I'm trying to add Dagger to an existing web application and am running into a design problem.
Currently our Handlers are created in a dispatcher with something like
registerHandler('/login', new LoginHandler(), HttpMethod.POST)
Inside the login handler we might call a function like
Services.loginService.login('username', 'password');
I want to be able to inject the loginService into the handler, but am having trouble figuring out the best approach. There is a really long list of handlers in the dispatcher, and injecting them all as instance variables seems like a large addition of code.
Is there a solution to this type of problem?
Based on your comment about having different services to inject. I would propose next solution.
ServicesProvider:
#Module(injects = {LoginHandler.class, LogoutHandler.class})
public class ServicesProvider {
#Provides #Singleton public LoginService getLoginService() {
return new LoginService();
}
}
LoginHandler.java:
public class LoginHandler extends Handler {
#Inject LoginService loginService;
}
HttpNetwork.java
public class HttpNetwork extends Network {
private ObjectGraph objectGraph = ObjectGraph.create(new ServicesProvider());
public registerHandler(String path, Handler handler, String methodType) {
getObjectGraph().inject(handler);
}
}
There is one week point in this solution - you can't easily change ServiceProvider for test purpose (or any other kind of purpose). But if you inject it also (for example with another object graph or just through constructor) you can fix this situation.
Related
I am trying to find out how I can pass the StructrueMap container to a class that I wrote that inherits from another (MS-Class).
namespace TheNamespace
{
public class DatabaseIssuerNameRegistry : ValidatingIssuerNameRegistry
{
/* **This can't be done**
public DatabaseIssuerNameRegistry(IPortalTenantManager portalTenantManager)
{
_someField= portalTenantManager;
}*/
protected override bool IsThumbprintValid(string thumbprint, string issuer)
{
//How does it work ???????????
var portalTenantManager = container.GetInstance<IPortalTenantManager>();
//Do something with the portalTenantManager
}
}
I need portalTenantManager to be the Instance that I have defined in my container in the Global.asax.
My Global Assax has these things setup:
protected void Application_Start()
{
var container = new Container();
container.Configure(x =>
{ ....
....
x.For<IPortalTenantManager>().Use<PortalTenantManager>();
});
...
...
ControllerBuilder.Current.SetControllerFactory(new StructureMapControllerFactory(container));
...
GlobalConfiguration.Configuration.DependencyResolver = new StructureMapApiControllerFactory(container);
...
}
Edit:
Because of the comments of #NightOwl888 I'll explain a bit further what this class does. (Hopefully explaining so why my hands are tied)
My application is able to authenticate a user with Azure Active Directory and is Multi-tenant capable. In the authentication pipeline I have the possibility to store the validation endpoints in my database instead of the default way on the web.config file. See MSDN
and this, which actually is explaining exactly what I'm doing.
So I registered my class in the web.config under the Tag issuerNameRegistry. At some point of the validation pipeline my class is instantiated and the overriden method IsThumbprintValid is beeing called. The problem is that the class registered in issuerNameRegistry expects a parameterless constructor (there it is! the constrained construction!), therefore I cannot create a constructor that would solve my problem.
Thanks for your help
It turns out that this question has been asked before on MSDN, the answer of which was provided by Travis Spencer in 2 different posts.
it is typical in my experience to have a single container and use that service- or Web-side-wide. In the startup of the service or Web app, you can create the container, register the dependencies, new up an instance of your SecurityTokenServiceConfiguration class, resolve your dependencies, use it to punch out a SecurityTokenService object, and host it.
After the first beta, we really pushed for DI support. We got a little hook in beta 2. You can now create a custom SecurityTokenServiceConfiguration class that overrides the virtual CreateSecurityTokenService method. The implementation in Microsoft's SecurityTokenServiceConfiguration does Activator.CreateInstance; yours can do IoC. This can include the resolution of an IssuerNameRegistiry. Something like this perhaps:
RequestSecurityTokenResponse Issue(IClaimsPrincipal principal, RequestSecurityToken request)
{
SecurityTokenServiceConfiguration config = new MyGoodSecurityTokenServiceConfiguration();
SecurityTokenService sts = config.CreateSecurityTokenService();
RequestSecurityTokenResponse rstr = sts.Issue(principal, request);
return rstr;
}
public class MyGoodSecurityTokenServiceConfiguration : SecurityTokenServiceConfiguration
{
public override SecurityTokenService CreateSecurityTokenService()
{
IssuerNameRegistry = IoC.Resolve<IssuerNameRegistry>();
var sts = IoC.Reslove<SecurityTokenService>();
return sts;
}
}
Of course, this means that you need to create a static instance of your DI container so it is accessible to your SecurityTokenServiceConfiguration class. Personally, I don't like that idea because it makes your DI container accessible throughout the application, which can lead to abuse of the DI container as a service locator.
