This is a Canonical Question because this is a common error with Dagger 2.
If your question was flagged as a duplicate please read this post carefully and make sure to understand what this error means and why it occured. If this post does not work for you make sure to include where and how you provide the mentioned classes and include the full error message in your question like the one here.
I tried to use a dependency with Dagger 2, but I receive the following error when I try to compile my project:
error: com.example.MyDependency cannot be provided without an #Inject constructor or from an #Provides-annotated method.
com.example.MyDependency is provided at
com.example.MyComponent.myDependency()
What does this mean and how can I fix it?
I have a component and tried to provide a dependency. My basic setup looks like this:
// this is the dependency I try to use
class MyDependency {}
#Component
interface MyComponent {
// I want to make it accessible to be used with my component
MyDependency myDependency();
}
tl;dr You forgot to either add an #Inject to your constructor so that Dagger can use Constructor Injection to provide the object, or you need some method in one of your Modules that creates or binds the object.
What's going on?
Have a good look at the error message: It states that you try to request a dependency but Dagger has no way to provide or create it. It simply does not know how to, because it cannot be provided without an #Inject constructor or from an #Provides-annotated method.
A close look at the error message shows the class (a) that you are trying to provide and the component (b) that needs it.
com.example.MyDependency (a) is provided at
com.example.MyComponent.myDependency() (b)
You have to make sure that (b) can create or provide (a) to fix your issue.
It looks a bit more complex if you tried to inject your dependency somewhere else, but you can still see the full stack of events—in this case a constructor injection missing a dependency. The class (a) that you are trying to provide and the location (b) where Dagger tried injecting it. It also tells you where that dependent class was created (c) and again the component (d) that failed providing (a).
com.example.MyDependency cannot be provided without an #Inject constructor or from an #Provides-annotated method.
com.example.MyDependency (a) is injected at
com.example.DependentClass.(dependency) (b)
com.example.DependentClass is provided at (c)
com.example.MyComponent.myDependency() (d)
The same applies here: Make sure that (d) knows how to provide (a) and you're good to go.
How do I fix this?
Have a look at the error as shown above. Make sure you understand where it occured and what you are trying to inject. Then tell Dagger how to provide your object.
an #Inject constructor
As the error states, you try to use MyDependency but MyComponent does not know how to do that. If we have a look at the example it becomes clear why:
class MyDependency {}
The class has no #Inject annotated constructor! And there is no other module in the component, so there is nothing Dagger could do.
If you want to use constructor injection you can just add an #Inject annotated constructor and are done. Dagger will see this constructor and know how to create your class.
class MyDependency {
#Inject
MyDependency() { /**/ }
}
That is all you have to do when you can make use of constructor injection.
from an #Provides-annotated method
The error message states a second option, which allows you to provide an object if you don't want—or can't—use constructor injection. You can also add a #Provides annotated method to a module and add this module to your component.
#Module
class MyModule {
#Provides
MyDependency provideMyDependency() {
return new MyDependency();
}
}
#Component(modules = MyModule.class)
interface MyComponent {
MyDependency myDependency();
}
This way Dagger can use your module to create and provide your dependency. It is a little bit more boilerplate than using Constructor Injection, but you will have to use Modules for everything that needs further setup or that does not have an annotated constructor, e.g. third party libraries like Retrofit, OkHttp, or Gson.
There are also other ways to provide a dependency from a component. A #SubComponent has access to its parents dependencies, and a component dependency can expose some of its dependencies to its dependent components. But at some point everything Dagger provides needs to either have an #Inject constructor or a Module providing it.
But I did add MyDependency!
Pay close attention to the details. You probably are using an interface when you are only providing the implementation, or try to use a parent class when Dagger only knows about the subclass.
Maybe you added a custom #Qualifier or used #Named("typeA") with it. To Dagger this is a completely different object! Double check that you actually provide and request the same dependency.
Read the error and make sure that you either have an #Inject annotated constructor, a module that has a #Provides method that provides that type, or a parent component that does.
What if I want to provide an implementation for my interface?
