Dagger 2 on Android @Singleton annotated class not

问题:

I am currently trying to integrate Dagger 2 into an Android application. My project setup is as follows:

• library
• app (depends on library)

In my library project I defined a class that I'll later inject into other classes that need it (Activities and regular classes) in the library as well as the app project.

@Singleton
public class MyManager{
  @Inject
  public MyManager(){
    //Do some initializing
  }
}

Now - for instance in my Fragments or Activities or regular classes I'd inject the above Singleton as follows:

public class SomeClass{

  @Inject
  MyManager myManager;
}

Or so I thought, because in practice myManager is always null. And apparently it's constructor is never called either, so I guess I must be missing something configuration-wise? Or maybe I misunderstood the documentation and it's not meant to work this way at all? The purpose of MyManager class is to be an application-wide accessible component-accumulating entity - that's why I went for the @Singleton.

UPDATE

To avoid confusion: I mentioned my having components somewhere in a comment I think - this refers to components in the sense of "component based design" and has nothing to do with dagger. The dagger-based code I have is all listed above - there is nothing else related to dagger in my code.

When I started adding @Component I had some compiler issues, because my dagger2 was not setup properly - check out this really helpful thread on how to setup dagger2 correctly: https://stackoverflow.com/a/29943394/1041533

UPDATE 2

Here is my updated code, based on G. Lombard's suggestions - I changed the code as follows - the original Singleton is in the library project:

@Singleton
public class MyManager{
  @Inject
  public MyManager(){
    //Do some initializing
  }
}

Also in the library project is the bootstrap class:

@Singleton
@Component
public interface Bootstrap {
    void initialize(Activity activity);
}

Then I use the above Bootstrap class in my activity (in my concrete app, NOT in the library project! I do however also have Classes/Activities in the library that'll access Bootstrap to inject MyManager):

public class MyActivity extends Activity{

    @Inject
    MyManager manager;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        //DONT DO THIS !!! AS EXPLAINED BY EpicPandaForce
        DaggerBootstrap.create().initialize(this);
    }
}

But even after this line:

        DaggerBootstrap.create().initialize(this);

the manager instance is still null, i.e. not injected.

I just found this: https://stackoverflow.com/a/29326023/1041533

Which if I don't misread, implies I need to specify every single class in the Bootstrap class that will use @Inject to have stuff injected. Sadly - this is not an option, as I have more than 40 classes and activities for which I'd have to do that.

Meaning my Bootstrap interface apparently would have to look something like this:

@Singleton
@Component
public interface Bootstrap {
    void initialize(ActivityA activity);
    void initialize(ActivityB activity);
    void initialize(ActivityC activity);
    void initialize(ActivityD activity);
    void initialize(ActivityE activity);
    void initialize(ActivityF activity);
    //and so on and so forth...
}

If the above is true, that would not be worth it for my use case. Plus: Seems there is no compile-time check, if I forgot to specify one of my 40+ classes here? It just wont work - i.e. crash the app at runtime.

回答1:

You're making a mistake in that you are using

DaggerBootstrap.create().initialize(this);

in your Activity, as scopes are not shared across multiple component instances. What I recommend is using a custom application class

public class CustomApplication extends Application {
    @Override
    public void onCreate() {
         super.onCreate();
         Bootstrap.INSTANCE.setup();
    }
}

@Component
@Singleton
public interface _Bootstrap {
    void initialize(ActivityA activityA);
    //void initiali...
}

public enum Bootstrap {
    INSTANCE;

    private _Bootstrap bootstrap;

    void setup() {
        bootstrap = Dagger_Bootstrap.create();
    }

    public _Bootstrap getBootstrap() {
        return bootstrap;
    }
}

Then you could call it as

Bootstrap.INSTANCE.getBootstrap().initialize(this);

This way, you share the component across your classes. I personally named Bootstrap as injector, and _Bootstrap as ApplicationComponent, so it looks like this:

Injector.INSTANCE.getApplicationComponent().inject(this);

But that's just my typical setup. Names don't really matter.

EDIT: To your last question, you can solve this by subscoping and component dependencies.