Ideally, there would be a way in any DI friendly framework to pass the container into an abstract factory in order to resolve service dependencies. However, since I am not familiar with WIF it is unclear whether that can be done - perhaps the class where the Issue method exists could have a constructor added? The trick is to keep walking up the chain until you find the first place in the framework where you can intervene and do all of your DI configuration there.
I'm using Ninject in an MVC project and am trying to implement Domain Events following Udi Dahan's pattern http://www.udidahan.com/2009/06/14/domain-events-salvation/
In the extract below, the "Container" is used to resolve all the event-handlers for the particular type of event that has been raised.
My question (& apologies if I am missing something basic) is how to do this with Ninject? In other words:
How does the "Container" get set in this static class?
Once I have a Container (Kernel?) what would be the Ninject syntax to resolve all the event handlers (which I'm assuming I would have to register before-hand in a Service Module)?
I keep reading in posts that only constructor injection should be used and everything recursively get resolved from that, and that accessing the Ninject Kernel is a no-no. So any advice on how to do this will be much appreciated.
Extract from the article
public static class DomainEvents
{
[ThreadStatic] //so that each thread has its own callbacks
private static List<Delegate> actions;
public static IContainer Container { get; set; } //as before
//Registers a callback for the given domain event
public static void Register<T>(Action<T> callback) where T : IDomainEvent
{
if (actions == null)
actions = new List<Delegate>();
actions.Add(callback);
}
//Clears callbacks passed to Register on the current thread
public static void ClearCallbacks ()
{
actions = null;
}
//Raises the given domain event
public static void Raise<T>(T args) where T : IDomainEvent
{
if (Container != null)
foreach(var handler in Container.ResolveAll<Handles<T>>())
handler.Handle(args);
if (actions != null)
foreach (var action in actions)
if (action is Action<T>)
((Action<T>)action)(args);
}
}
How does the "Container" get set in this static class?
You will have to set it during application startup:
DomainEvents.Container = kernel;
what would be the Ninject syntax to resolve all the event handlers:
You can do it like this, for instance:
Container.Get<IEnumerable<Handles<T>>>())
Udi's static DomainEvents class is an implementation of the Ambient Context anti-pattern (see DI PP&P chapter 5.3). In this case I would rather use dependency injection to inject an IDomainEvents abstraction into code that needs it, instead of letting code depend on a static instance.
The problem however is that your domain objects will need a dependency on the IDomainEvents and constructor injection is (probably) not possible. The trick is to use method injection in that case.
In other words, use constructor injection to inject the IDomainEvents into command handlers or services (or what ever you call your business logic that uses the methods on your domain objects) and pass that dependency into the domain object when calling a method that needs it (method injection).
I am writing a library that will provide a collection of public types to its consumers.
I want to make types from this library dependency injection friendly. This means that every class needs to have a constructor through which it is possible to specify every single dependency of the object being initialized. I also want the library to adhere to the convention over configuration principle. This means that if a consumer wants the default behavior, he may use a parameterless constructor and the object will somehow construct the dependencies for itself.
In example (C#):
public class Samurai {
private readonly IWeapon _weapon;
// consumers will use this constructor most of the time
public Samurai() {
_weapon = ??? // get an instance of the default weapon somehow
}
// consumers will use this constructor if they want to explicitly
// configure dependencies for this instance
public Samurai(IWeapon weapon) {
_weapon = weapon;
}
}
My first solution would be to use the service locator pattern.
The code would look like this:
...
public Samurai() {
_weapon = ServiceLocator.Instance.Get<IWeapon>();
}
...
I have a problem with this, though. Service locator has been flagged as an anti-pattern (link) and I completely agree with these arguments. On the other hand, Martin Fowler advocates use of the service locator pattern exactly in this situation (library projects) (link). I want to be careful and eliminate the possible necessity to rewrite the library after it shows up that service locator really was a bad idea.
So in conclusion - do you think that service locator is fine in this scenario? Should I solve my problem in a completely different way? Any thought is welcome...
If you want to make life easier for users who are not using a DI container, you can provide default instances via a dedicated Defaults class which has methods like this:
public virtual Samurai CreateDefaultSamurai()
{
return new Samurai(CreateDefaultWeapon());
}
public virtual IWeapon CreateDefaultWeapon()
{
return new Shuriken();
}
This way you don't need to pollute the classes themselves with default constructors, and your users aren't at risk of using those default constructors unintentionally.
