I have a Domain Specific Language, and I would like to register objects that can be instantiated inside.
For instance a class that can do httprequests.
[IoC("HttpRequest", typeof(DslScriptObject), IoCAttribute.IoCLifestyleType.Transient)]
internal class WebRequestDslObj : DslScriptObject
{
[DslNew]
public WebRequestDslObj() : this(null, null)
{}
[DslNew]
public WebRequestDslObj([DslParam("uri")]string uristring, [DslOptionalParam("contenttype")] string contenttype) : this(uristring, null)
{}
}
I then have a class that maps types from my dsl datatypes to c# datatypes (I have them as an IList if that makes any difference), and this works ok, if I do not use Castle to instantiate the object.
But as soon as I want to use IoC to autoregister the various types, then I dont know what to do about the constructors. I have tried to look at setting a CustomComponentActivator, but I got stuck at not being able to find any good example or documentation. Is that a viable path to take? (and will I be able to get around the funny special case for null parameters?)
Anyone have an example of where I can start?
So what are you trying to do with Windsor, because I'm not sure I see where you're going with it...
If you want to affect how component gets register in Windsor, for example rename parameters, you can write custom ComponentModel construction contributor to do it.
Related
I would like to define some properties in my logback.xml config file and saw that by implementing the PropertyDefiner was a great way to set properties in a customizable way.
After starting to implement it I began to wonder how to access the value of the name attribute of the element within the tag. I'm not seeing anyway to do this and I'm scratching my head. Would this PropertyDefiner really make you create a new implementation for every single property? Why not just hard code it? I didn't see much discussion about this out on the web.
I hope I'm just not seeing it and that the brains of stackoverflow can help me out. Does anyone know how to do this? Thanks!
I found this discussion: essentially the same question is asked, but no answer was returned.
fyi: I want to customize how I get my properties because I am pulling it from a database. I have a helper class which pulls the properties in on server startup. These properties vary based on environment (dev, test, prod, etc.)
As of logback version 1.0.6, the value of the name attribute cannot be accessed directly. However, nothing prevents you from passing the value of the name attribute in a property of your choice. Example:
<define name="rootLevel" class="Your.PropertyDefiner">
<myKey>rootLevel</myKey>
</define>
where myKey is a property of Your.PropertyDefiner. For example:
class Your.PropertyDefiner implements PropertyDefiner {
String myKey;
public void setMyKey(String k) {
this.myKey= k;
}
public String getPropertyValue() {
return ...
}
}
Joran, logback's configuration framework, takes care of the wiring. Joran will inject the value of the myKey element into the myKey property of Your.PropertyDefiner. If you are curious about the technical details, see the documentation on implicit actions and implicit actions in practice.
I'm using Spring MVC to handle JSON POST requests. Underneath the covers I'm using the MappingJacksonHttpMessageConverter built on the Jackson JSON processor and enabled when you use the mvc:annotation-driven.
One of my services receives a list of actions:
#RequestMapping(value="/executeActions", method=RequestMethod.POST)
public #ResponseBody String executeActions(#RequestBody List<ActionImpl> actions) {
logger.info("executeActions");
return "ACK";
}
I have found that Jackson maps the requestBody to a List of java.util.LinkedHashMap items (simple data binding). Instead, I would like the request to be bound to a List of typed objects (in this case "ActionImpl").
I know this is easy to do if you use Jackson's ObjectMapper directly:
List<ActionImpl> result = mapper.readValue(src, new TypeReference<List<ActionImpl>>() { });
but I was wondering what's the best way to achieve this when using Spring MVC and MappingJacksonHttpMessageConverter. Any hints?
Thanks
I have found that you can also work around the type erasure issue by using an array as the #RequestBody instead of a collection. For example, the following would work:
public #ResponseBody String executeActions(#RequestBody ActionImpl[] actions) { //... }
I suspect problem is due to type erasure, i.e. instead of passing generic parameter type, maybe only actions.getClass() is passed; and this would give type equivalent of List< ?>.
If this is true, one possibility would be to use an intermediate sub-class, like:
public class ActionImplList extends ArrayList<ActionImpl> { }
because this will the retain type information even if only class is passed.