A simple example like the following shows how one class extends another:
class MyDependency extends MyBaseDependency {
#Inject MyDependency() { super(); }
}
This will inform Dagger about MyDependency, but not about MyBaseDependency.
If you have one class implementing an interface or extending a super class you have to declare that. If you provide MyDependency this does not mean that Dagger can provide MyBaseDependency. You can use #Binds to tell Dagger about your implementation and provide it when the super class is required.
#Module
interface MyModule {
#Binds
MyBaseDependency provideMyBaseDependency(MyDependency implementation);
}
Related
I am having problems injecting a generic type interface. Not sure how to do this or google it since I don't know the exact terms to search for. Sorry if i'm completely wrong just getting started with dagger.
Basically I have a use case class
public class LoadConversations<C extends IConversation>
extends UseCase<List<C>, LoadConversations.Type> {
private final IConversationRepository<C> messageRepository;
#Inject LoadConversations(#NonNull IConversationRepository<C> messageRepository) {
this.messageRepository = messageRepository;
}
....
public enum Type {
ALL, NEWER, OLDER
}
}
With IConversationRepository being an interface.
public interface IConversationRepository<C extends IConversation> {
Observable<List<C>> conversations(LoadConversations.Type params);
}
IConversation being a blank interface and ConversationModule where i provide the IConversationRepository.
Im having problems injecting with the following code. Am i missing something or doing something completey wrong. Thanks in advance.
Trying to provide as follows:
#Provides IConversationRepository<Conversation> provideConversationRepository(
ConversationRepository conversationRepository) {
return conversationRepository;
}
And I'm trying to inject this to my presenter as
private final LoadConversations<Conversation> loadConversations;
#Inject public ConversationListPresenter(LoadConversations<Conversation> loadConversations) {
this.loadConversations = loadConversations;
}
Implementation of ConversationRepository
public class ConversationRepository implements IConversationRepository<Conversation> {
#Override public Observable<List<Conversation>> conversations(LoadConversations.Type params) {
....
}
}
Error Log:
Error:(15, 10) error: com.rbttalk.android.data.repository.ConversationRepository cannot be provided without an #Inject constructor or from an #Provides-annotated method.
com.rbttalk.android.data.repository.ConversationRepository is injected at
com.rbttalk.android.di.module.sub_modules.ConversationModule.provideConversationRepository(conversationRepository)
com.rbttalk.android.domain.repository.IConversationRepository<com.rbttalk.android.domain.models.Conversation> is injected at
com.rbttalk.android.domain.usecase.conversation.LoadConversations.<init>(arg0, …)
com.rbttalk.android.domain.usecase.conversation.LoadConversations<com.rbttalk.android.domain.models.Conversation> is injected at
com.rbttalk.android.ui.main.conversation.ConversationListPresenter.<init>(loadConversations)
com.rbttalk.android.ui.main.conversation.ConversationListPresenter is injected at
com.rbttalk.android.ui.main.conversation.ConversationListFragment.userListPresenter
com.rbttalk.android.ui.main.conversation.ConversationListFragment is injected at
com.rbttalk.android.di.component.ConversationComponent.inject(conversationListFragment)
You're very close! The error message says it all:
com.rbttalk.android.data.repository.ConversationRepository cannot be provided without an #Inject constructor or from an #Provides-annotated method.
Note that this is not IConversationRepository; you've provided a binding for that with your #Provides method (which you can eventually consider converting to a #Binds method). However, that #Provides method has a parameter, ConversationRepository, which effectively asks Dagger to create an instance of that concrete ConversationRepository type for you. You've made that binding correctly, but now Dagger needs to instantiate ConversationRepository for you, and it simply doesn't know how.
You'll need to create an #Inject-annotated constructor for ConversationRepository using the annotation type javax.inject.Inject, even if it just looks like this:
#Inject ConversationRepository() {}
This allows Dagger to know that yes, it is safe to call that constructor. (This differs from Guice, which was willing to call a public parameterless constructor including the default constructor provided by Java.) Though you are welcome to accept injector-provided parameters in that annotated constructor (which might be nice if your repository has dependencies, because then you can keep the fields final), you may also choose to simply annotate some fields with #Inject and let the injector populate those after creation.