There is an alternative, that is injecting a specific provider, let's say a WeaponProvider in your case into your class so it can do the lookup for you:
public interface IWeaponProvider
{
IWeapon GetWeapon();
}
public class Samurai
{
private readonly IWeapon _weapon;
public Samurai(IWeaponProvider provider)
{
_weapon = provider.GetWeapon();
}
}
Now you can provide a local default provider for a weapon:
public class DefaultWeaponProvider : IWeaponProvider
{
public IWeapon GetWeapon()
{
return new Sword();
}
}
And since this is a local default (as opposed to one from a different assembly, so it's not a "bastard injection"), you can use it as part of your Samurai class as well:
public class Samurai
{
private readonly IWeapon _weapon;
public Samurai() : this(new DefaultWeaponProvider())
{
}
public Samurai(IWeaponProvider provider)
{
_weapon = provider.GetWeapon();
}
}
I have used the following approach in my C# project. The goal was to achieve dependency injection (for unit / mock testing) whilst not crippling the implementation of the code for a "normal use case" (i.e. having a large amount of new()'s that are cascaded through the execution flow).
public sealed class QueueProcessor : IQueueProcessor
{
private IVbfInventory vbfInventory;
private IVbfRetryList vbfRetryList;
public QueueProcessor(IVbfInventory vbfInventory = null, IVbfRetryList vbfRetryList = null)
{
this.vbfInventory = vbfInventory ?? new VbfInventory();
this.vbfRetryList = vbfRetryList ?? new VbfRetryList();
}
}
This allows DI but also means any consumer doesn't have to worry about what the "default instance flow" should be.
Let's say we have class:
public class WithDependencies
{
public WithDependencies(IAmDependencyOne first, IAmDependencyTwo second)
// ...
}
Now the question. How do you create objects of WithDependencies class in an application?
I know there are many ways.
new WithDependencies(new DependencyOne(), new DependencyTwo());
new WithDependencies(IoC.Resolve(IDependencyOne), IoC.Resolve(IDependencyTwo());
// register IDependencyOne, IDependencyTwo implementations at app start
IoC.Resolve(WithDependencies);
// register IDependencyOne, IDependencyTwo implementations at app start
// isolate ourselves from concrete IoC Container
MyCustomWithDependenciesFactory.Create();
and so on...
What do you think is the way to do it?
Edit:
Because I don't get answers or I don't understand them I'll try to ask again. Let's say that on some event (button, timer, whatever) I need new object WithDependencies(). How do I create it? Assume IoC container is already configured.
It depends on the context, so it's impossible to provide a single answer. Conceptually you'd be doing something like this from the Composition Root:
var wd = new WithDependencies(new DependencyOne(), new DependencyTwo());
However, even in the absence of a DI Container, the above code isn't always unambiguously the correct answer. In some cases, you might want to share the same dependency among several consumers, like this:
var dep1 = new DependencyOne();
var wd = new WithDependencies(dep1, new DependencyTwo());
var another = AnotherWithDependencies(dep1, new DependencyThree());
In other cases, you might not want to share dependencies, in which case the first option is more correct.
This is just a small glimpse of an entire dimension of DI concerned with Lifetime Management. Many DI Containers can take care of that for you, which is one excellent argument to prefer a DI Container over Poor Man's DI.
Once you start using a DI Container, you should follow the Register Resolve Release pattern when resolving types, letting Auto-wiring take care of the actual composition:
var wd = container.Resolve<WithDependencies>();
The above example assumes that the container is already correctly configured.
If you need to create a dependency which has its own dependencies, you can either A) do it yourself, or B) ask something else to do it for you. Option A negates the benefits of dependency injection (decoupling, etc.), so I would say option B is a better starting point. Now, we have chosen to use the factory pattern, no matter whether it takes the form of a service locator (i.e. IoC.Resolve), a static factory, or an instance factory. The point is that we have delegated that responsibility to an external authority.
There are a number of trade-offs required for static accessors. (I went over them in another answer, so I won't repeat them here.) In order to avoid introducing a dependency on the infrastructure or the container, a solid option is to accept a factory for creating WithDependencies when we need an instance somewhere else:
public class NeedsWithDependencies
{
private readonly IWithDependenciesFactory _withDependenciesFactory;
public NeedsWithDependencies(IWithDependenciesFactory withDependenciesFactory)
{
_withDependenciesFactory = withDependenciesFactory;
}
public void Foo()
{
var withDependencies = _withDependenciesFactory.Create();
...Use the instance...
