I am using Jackson 2.3 in a JAX-RS application deployed (currently) to GlassFish 4.1.
I would like to be relieved of the burden of having to know of all the mixin classes ahead of time that might show up in my application as it matures. I'd like for my ObjectMapper to be able to discover them as things roll along.
I've written a ContextResolver<ObjectMapper> that, in its getContext(Class) method, does some configuration lookup to figure out what a mixin class might be for a given target class (following some naming conventions). If it finds such a class, then it loads it and adds it to the ObjectMapper using addMixInAnnotations().
It occurs to me that this on-demand discovery of mixin annotations probably doesn't work well with a shared instance of an ObjectMapper (perhaps changing configuration at this stage in the game is too late, or perhaps there are thread issues) and that there must be a better way. What is it?
Related
I have been playing around the last couple of days with different solutions for mapping DTO's to entities for a VS2013, EF6, WCF Service App project.
It is a fairly large project that is currently undergoing a major refactoring to bring the legacy code under test (as well as port the ORM from OpenAccess to EF6).
To be honest I had never used AutoMapper before but what I saw I really liked so I set out to test it out in a demo app and to be honest I am a bit ashamed that I have been unable to achieve a working solution after hours of tinkering and Googling. Here is a breakdown of the project:
WCF Service Application template based project (.svc file w/code behind).
Using Unity 3.x for my IoC container and thus creating my own ServiceHostFactory inheriting from UnityServiceHostFactory.
Using current AutoMapper nuget package.
DTO's and DAL are in two separate libraries as expected, both of which are referenced by the service app project.
My goal is simple (I think): Wire up and create all of my maps in my composition root and inject the necessary objects (using my DI container) into the class that has domain knowledge of the DTO's and a reference to my DAL library. Anyone that needs a transformation would therefore only need to reference the transformation library.
The problem: Well, there are a couple of them...
1) I cannot find a working example of AutoMapper in Unity anywhere. The code snippet that is referenced many times across the web for registering AutoMapper in Unity (see below) references a Configuration class that doesn't seem to exist anymore and I cannot find any documentation on its deprecation:
container.RegisterType<AutoMapper.Configuration, AutoMapper.Configuration>(new PerThreadLifetimeManager(), new InjectionConstructor(typeof(ITypeMapFactory),
AutoMapper.Mappers.MapperRegistry.AllMappers())).RegisterType<ITypeMapFactory,
TypeMapFactoy>().RegisterType<IConfiguration, AutoMapper.Configuration>().RegisterType<IConfigurationProvider,
AutoMapper.Configuration>().RegisterType<IMappingEngine, MappingEngine>();
2) Where to create the maps themselves... I would assuming that I could perform this operation right in my ServiceHostFactory but is that the correct place? There is a Bootstrapper project out there but I have not gone down that road (yet) and would like to avoid it if possible.
3) Other than the obviously necessary reference to AutoMapper in the DTO lib, what would I be injecting into the instantition, the configuration object (assuming IConfiguration or IConfigurationProvider) and which class I am injecting into the constructor of the WCF service to gain access to the necessary object.
I know #3 is a little vague but since I cannot get AutoMapper bound in my Unity container, I cannot test/trial/error to figure out the other issues.
Any pointers would be greatly appreciated.
UPDATE
So I now have a working solution that is testing correctly but would still like to get confirmation that I am following any established best practices.
First off, the Unity container registration for AutoMapper (as of 11/13/2013) v3.x looks like this:
container
.RegisterType<ConfigurationStore, ConfigurationStore>
(
new ContainerControlledLifetimeManager()
, new InjectionConstructor(typeof(ITypeMapFactory)
, MapperRegistry.AllMappers())
)
.RegisterType<IConfigurationProvider, ConfigurationStore>()
.RegisterType<IConfiguration, ConfigurationStore>()
.RegisterType<IMappingEngine, MappingEngine>()
.RegisterType<ITypeMapFactory, TypeMapFactory>();
Right after all of my container registrations, I created and am calling a RegisterMaps() method inside of ConfigureContainer(). I created a test mapping that does both an auto mapping for like named properties as well as a custom mapping. I did this in my demo app for two reasons primarily:
I don't yet know AutoMapper in a WCF app hosted in IIS and injected with Unity well enough to fully understand its behavior. I do not seem to have to inject any kind of configuration object into my library that does the transformations and I am still reading through the source to understand its implementation.