I'm starting to use Dagger 2 in an application I'm developing but I have some questions about how Dagger 2 works.
I get the all the logic behind the #Provides methods and the #Inject annotation for initialising your dependencies, but the #Inject annotation to class constructors kind of bugs my mind.
For example:
Im my app, I have one module defined, the ContextModule, to retrieve the context of my application:
ContextModule.java
#Module
public class ContextModule {
private final Context context;
public ContextModule(Context context) {
this.context = context;
}
#Provides
public Context context() {
return this.context;
}
}
This module is used by my BaseActivityComponent:
BaseActivityComponent.java
#BaseActivityScope
#Component(modules = ContextModule.class)
public interface BaseActivityComponent {
void injectBaseActivity(BaseActivity baseActivity);
}
So far so good.. then I have an AuthController class, that depends on the context and I want to inject it in my BaseActivity. So in my AuthControllers.class I have something like:
public class AuthController {
private Context context;
#Inject
public AuthController(Context context) {
this.context = context;
}
public void auth() {
// DO STUFF WITH CONTEXT
}
}
And I inject it in my BaseActivity like:
public class BaseActivity extends AppCompatActivity {
#Inject
AuthController authController;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
BaseActivityComponent component = DaggerBaseActivityComponent.builder()
.contextModule(new ContextModule(this))
.build();
component.injectBaseActivity(this);
authController.auth();
}
}
Now my question is, how does dagger knows that my AuthControllers is a dependency for BaseActivity? Just by declaring
#Inject
AuthController authController;
it's like the same thing as if I created a ControllerModule like:
#Module(includes = ContextModule.class)
public class ControllerModule {
#Provides
AuthController authController(Context context) {
return new AuthController(context);
}
}
And then in my BaseActivityComponent I would add my AuthController getter and change my dependency module to ControllersModule:
#BaseActivityScope
#Component(modules = ControllersModule.class)
public interface BaseActivityComponent {
void injectBaseActivity(BaseActivity baseActivity);
AuthController getAuthController();
}
When I call injectBaseActivity(this) it "tells" dagger that all #Inject annotations are dependencies of my class, and then it searchers my project for #Inject annotated constructors that matches that type?
I thought a good thing about Dagger 2 is that the Module files could be used as a "documentation" of my dependencies three. But if just add #Inject in all the constructors I have control of, couldn't it get a little confusing in the future, since you don't know what actually depends on what? (I mean, you know what depends on what, you just have to browse a lot of files to really find out)
Is there any best practices for when using #Inject annotations in constructors or when to add the #Provides method in Modules files?
I get that using #Inject in constructor I don't need to change the constructor definition in my Module file, but is there any downside?
Thanks.
When I call injectBaseActivity(this) it "tells" dagger that all #Inject annotations are dependencies of my class, and then it searches my project for #Inject annotated constructors that matches that type?
Exactly. But it's not done when you call injectBaseActivity, but it all happens during compile time. This is one way of annotation processing (another makes use of reflection at runtime).
When you build your project the dagger-annotation-processor you include (as a dependency)in your build.gradle file gets called with a list of all your fields, classes, etc annotated by the #Inject annotation and builds a dependency graph with it. It then resolves the graph, generating source code that provides all the dependencies for the items on the graph.
injectBaseActivity just executes the code which was generated before, and assigns all the dependencies to your object. It is proper source code, which you can read, and debug.
The reason this is a compile step—simply put—is performance and validation. (e.g. If you have some dependency cycle, you get a compile error)
how does dagger knows that my AuthControllers is a dependency for BaseActivity?
#Inject
AuthController authController;
By annotating the field #Inject dagger knows you want an AuthController. So far so good. Now dagger will look for some means to provide the controller, looking for it within the component, the components dependencies, and the components modules. It will also look whether the class can be supplied on its own, because it knows about its constructor.
How does dagger know about the objects constructor if you don't include it in any module?
#Inject
public AuthController(Context context) { /**/ }
By annotating the constructor with inject you also told dagger that there is a class called AuthController and you need a context for it to be instantiated. It is basically the same as adding it to your module.