}
}
Next, we can create a container-specific implementation of the factory:
public class WithDependenciesFactory : IWithDependenciesFactory
{
private readonly IContainer _container;
public WithDependenciesFactory(IContainer container)
{
_container = container
}
public WithDependencies Create()
{
return _container.Resolve<WithDependencies>();
}
}
Now NeedsWithDependencies is completely isolated from any knowledge of how WithDependencies gets created; it also exposes all its dependencies in its constructor, instead of hiding dependencies on static accessors, making it easy to reuse and test.
Defining all those factories can get a little cumbersome, though. I like Autofac's factory relationship type, which will detect parameters of the form Func<TDependency> and automatically inject a function which serves the same purpose as the hand-coded factory above:
public class NeedsWithDependencies
{
private readonly Func<WithDependencies> _withDependenciesFactory;
public NeedsWithDependencies(Func<WithDependencies> withDependenciesFactory)
{
_withDependenciesFactory = withDependenciesFactory;
}
public void Foo()
{
var withDependencies = _withDependenciesFactory();
...Use the instance...
}
}
It also works great with runtime parameters:
public class NeedsWithDependencies
{
private readonly Func<int, WithDependencies> _withDependenciesFactory;
public NeedsWithDependencies(Func<int, WithDependencies> withDependenciesFactory)
{
_withDependenciesFactory = withDependenciesFactory;
}
public void Foo(int x)
{
var withDependencies = _withDependenciesFactory(x);
...Use the instance...
}
}
Sometimes I try to get rid of factories or at least not depend directly on them, so Dependency Injection (without factories) is useful of course.
Therefore I use Google Juice, cause its a small little framework using Java Annotations and you can quickly change your injections / dependencies. Just take a look at it:
http://code.google.com/p/google-guice/
We are trying to figure out how to setup Dependency Injection for situations where service classes can have different dependencies based on how they are used. In our specific case, we have a web app where 95% of the time the connection string is the same for the entire Request (this is a web application), but sometimes it can change.
For example, we might have 2 classes with the following dependencies (simplified version - service actually has 4 dependencies):
public LoginService (IUserRepository userRep)
{
}
public UserRepository (IContext dbContext)
{
}
In our IoC container, most of our dependencies are auto-wired except the Context for which I have something like this (not actual code, it's from memory ... this is StructureMap):
x.ForRequestedType().Use()
.WithCtorArg("connectionString").EqualTo(Session["ConnString"]);
For 95% of our web application, this works perfectly. However, we have some admin-type functions that must operate across thousands of databases (one per client). Basically, we'd want to do this:
public CreateUserList(IList<string> connStrings)
{
foreach (connString in connStrings)
{
//first create dependency graph using new connection string
????
//then call service method on new database
_loginService.GetReportDataForAllUsers();
}
}
My question is: How do we create that new dependency graph for each time through the loop, while maintaining something that can easily be tested?
To defer the creation of an object until runtime, you can use a factory:
public interface ILoginServiceFactory
{
ILoginService CreateLoginService(string connectionString);
}
Usage:
public void CreateUserList(IList<string> connStrings)
{
foreach(connString in connStrings)
{
var loginService = _loginServiceFactory.CreateLoginService(connString);
loginService.GetReportDataForAllUsers();
}
}
Within the loop, do:
container.With("connectionString").EqualTo(connString).GetInstance<ILoginService>()
where "connectionString" is the name of a string constructor parameter on the concrete implementation of ILoginService.
So most UserRepository methods use a single connection string obtained from session, but several methods need to operate against a list of connection strings?
You can solve this problem by promoting the connection string dependency from IContext to the repository and adding two additional dependencies - a context factory and a list of all the possible connections strings the repository might need to do its work:
public UserRepository(IContextFactory contextFactory,
string defaultConnectionString,
List<string> allConnectionStrings)
Then each of its methods can build as many IContext instances as they need:
// In UserRepository
public CreateUserList() {
foreach (string connString in allConnectionStrings) {
IContext context = contextFactory.CreateInstance(connString);
// Build the rest of the dependency graph, etc.
_loginService.GetReportDataForAllUsers();
}
}
public LoginUser() {
IContext context = contextFactory.CreateInstance(defaultConnectionString);
// Build the rest of the dependency graph, etc.
}
We ended up just creating a concrete context and injecting that, then changing creating a wrapper class that changed the context's connection string. Seemed to work fine.