Sorry for this basic question I am asking but can someone tell me if dagger dependency injection can work just with #module, #provides #Inject etc. That is without using #component annotation?
In spring DI, we use #componentScan to let spring know where to find beans. What is the equivalent of this in dagger?
You always need #Component. This is the entry point for dependencies from "outside".
When I say "outside" I mean the case where after the dependency graph inside the Component is created and now it is a time for some Activity to be Injected for example. So the component exposed dependencies for the "outside".
At the same time, a component can share its dependencies with other Subcomponents for example.
The Components depend on the Modules to know how to create dependencies.
Related
in my opinion, the get_it package is one of ways to dependency injection in flutter, is that so? I'm just not sure about it, please help me figure it out
Rather not. get_it describes itself as a Service Locator, as can be seen it its documentation. A service locator is a different pattern than Dependency Injection, although they can be used for the same purpose.
Both approaches aim towards abstracting object interfaces from their actual implementation, but do it in slightly different ways. In classical dependency injection (e.g. via constructor or interface injection), each object holds a reference to the objects that implements its specific behavior. With the service locator pattern, there is just one single object - getIt() in our case - that can be used to find implementations across the whole application.
A great and more extensive discussion of these appraoches can be found in this article by Martin Fowler.
I have been working with for a while but somehow never thought about this. Every aem project that I have worked on, has one similarity in their code structure. There is an interface for every service written.
My question is why do we need a interface for every service?
Can #reference or #inject not use the services without an interface?
Using interfaces is a good practice to decouple the user of a service from the implementation. In many cases you even want to have an API bundle so the user of the service does not need a maven dependency to the implementing bundle.
On the other hand you are not required to use interfaces. Especially when I wire components inside a bundle interfaces are often an unnecessary layer. In this case simply export the service directly with the class.
See here for an example:
#Component(service = DistributionMetricsService.class)
public class DistributionMetricsService {
...
}
and here for the client code:
#Reference
private DistributionMetricsService distributionMetricsService;
So the main difference is that you have to specify the service property if you want to export a component with its implementation class.
After migrating from PlayFramework version 2.4 to version 2.5, I need to inject dependencies.
I'm avoiding to use #Inject for constructor DI as in this the caller class of the class that has #Inject also needs to get the same dependencies injected to call the callee's constructor. This increases code complexity.
Please suggest any other way of DI which does not involve injecting to constructor.
MacWire looks like a great tool for constructor injection. It is macro-based and thus typesafe and non invasive. The README page has a very good documentation, definitely worth a look IMHO.
Let me ask two coupled questions that might boil down to one about good application design ;-)
What is the best practice for using event based communication in an e4 RCP application?
How can I write simple unit tests (using JUnit) for classes that send/receive events using dependency injection and IEventBroker ?
Let’s be more concrete: say I am developing an Eclipse e4 RCP application consisting of several plugins that need to communicate. For communication I want to use the event service provided by org.eclipse.e4.core.services.events.IEventBroker so my plugins stay loosely coupled. I use dependency injection to inject the event broker to a class that dispatches events:
#Inject static IEventBroker broker;
private void sendEvent() {
broker.post(MyEventConstants.SOME_EVENT, payload)
}
On the receiver side, I have a method like:
#Inject
#Optional
private void receiveEvent(#UIEventTopic(MyEventConstants.SOME_EVENT) Object payload)
Now the questions:
In order for IEventBroker to be successfully injected, my class needs access to the current IEclipseContext. Most of my classes using the event service are not referenced by the e4 application model, so I have to manually inject the context on instantiation using e.g. ContextInjectionFactory.inject(myEventSendingObject, context);
This approach works but I find myself passing around a lot of context to wherever I use the event service. Is this really the correct approach to event based communication across an E4 application?
how can I easily write JUnit tests for a class that uses the event service (either as a sender or receiver)? Obviously, none of the above annotations work in isolation since there is no context available. I understand everyone’s convinced that dependency injection simplifies testability. But does this also apply to injecting services like the IEventBroker?