A module #Provides method should be used if you don't have the source code to just add the #Inject annotation to the constructor, or if the object needs further initialization. Or in your case...
[...]the Module files could be used as a "documentation" of my dependencies tree [...]
Yes, of course you could do that. But as your project grows you will have to maintain a lot of unnecessary code, since the same could have been done with a simple annotation on the constructor.
Is there any best practices for when using #Inject annotations in constructors or when to add the #Provides method in Modules files?
If you want to provide different versions for a different context (e.g. implementing an interface in 2 different ways) there is also the #Binds annotation that tells dagger which class you wish to provide as implementation.
Other than that I believe you should always use constructor injection when possible. If something changes you don't have to touch any other parts of your code, and it is just less code that you write, and hence less places where you could include a bug.
Also Dagger can and does optimize a lot by knowing more, and if you implement unnecessary code it will have to work with the overhead you introduced
Of course in the end it is all up to what you think is best. After all it is you that has to work with your code ;)
I am trying to make my app better and code more maintainable using Dagger2 I caught general idea, but still cannot figure out how scopes are managed by Dagger2
I injected dagger into my project (sounds funny).
I created ApplicationComonent component and it works perfectly in my project.
Here is my code.
#Singleton
#Component(modules = {
ApplicationModule.class,
ThreadingModule.class,
NetworkModule.class,
DatabaseModule.class,
ServiceModule.class,
ParseModule.class,
PreferencesSessionModule.class})
public interface ApplicationComponent {
ActivityComponent activityComponent(ActivityModule activityModule);
void inject(BaseActivity baseActivity);
void inject(MainAppActivity mainAppActivity);
void inject(MyApplication application);
void inject(BaseFragment baseFragment);
void inject(MyService service);
void inject(RegistrationIntentService service);
}
I create my component instance in MyApplication class like this
private void initializeAndInjectComponent() {
mApplicationComponent =
DaggerApplicationComponent
.builder()
.threadingModule(new ThreadingModule(1))
.applicationModule(new ApplicationModule(this))
.networkModule(new NetworkModule(
MyService.API_SERVER_BASE_URL,
MyService.TIMEOUT))
.build();
mApplicationComponent.inject(this);
}
And I can obtain component in order to inject in in my Activities
MyApplication application = MyApplication.get(this);
application.getApplicationComponent().inject(this);
Everything works perfectly.
To add each method as well as module class is annotated with #Singleton scope, all modules related to the ApplicationComponent
Now I want to make dependencies better and I have seen a lot of examples with custom scopes like #PerActivity, #PerFragment. I have a lot of questions, but about this later.
So I created ActivityComponent
#PerActivity
#Subcomponent(
modules = {
NetworkServiceModule.class,
ActivityModule.class,
PermissionModule.class
})
public interface ActivityComponent {
Activity activity();
void inject(BaseActivity baseActivity);
}
All modules looks like this
#PerActivity
#Module
public class ActivityModule {
private Activity mActivity;
public ActivityModule(Activity activity) {
this.mActivity = activity;
}
#Provides
#PerActivity
Activity provideActivity() {
return this.mActivity;
}
}
I have following dependencies in my BaseActivity
// Dependencies from ApplicationComponent
#Inject
protected ApplicationSettingsManager mApplicationSettingsManager;
#Inject
protected ScheduledThreadPoolExecutor mPoolExecutor;
// Dependencies from ActivityComponent
#Inject
protected SpiceManager mSpiceManager;
#Inject
protected PermissionController mPermissionController;
And in my onCreate() method I am injecting as following
MyApplication application = MyApplication.get(this);
application.getApplicationComponent().activityComponent(new ActivityModule(this)).inject(this);
Before creating subcomponent ActivityComponent it was
MyApplication application = MyApplication.get(this);
application.getApplicationComponent().inject(this);
Now I got an error
Error:(34, 10) error: com.octo.android.robospice.SpiceManager cannot be provided without an #Inject constructor or from an #Provides- or #Produces-annotated method.