As I understand it, there is a caching mechanism at play here and that if a mapping is not found in cache that it will create it on the fly. While this is great in theory, the only way I could then test my mappings that were occurring in my composition root was to do some sort of custom mapping and then call Mapper.Map in the library that performs mapping and returns the DTO.
All of that blathering aside, here is what I was able to accomplish.
WCF Service App (composition root) injects all of the necessary objects including my DtoConversionMapper instance.
The project is made up of the WCF Service App (comp root), DtoLib, DalLib, ContractsLib (interfaces).
In my ServiceFactoryHost I am able to create mappings, including custom mappings (i.e. map unlike named properties between my DTO and EF 6 entity).
The DtoConversionMapper class lives in the DtoLib library and looks like this: IExampleDto GetExampleDto(ExampleEntity entity);
Any library with a reference to the DtoLib can convert back and forth, including the Service App where the vast majority of these calls will take place.
Any guiding advice would be greatly appreciated but I do have a working demo now that I can test things out with while I work through this large refactoring.
Final Update
I changed the demo project just a little by adding another library (MappingLib) and moved all of my DTO conversions and mappings to it in a static method. While I still call the static method in my composition root after the Unity container is initialized, this gives me the added flexibility of being able to call that same map creation method in my NUnit unit test libraries, effectively eliminating any duplication of code surrounding auto mapper and makes it very testable.
I am currently experiencing a problem in my RCP application and wanted to ask, if someone stumbled over the same problem and can give me some valuable hints:
My RCP application allows plugins to provide implementations of a specific abstract class of my model (singleton) to extend my model during runtime via the update manager. I instantiate these classes via
extensionPointImplementation.createExecutableExtension(..)
after parsing the Eclipse registry. I can serialize the created instances using the default Java serialization API.
Now to the problem: The plugin trying to deserialize the objects cannot find the class implementations of the model extensions due to the fact, that there is no plugin dependency between the plugins. Nevertheless, it is not possible for me to create such a dependency which would make the idea of extending the model during runtime obsolete.
Is it possible to solve this problem by using the default Java serialization API or do I have to implement my own serialization (which parses the Eclipse registry and creates the instances via the line shown above if all necessary plugins are available, otherwise throw an exception) which might be based on the default Java serialization API (if possible I do not want to create the serialization completely by myself)?
Thanks.
You need to define a so called buddy policy.
In the bundle trying to instantiate the class add
Eclipse-BuddyPolicy: registered
to the manifest.mf.
In the bundle providing the class add
Eclipse-RegisterBuddy: <symbolic name of the bundle instantiating the class>
to the manifest.mf.
Simple question... I want to use Ninject Interceptors to take care of my NFRs, however alot of my classes require arguments in the constructors. I read that they are looking at allowing constructors with arguments but currently I get an error:
Can not instantiate proxy of class: myClass
Could not find a parameterless constructor.
Parameter name: constructorArguments
I am using version 2.2.1 I think, noticed there is a tagged 2.3 version on the extensions site, but will any of this solve my problems? if not is there any way around this?
2.3 adds support for Interface proxies. This means it will solve the problem for all types that are resolved by interface. It's also planned to add support for classes without default constructor.
But be aware that 2.3 is work in progress. While there aren't any known new problems it is not tested yet against real applications as 2.2 is and interface changes of new stuff can still change. Also InRequestScope support for XML and Conventions has temporarily been disabled.
I have been a web developer for some time now using ASP.NET and C#, I want to try and increase my skills by using best practices.
I have a website. I want to load the settings once off, and just reference it where ever I need it. So I did some research and 50% of the developers seem to be using the singleton pattern to do this. And the other 50% of the developers are ant-singleton. They all hate singletons. They recommend dependency injection.
Why are singletons bad? What is best practice to load websites settings? Should they be loaded only once and referenced where needed? How would I go about doing this with dependency injection (I am new at this)? Are there any samples that someone could recommend for my scenario? And I also would like to see some unit test code for this (for my scenario).
Thanks
Brendan
Generally, I avoid singletons because they make it harder to unit test your application. Singletons are hard to mock up for unit tests precisely because of their nature -- you always get the same one, not one you can configure easily for a unit test. Configuration data -- strongly-typed configuration data, anyway -- is one exception I make, though. Typically configuration data is relatively static anyway and the alternative involves writing a fair amount of code to avoid the static classes the framework provides to access the web.config anyway.