This article describes creation of your own IEclipseContext to include the process of DI in tests. Not sure if this could resolve my 2nd issue but I also hesitate running all my tests as JUnit Plug-in tests as it appears impractible to fire up the PDE for each unit test. Maybe I just misunderstand the approach.
This article speaks about “simply mocking IEventBroker”. Yes, that would be great! Unfortunately, I couldn’t find any information on how this can be achieved.
All this makes me wonder whether I am still on the right track or if this is already a case of bad design? And if so, how would you go about redesigning? Move all event related actions to dedicated event sender/receiver classes or a dedicated plugin?
Actually, running a JUnit plug-in test is not that expensive. You can configure the launch configuration to run in headless mode so the only thing loaded is a lightweight PDE without workbench. The same happens when you run a headless build with for example Tycho. Surefire launches your test-bundle as headless plug-in test by default.
The advantage over isolated unit tests is that you can access your plug-in's resources and, most importantly, use dependency injection. If you want to mock an injected object you have to run a plug-in test so you can use InjectorFactory.
This is how you would go about mocking the event service: IEventBroker is an interface, so the only thing you need to do is writing a mock implementation for it
public class IEventBrokerMock implements IEventBroker {
#Override
// Implemented Methods
}
In your test method you would have something like
InjectorFactory.getDefault().addBinding(IEventBroker.class).implementedBy(IEventBrokerMock.class);
ClassUnderTest myObject = InjectorFactory.getDefault().make(ClassUnderTest.class, null);
If you want to work with a context the test method would instead contain
IEclipseContext context = EclipseContextFactory.create();
context.set(IEventBroker.class, new IEventBrokerMock());
ClassUnderTest myObject = ContextInjectionFactory.make(ClassUnderTest.class, context);
If you run this as JUnit plug-in test your object will have the mocked event service injected.
for testing, instead of DI, i use "eventBroker=new org.eclipse.e4.ui.services.internal.events.EventBroker();" to get a eventbroker object to use, it works ok
I was looking for some help on CDI APIs. I'm sorry if my question looks naive, I tried looking on net for some high level description on CDI APIs, but couldn't get it right.
The javadoc says:
Contexts and Dependency Injection (CDI) defines a set of complementary services that help improve the structure of application code.
My question is in EJB 3.X we already have Annotations for helping with DI and injecting resources like PersistenceContext and other kind of resources. So where exactly the CDI APIs will be helpful? In plain web-app/standalone Java programs using J2SE?
EJB 3.0 comes with dependency injection on resource- and EJB-level - which is pretty cool already :-)
What CDI does (and which is even cooler) - it lowers the barrier to dependency injection to so-called "managed beans" (JSR 316) - which (among others) defines the minimal set of preconditions a class needs to benefit from dependency injection. Just slightly simplifying, one can say that all classes in a CDI project are managed beans and therefore are eligable for DI.
To summarize what CDI brings over EJB 3.0 in terms of DI:
you don't need EJBs anymore, CDI basically works with POJOs. That's truly lightweight, because it allows you to use EJBs when you need EJB, not when you need DI.
DI turns stateful - different dependencies live in different scopes - something EJB 3.0 completely fails to deliver.
you can benefit from a typesafe and loosely coupled interceptor mechanism
you can benefit from a typesafe and loosely coupled mechanism
Have a look at the first chapter here, and you'll get the idea :-)
DI in Java EE5 allows you inject only resources like JDBC DataSource, JPA EntityManager, UserTransaction, Web Services, EJBs etc. All this resources was managed by container.
With EE6 and with CDI in particular you aren't restricted to inject only resources - you can inject everything (every bean). Look at annotations which come with CDI specification: #Inject, #Named, #Scope, #Singleton etc.
CDI give you features like events, decorators etc.
Look at this tutorial, it should help you to understand CDI: http://java.dzone.com/articles/cdi-di-p2