BaseActivity.mSpiceManager
[injected field of type: com.octo.android.robospice.SpiceManager mSpiceManager]
I cannot figure out where is problem, what I missed.
My questions about scopes in dagger2.
Everything but #Singleton is ignored by Dagger 2, am I right ?
I don't understand how life of component is managed ? I have only one idea
When you use #Singleton annotation dagger is creating object in some static pool that will exist during whole application lifecycle, and will be destroyed when JVM process (dalvik VM,ART) instance will be destroyed.
When you use any other annotation is just for you as developer to better maintain code, #PerActivity, #PerFragment is just custom annotation nothing more. And in case you place #PerFragment component in Application class it will live as long as Application lives. Am I right ?
So I understand this like this, if dagger finds #Singleton annotation it will add static reference to component when it is created first time and in case of any other annotation it won't hold reference to component.
I would be very grateful for any help with problems described above.
UPDATE
Thank you David Medenjak for great answer, I got much better understanding of Dagger2.
I have also just found the problem, as far as I am using separate Activity component now, I forgot about two lines in ApplicationComponent and change inejction in my MainActivity to ActivityComponent instead of ApplicationComponent, so for sure it couldn't resolve dependencies from subcomponent.
void inject(BaseActivity baseActivity);
void inject(MainAppActivity mainAppActivity);
Now everything works perfectly, I like Dagger2 and separated architecture.
A bit radical, but to simplify things:
All Scope annotations are nothing but syntactic sugar—including #Singleton.
Scopes mostly just provide compile time checks. Cyclic dependencies, or errors about things that you might have missed. #Singleton is just like any other scope, the only difference is that it is an already existing annotation and you don't have to create it yourself. You could just use #MySingleton instead.
[...] dagger is creating object in some static pool that will exists during whole application lifecycle
No. Dagger does nothing static. You have component objects. Those components hold your objects created by modules. If an object in a component has the scope of the component, it will only be created once in that exact component. If you decide to create 2 AppComponent objects, you will have 2 objects of each #Singleton annotated object, each within its component. This is why you should keep the reference to the component. Most implementations that I have seen or used hence keep their AppComponent within their Application. If you do this, you can use it like a singleton—it is still just a POJO.
[...]you place #PerFragment component in Application class it will live as long as Application lives.
Yes. As already covered by the paragraph above, it is just an object. Keep the reference, you keep the objects. Throw it away or create a new one and you have new objects (defined within in this component / scope). You should although not keep activity or fragment scoped components any place besides in activities or fragments respectively, since keeping them e.g. in your app component will most likely lead to a memory leak. (If it doesn't, you probably would not have needed the activity or fragment scope.)
if dagger finds #Singleton annotation it will add static reference to component when it is created first time and in case of any other annotation it won't hold reference to component.
Again, no. Nothing static. Plain old java objects. You can have multiple #Singleton components with their own objects, but you probably shouldn't (Although this is what makes instrumentation testing possible / easy—just swap components.)
Your mentioned error
SpiceManager cannot be provided without an #Inject constructor or from an #Provides- or #Produces-annotated method.
This means that the component you are trying to inject your object with can not find any way to produce or provide a SpiceManager. Make sure you provide it from your AppComponent or some other place, are not missing any annotations, etc.
I'm having a project based on Dagger 2 which consists of two modules. The core module includes some interfaces and some classes that have member injections declared for these interfaces.
The actual implementations of these interfaces are included in the second module which is an Android project. So, naturally the provide methods for these are included in the Android project.
Dagger will complain during compilation about not knowing how to inject these in the core module.
Any thoughts on how to achieve this without using constructor injections?
In short, I just tried this, and it works. Be sure to check the exact error messages and make sure you are providing these interfaces and #Inject annotations are present.
There is probably just some wrong named interface or a missing annotation. Following up is a full sample using your described architecture that is compiling just fine. The issue you are currently experiencing is probably the one described in the last part of this post. If possible, you should go with the first solution though and just add those annotations.
The library
For reproducability this sample has minimalist models. First, the interface needed by my class in the library module:
public interface MyInterface {
}
Here is my class that needs that interface. Make sure to declare it in the constructor and provide the #Inject annotation!