So then:
public #ResponseBody String executeActions(#RequestBody ActionImplList actions)
would do the trick. Not optimal but should work.
I hope someone with more Spring MVC knowledge can shed light on why parameter type is not being passed (perhaps it's a bug?), but at least there is a work around.
For your information, the feature will be available in Spring 3.2 (see https://jira.springsource.org/browse/SPR-9570)
I just tested it on current M2 and it works like a charm out of the box (no need to provide additionnal annotation to provide the parameterized type, it will be automatically resolved by new MessageConverter)
This question is already old, but I think I can contribute a bit anyway.
Like StaxMan pointed out, this is due to type erasure. It definitely should be possible, because you can get the generic arguments via reflection from the method definition. However, the problem is the API of the HttpMessageConverter:
T read(Class<? extends T> clazz, HttpInputMessage inputMessage);
Here, only List.class will be passed to the method. So, as you can see, it is impossible to implement a HttpMessageConverter that calculates the real type by looking at the method parameter type, as that is not available.
Nevertheless, it is possible to code your own workaround - you just won't be using HttpMessageConverter. Spring MVC allows you to write your own WebArgumentResolver that kicks in before the standard resolution methods. You can for example use your own custom annotation (#JsonRequestBody?) that directly uses an ObjectMapper to parse your value. You will be able to provide the parameter type from the method:
final Type parameterType= method.getParameterTypes()[index];
List<ActionImpl> result = mapper.readValue(src, new TypeReference<Object>>() {
#Override
public Type getType() {
return parameterType;
}
});
Not really the way TypeReference was intended to be used I presume, but ObjectMapper doesn't provide a more suitable method.
Have you tried declaring the method as:
executeActions(#RequestBody TypeReference<List<ActionImpl>> actions)
I haven't tried it, but based on your question it's the first thing I would try.
I've got a bit of a problem. I'm working in the Castle Windsor IOC Container. Now what i wanted to do is just mess about with some AOP principles and what i specifically want to do is based on a method name perform some logging. I have been looking at Interceptors and at the moment i am using the IInterceptor interface implemented as a class to perform this logging using aspects. The issue is if i want to perform the logging on a specific method then it gets messy as i need to put in some logic into my implemented aspect to check the method name etc...
I have read that you can do all of this using Dynamic Proxies and the IInterceptorSelector interface and the IProxyGenerationHook interface. I have seen a few examples of this done on the net but i am quite confused how this all fits into the Windsor container. I mean i am using the windsor container which in my code is actually a ref to the IWindsorContainer interface to create all my objects. All my configuration is done in code rather than XML.
Firstly does anyone know of a way to perform method specific AOP in the windsor container besides the way i am currently doing it.
Secondly how do i use the Dynamic Proxy in the windsor container ?
Below i have added the code where i am creating my proxy and registering my class with
the interceptors
ProxyGenerator _generator = new ProxyGenerator(new PersistentProxyBuilder());
IInterceptorSelector _selector = new CheckLoggingSelector();
var loggingAspect = new LoggingAspect();
var options = new ProxyGenerationOptions(new LoggingProxyGenerationHook())
{ Selector = _selector };
var proxy = _generator.CreateClassProxy(typeof(TestClass), options, loggingAspect);
TestClass testProxy = proxy as TestClass;
windsorContainer.Register(
Component.For<LoggingAspect>(),
Component.For<CheckLoggingAspect>(),
Component.For<ExceptionCatchAspect>(),
Component.For<ITestClass>()
.ImplementedBy<TestClass>()
.Named("ATestClass")
.Parameters(Parameter.ForKey("Name").Eq("Testing"))
.Proxy.MixIns(testProxy));
The Test Class is below:
public class TestClass : ITestClass
{
public TestClass()
{
}
public string Name
{
get;
set;
}
public void Checkin()
{
Name = "Checked In";
}
}
as for the interceptors they are very simple and just enter a method if the name starts with Check.
Now when i resolve my TestClass from the container i get an error.
