequence of operations and on each method call the state machine and verify if the sequence of operation is allowed or throw an exception if not.
I will not suggest this approach for Configurations though, it can get very messy and not readable
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.