#MyScope // be sure to add scopes in your class if you use constructor injection!
public class MyClassUsingMyInterface {
private MyInterface mMyInterface;
#Inject
public MyClassUsingMyInterface(MyInterface myInterface) {
mMyInterface = myInterface;
}
}
The idea is that the interface will be implemented by the app using MyClassUsingMyInterface and provided by dagger. The code is nicely decoupled, and my awesome library with not so many features is complete.
The application
Here need to supply the actual coupling. This means to get MyClassUsingMyInterface we have to make sure we can supply MyInterface. Let's start with the module supplying that:
#Module
public class MyModule {
#Provides
MyInterface providesMyInterface() {
return new MyInterface() {
// my super awesome implementation. MIT license applies.
};
}
}
And to actually use this, we provide a component that can inject into MyTestInjectedClass that is going to need MyClassUsingMyInterface.
#Component(modules = MyModule.class)
public interface MyComponent {
void inject(MyTestInjectedClass testClass);
}
Now we have a way to provide the requested interface. We declared that interface needed by the library class in a constructor marked with #Inject. Now I want a class that requires my awesome library class to use. And I want to inject it with dagger.
public class MyTestInjectedClass {
#Inject
MyClassUsingMyInterface mMyClassUsingMyInterface;
void onStart() {
DaggerMyComponent.create().inject(this);
}
}
Now we hit compile...and dagger will create all the factories needed.
Inject Libraries you can not modify
To just provide the full scale of dagger, this sample could also have been without actual access to the source code of the library. If there is no #Inject annotation dagger will have a hard time creating the object. Notice the missing annotation:
public class MyClassUsingMyInterface {
private MyInterface mMyInterface;
public MyClassUsingMyInterface(MyInterface myInterface) {
mMyInterface = myInterface;
}
}
In that case we have to manually provide the class. The module would be needed to be modified like the following:
#Module
public class MyModule {
#Provides
MyInterface providesMyInterface() {
return new MyInterface() {
};
}
#Provides
MyClassUsingMyInterface providesMyClass(MyInterface myInterface) {
return new MyClassUsingMyInterface(myInterface);
}
}
This introduces more code for us to write, but will make those classes available that you can not modify.
Let's say I have the following classes and dagger module
public class Base implements IBase {
private IDependency dependency; //IDependency is an interface
Base(IDependency dependency) {
this.dependency = dependency
}
}
public class SubClass extends Base implements ISubclass {
Base(IDependency dependency) {
super(dependency)
}
}
#Module
public class MyModule {
// Let's assume some other class use SubClass and requires this
#Provides
ISubclass providesSubclass(IDependency dependency) {
return new SubClass(dependency);
}
}
If I add a new parameter to Base constructor, I'll have to go to MyModule and modify provides method to include this new parameter (besides obviusly chaging Base and Subclass constuctors). It seems to me that using propery injection I don't have this problem since I'm not using any constructor.
My feeling is that I might be doing something wrong or I have some concept wrong. I prefer constructor injection over property injection but right now I have to add a constructor parameter to a base class used by 40 other classes and not only I have to modify those 40 classes constructors, I also have to modify modules to reflect new constructors parameters.
Am I missing something? Am I correct if I say that doing constructor injection I'll write much more less code and maintenance will be easier?
Yes, you are missing some awesome feature: You can still use constructor injection in this case! And you don't even have to write it yourself.
If all of the dependencies can be provided, dagger can and will create the object for you. Given that you can provide IDependency you just need to modify your code like the following:
public class SubClass extends Base implements ISubclass {
#Inject // Don't forget the annotation!
public Base(IDependency dependency) {
super(dependency)
}
}
#Module
public class MyModule {
#Provides
ISubclass providesSubclass(SubClass subclass) {
return subclass;
}
}
You provide the interface, yet you depend on your implementation to provide it. Dagger will resolve this, and you can merrily add as many parameters to the constructor as you like. (Apart from the obvious changes to the actual constructors you already pointed out)
Don't forget the #Inject annotation!