{"This is a DynamicProxy2 error: Mixin type TestClassProxy implements IProxyTargetAccessor which is a DynamicProxy infrastructure interface and you should never implement it yourself. Are you trying to mix in an existing proxy?"}
I know i'm using the proxy in the wrong way but as i haven't seen any concrete example in how to use a proxy with the windsor container it's kind of confusing.
I mean if i want to use the LoggingProxyGenerationHook which just tell the interceptors to first for methods that start with the word "check" then is this the correct way to do it or am i completely on the wrong path. I just went down the proxy way as it seems very powerfull and i would like to understand how to use these proxies for future programming efforts.
By using .Interceptors() you already are using Dynamic Proxy. When component has specified interceptors Windsor will create proxy for it, and use these interceptors for it. You can also use method .SelectedWith and .Proxy property to set other options you already know from DynamicProxy.
I just added a website about Windsor AOP to documentation wiki. There's not much there yet, but I (and Mauricio ;) ) will put there all the information you need. Take a look, and let us know if everything is clear, and if something is missing.
As we all know, when we derive a class and use polymorphism, someone, somewhere needs to know what class to instanciate. We can use factories, a big switch statement, if-else-if, etc. I just learnt from Bill K this is called Dependency Injection.
My Question: Is it good practice to use reflection and attributes as the dependency injection mechanism? That way, the list gets populated dynamically as we add new types.
Here is an example. Please no comment about how loading images can be done other ways, we know.
Suppose we have the following IImageFileFormat interface:
public interface IImageFileFormat
{
string[] SupportedFormats { get; };
Image Load(string fileName);
void Save(Image image, string fileName);
}
Different classes will implement this interface:
[FileFormat]
public class BmpFileFormat : IImageFileFormat { ... }
[FileFormat]
public class JpegFileFormat : IImageFileFormat { ... }
When a file needs to be loaded or saved, a manager needs to iterate through all known loader and call the Load()/Save() from the appropriate instance depending on their SupportedExtensions.
class ImageLoader
{
public Image Load(string fileName)
{
return FindFormat(fileName).Load(fileName);
}
public void Save(Image image, string fileName)
{
FindFormat(fileName).Save(image, fileName);
}
IImageFileFormat FindFormat(string fileName)
{
string extension = Path.GetExtension(fileName);
return formats.First(f => f.SupportedExtensions.Contains(extension));
}
private List<IImageFileFormat> formats;
}
I guess the important point here is whether the list of available loader (formats) should be populated by hand or using reflection.
By hand:
public ImageLoader()
{
formats = new List<IImageFileFormat>();
formats.Add(new BmpFileFormat());
formats.Add(new JpegFileFormat());
}
By reflection:
public ImageLoader()
{
formats = new List<IImageFileFormat>();
foreach(Type type in Assembly.GetExecutingAssembly().GetTypes())
{
if(type.GetCustomAttributes(typeof(FileFormatAttribute), false).Length > 0)
{
formats.Add(Activator.CreateInstance(type))
}
}
}
I sometimes use the later and it never occured to me that it could be a very bad idea. Yes, adding new classes is easy, but the mechanic registering those same classes is harder to grasp and therefore maintain than a simple coded-by-hand list.
Please discuss.
My personal preference is neither - when there is a mapping of classes to some arbitrary string, a configuration file is the place to do it IMHO. This way, you never need to modify the code - especially if you use a dynamic loading mechanism to add new dynamic libraries.
In general, I always prefer some method that allows me to write code once as much as possible - both your methods require altering already-written/built/deployed code (since your reflection route makes no provision for adding file format loaders in new DLLs).
Edit by Coincoin:
Reflection approach could be effectively combined with configuration files to locate the implmentations to be injected.
The type could be declared explicitely in the config file using canonical names, similar to MSBuild <UsingTask>
The config could locate the assemblies, but then we have to inject all matching types, ala Microsoft Visual Studio Packages.
Any other mechanism to match a value or set of condition to the needed type.
My vote is that the reflection method is nicer. With that method, adding a new file format only modifies one part of the code - the place where you define the class to handle the file format. Without reflection, you'll have to remember to modify the other class, the ImageLoader, as well
Isn't this pretty much what the Dependency Injection pattern is all about?