There are a couple of different ways to use it that will still allow you to unit test you application. One way (maybe both ways, if your singleton doesn't lazily read the app.cofnig) is to have a default app.config file in your unit test project providing the defaults required for your tests. You can use reflection to replace any specific values as needed in your unit tests. Typically, I'd configure a private method that allows the private singleton instance to be deleted in test set up if I do make changes for particular tests.
Another way is to not actually use the singleton directly, but create an interface for it that the singleton class implements. You can use hand injection of the interface, defaulting to the singleton instance if the supplied value is null. This allows you to create a mock instance that you can pass to the class under test for your tests, but in your real code use the singleton instance. Essentially, every class that needs it maintains a private reference to the singleton instance and uses it. I like this way a little better, but since the singleton will be created you may still need the default app.config file, unless all of the values are lazily loaded.
public class Foo
{
private IAppConfiguration Configuration { get; set; }
public Foo() : this(null) { }
public Foo( IAppConfiguration config )
{
this.Configuration = config ?? AppConfiguration.Instance;
}
public void Bar()
{
var value = this.Config.SomeMaximum;
...
}
}
There's a good discussion of singleton patterns, and coding examples here... http://en.wikipedia.org/wiki/Singleton_pattern See also here... http://en.wikipedia.org/wiki/Dependency_injection
For some reason, singletons seem to divide programmers into strong pro- and anti- camps. Whatever the merits of the approach, if your colleagues are against it, it's probably best not to use one. If you're on your own, try it and see.
Design Patterns can be amazing things. Unfortunately, the singleton seems to stick out like a sore thumb and in many cases can be considered an anti-pattern (it promotes bad practices). Bizarely, the majority of developers will only know one design pattern, and that is the singleton.
Ideally your settings should be a member variable in a high level location, for example the application object which owns the webpages you are spawning. The pages can then ask the app for the settings, or the application can pass the settings as pages are constructed.
One way to approach this problem, is to flog it off as a DAL problem.
Whatever class / web page, etc. needs to use config settings should declare a dependency on an IConfigSettingsService (factory/repository/whatever-you-like-to-call-them).
private IConfigSettingsService _configSettingsService;
public WebPage(IConfigSettingsService configSettingsService)
{
_configSettingsService = configSettingsService;
}
So your class would get settings like this:
ConfigSettings _configSettings = _configSettingsService.GetTheOnlySettings();
the ConfigSettingsService implementation would have a dependency which is Dal class. How would that Dal populate the ConfigSettings object? Who cares.
Maybe it would populate a ConfigSettings from a database or .config xml file, every time.
Maybe it do that the first time but then populate a static _configSettings for subsequent calls.
Maybe it would get the settings from Redis. If something indicates the settings have changed then the dal, or something external, can update Redis. (This approach will be useful if you have more than one app using the settings.
Whatever it does, your only dependency is a non-singleton service interface. That is very easy to mock. In your tests you can have it return a ConfigSettings with whatever you want in it).
In reality it would more likely be MyPageBase which has the IConfigSettingsService dependency, but it could just as easily be a web service, windows service, MVC somewhatsit, or all of the above.
We've begun using Dependency Injection recently, and we've chosen Ninject 2 (for now) as our IoC Container. As I refactor our solution to incorporate DI principles, I've run into something that bugs me a little, and I'm wondering if there's an easy way to get around it.
For our data layer, we have a whole bunch of data-access classes that inherit the same generic class (EntityMapper). While in the past we've always constructed a new instance of these classes when we needed one, they really could all be changed into Singletons. We've overridden ObjectDataSource to use Ninject to instantiate its data-access object, so any time we create an ObjectDataSource that points to one of our EntityMapper classes, Ninject will use its default self-binding strategy to inject the necessary dependencies. Since there are so many of these classes, we'd rather not have to create an explicit binding for each of our EntityMapper classes, and we'd rather not have to put a special attribute on every one of them either. However, I would like to be able to instruct Ninject to make any instance of EntityMapper into a singleton class. Something like this:
Bind(t => typeof(IEntityMapper).IsAssignableFrom(t)).InSingletonScope();
Is there any way to do this?
You can use the conventions extension to do the following
var kernel = new StandardKernel();
kernel.Scan( x=>
{
x.FromAssemblyContaining<MyEntityMapper>();
x.FromCallingAssembly();
x.WhereTypeInheritsFrom<IEntityMapper>();
x.InSingletonScope();
} );