If you can isolate the dependencies then the mechanics will almost certainly be reflection based, but it will be configuration file driven so the messiness of the reflection can be pretty well encapsulated and isolated.
I believe with DI you simply say I need an object of type <interface> with some other parameters, and the DI system returns an object to you that satisfies your conditions.
This goes together with IoC (Inversion of Control) where the object being supplied may need something else, so that other thing is automatically created and installed into your object (being created by DI) before it's returned to the user.
I know this borders on the "no comment about loading images other ways", but why not just flip your dependencies -- rather than have ImageLoader depend on ImageFileFormats, have each IImageFileFormat depend on an ImageLoader? You'll gain a few things out of this:
Each time you add a new IImageFileFormat, you won't need to make any changes anywhere else (and you won't have to use reflection, either)
If you take it one step further and abstract ImageLoader, you can mock it in Unit Tests, making testing the concrete implementations of each IImageFileFormat that much easier
In vb.net, if all the image loaders will be in the same assembly, one could use partial classes and events to achieve the desired effect (have a class whose purpose is to fire an event when the image loaders should register themselves; each file containing image loaders can have use a "partial class" to add another event handler to that class); C# doesn't have a direct equivalent to vb.net's WithEvents syntax, but I suspect partial classes are a limited mechanism for achieving the same thing.
Method chaining is the only way I know to build fluent interfaces.
Here's an example in C#:
John john = new JohnBuilder()
.AddSmartCode("c#")
.WithfluentInterface("Please")
.ButHow("Dunno");
Assert.IsNotNull(john);
[Test]
public void Should_Assign_Due_Date_With_7DayTermsVia_Invoice_Builder()
{
DateTime now = DateTime.Now;
IInvoice invoice = new InvoiceBuilder()
.IssuedOn(now)
.WithInvoiceNumber(40)
.WithPaymentTerms(PaymentTerms.SevenDays)
.Generate();
Assert.IsTrue(invoice.DateDue == now.AddDays(7));
}
So how do others create fluent interfaces. How do you create it? What language/platform/technology is needed?
The core idea behind building a fluent interface is one of readability - someone reading the code should be able to understand what is being achieved without having to dig into the implementation to clarify details.
In modern OO languages such as C#, VB.NET and Java, method chaining is one way that this is achieved, but it's not the only technique - two others are factory classes and named parameters.
Note also that these techniques are not mutually exclusive - the goal is to maximize readabilty of the code, not purity of approach.
Method Chaining
The key insight behind method chaining is to never have a method that returns void, but to always return some object, or more often, some interface, that allows for further calls to be made.
You don't need to necessarily return the same object on which the method was called - that is, you don't always need to "return this;".
One useful design technique is to create an inner class - I always suffix these with "Expression" - that exposes the fluent API, allowing for configuration of another class.
This has two advantages - it keeps the fluent API in one place, isolated from the main functionality of the class, and (because it's an inner class) it can tinker with the innards of the main class in ways that other classes cannot.
You may want to use a series of interfaces, to control which methods are available to the developer at a given point in time.
Factory Classes
Sometimes you want to build up a series of related objects - examples include the NHibernate Criteria API, Rhino.Mocks expectation constraints and NUnit 2.4's new syntax.
In both of these cases, you have the actual objects you are storing, but to make them easier to create there are factory classes providing static methods to manufacture the instances you require.
For example, in NUnit 2.4 you can write:
Assert.That( result, Is.EqualTo(4));
The "Is" class is a static class full of factory methods that create constraints for evaluation by NUnit.
In fact, to allow for rounding errors and other imprecision of floating point numbers, you can specify a precision for the test:
Assert.That( result, Is.EqualTo(4.0).Within(0.01));
(Advance apologies - my syntax may be off.)
Named Parameters
In languages that support them (including Smalltalk, and C# 4.0) named parameters provide a way to include additional "syntax" in a method call, improving readability.
Consider a hypothetical Save() method that takes a file name, and permissions to apply to the file after saving:
myDocument.Save("sampleFile.txt", FilePermissions.ReadOnly);
with named parameters, this method could look like this:
myDocument.Save(file:"SampleFile.txt", permissions:FilePermissions.ReadOnly);
or, more fluently:
myDocument.Save(toFile:"SampleFile.txt", withPermissions:FilePermissions.ReadOnly);
You can create a fluent interface in any version of .NET or any other language that is Object Oriented. All you need to do is create an object whose methods always return the object itself.
For example in C#:
public class JohnBuilder
{
public JohnBuilder AddSmartCode(string s)
{
// do something
return this;
}
public JohnBuilder WithfluentInterface(string s)
{
// do something
return this;
}
public JohnBuilder ButHow(string s)
{
// do something
return this;
}
}
Usage:
John = new JohnBuilder()
.AddSmartCode("c#")
.WithfluentInterface("Please")
.ButHow("Dunno");
AFAIK, the term fluent interface does not specify a specific technology or framework, but rather a design pattern. Wikipedia does have an extensive example of fluent interfaces in C♯.
In a simple setter method, you do not return void but this. That way, you can chain all of the statements on that object which behave like that. Here is a quick example based on your original question:
public class JohnBuilder
{
private IList<string> languages = new List<string>();
private IList<string> fluentInterfaces = new List<string>();
private string butHow = string.Empty;
public JohnBuilder AddSmartCode(string language)
{
this.languages.Add(language);
return this;
}
public JohnBuilder WithFluentInterface(string fluentInterface)
{
this.fluentInterfaces.Add(fluentInterface);
return this;
}
public JohnBuilder ButHow(string butHow)
{
this.butHow = butHow;
return this;
}
}
public static class MyProgram
{
public static void Main(string[] args)
{
JohnBuilder johnBuilder = new JohnBuilder().AddSmartCode("c#").WithFluentInterface("Please").ButHow("Dunno");
}
}
Sometime ago I had the same doubts you are having now. I've done some research and now I'm writing a series of blog posts about techinics of designing a fluent interface.
Check it out at:
Guidelines to Fluent Interface design in C# part 1
I have a section there about Chaining X Nesting that can be interesting to you.
In the following posts I will talk about it in a deeper way.
Best regards,
André Vianna
Fluent interface is achieved in object oriented programming by always returning from your methods the same interface that contains the method. Consequently you can achieve this effect in java, javascript and your other favorite object oriented languages, regardless of version.
I have found this technique easiest to accomplish through the use of interfaces:
public interface IFoo
{
IFoo SetBar(string s);
IFoo DoStuff();
IFoo SetColor(Color c);
}
In this way, any concrete class that implements the interface, gets the fluent method chaining capabilities. FWIW.. I wrote the above code in C# 1.1
You will find this technique littered throughout the jQuery API
A couple of things come to mind that are possible in .Net 3.5/C# 3.0:
If an object doesn't implement a fluent interface, you could use Extension Methods to chain your calls.
You might be able to use the object initialization to simulate fluent, but this only works at instantiation time and would only work for single argument methods (where the property is only a setter). This seems hackish to me, but the there it is.
Personally, I don't see anything wrong with using function chaining if you are implementing a builder object. If the builder object has chaining methods, it keeps the object you are creating clean. Just a thought.
This is how I've built my so called fluent interfaces or my only forary into it
Tokenizer<Bid> tkn = new Tokenizer<Bid>();
tkn.Add(Token.LambdaToken<Bid>("<YourFullName>", b => Util.CurrentUser.FullName))
.Add(Token.LambdaToken<Bid>("<WalkthroughDate>",
b => b.WalkThroughDate.ToShortDateString()))
.Add(Token.LambdaToken<Bid>("<ContactFullName>", b => b.Contact.FullName))
.Cache("Bid")
.SetPattern(#"<\w+>");
My example required .net 3.5 but that's only cause of my lambda's. As Brad pointed out you can do this in any version of .net. Although I think lambda's make for more interesting possibilities such as this.
======
Some other good examples are nHibernate's Criteria API, there is also a fluent nhibernate extension for configuring nhibernate but I've never used it
Dynamic keyword in C# 4.0 will make it possible to write dynamic style builders. Take a look at following article about JSON object construction.