Your library project should be able to see only the library classes, correct? In that case, all you do is

@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface LibraryScope {
}

@Component(modules={LibraryModule.class})
@LibraryScope
public interface LibraryComponent {
    LibraryClass libraryClass(); //provision method for `MyManager`
}

@Module
public class LibraryModule {
    @LibraryScope
    @Provides
    public LibraryClass libraryClass() { //in your example, LibraryClass is `MyManager`
        return new LibraryClass(); //this is instantiation of `MyManager`
    }
}

public enum LibraryBootstrap {
    INSTANCE;

    private LibraryComponent libraryComponent;

    static {
        INSTANCE.libraryComponent = DaggerLibraryComponent.create();
    }

    public LibraryComponent getLibraryComponent() {
        return libraryComponent;
    }
}

@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface ApplicationScope {
}

@Component(dependencies={LibraryComponent.class}, modules={AdditionalAppModule.class})
@ApplicationScope
public interface ApplicationComponent extends LibraryComponent {
    AdditionalAppClass additionalAppClass();

    void inject(InjectableAppClass1 injectableAppClass1);
    void inject(InjectableAppClass2 injectableAppClass2);
    void inject(InjectableAppClass3 injectableAppClass3);
}

@Module
public class AdditionalAppModule {
    @ApplicationScope
    @Provides
    public AdditionalAppClass additionalAppClass() { //something your app shares as a dependency, and not the library
        return new AdditionalAppClass();
    }
}

public enum ApplicationBootstrap {
    INSTANCE;

    private ApplicationComponent applicationComponent;

    void setup() {
        this.applicationComponent = DaggerApplicationComponent.builder()
                                        .libraryComponent(LibraryBootstrap.INSTANCE.getLibraryComponent())
                                        .build();
    }

    public ApplicationComponent getApplicationComponent() {
        return applicationComponent;
    }
}

Then

@Inject
LibraryClass libraryClass; //MyManager myManager;

...
    ApplicationBootstrap.INSTANCE.getApplicationComponent().inject(this);

回答2:

It's hard to say what your problem was, since you didn't show what your Component looks like and whether you have multiple components etc.

Assuming this logical structure:

/app
   MainComponent
   SomeClass    // where MyManager is to be injected
   MainActivity // where SomeClass is to be injected
/library
   LibraryComponent
   MyManager    // Singleton

Then your classes as listed would inject correctly with the following configuration:

@Singleton
@Component
public interface LibraryComponent {
    MyManager getMyManager();
}

and the app-level component to inject dependencies into the activity:

@ActivityScope
@Component(dependencies = LibraryComponent.class)
public interface MainComponent {
    void inject(MainActivity mainActivity);
}

Note that MainComponent depends on LibraryComponent, but because the latter has singleton scope, you need to define a scope for the other one too, which I was I used the "activity scope" here. (Or you could also just make the MainComponent a singleton and get rid of the LibraryComponent completely if that suits your needs.)

Finally it's all injected into the activity like this:

@Inject
SomeClass someClass;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    DaggerMainComponent.builder()
            .libraryComponent(DaggerLibraryComponent.create())
            .build()
            .inject(this);

    someClass.doSomething();
}

I've put a working sample here on GitHub

Update 1:

If I understand your setup correctly, you've so far only used the @Singleton and @Inject annotations on the two classes listed (MyManager and SomeClass), and there is no other Dagger-related code in your project.

In that case, the reason your MyManager isn't getting injected, is because Dagger doesn't know how to provide/instantiate the dependencies. This is where the "components" come in that I mentioned above. Without any Dagger 2 components (interface or abstract class annotated with @Component), your dependencies won't get injected automatically.

I don't know if you have experience with Dependency Injection concepts, but assuming you don't, I'll step through the minimum basics you'll need to understand to get your MyManager injected into SomeClass:

First: when you use DI, you need to understand the difference between "newables" and "injectables". This blogpost by Misko Hevery explains the details.

This means, you can't new up your SomeClass. This won't work:

mSomeClass = new SomeClass();

Because if you did that (say in an activity or fragment), Dagger will have no idea that you expected a dependency to get injected into SomeClass and it has no opportunity to inject anything.

In order for its dependencies to get injected, you have to instantiate (or inject) SomeClass itself through Dagger too.

In other words, say in your Activity where SomeClass is used, you'll need:

@Inject
SomeClass mSomeClass;

Next, you need a Dagger component to perform the actual injection. To create a component, you create an interface with a method that takes your root object (say MainActivity) as argument, e.g.:

@Singleton
@Component
public interface Bootstrap {
    void initialize(MainActivity activity);
}

Now when you build your project, Dagger 2 generates a class called DaggerBootstrap that implements this interface. You use this generated class to perform the injection, say in your activity's onCreate:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    DaggerBootstrap.create().initialize(this);

    mSomeClass.doSomething();
}

I believe this generated component is the key part you're missing. Full code for above here.

Some useful Dagger 2 resources:

• the official Dagger 2 guide
• reddit page with lots of links

Update 2:

Seems like the final missing piece of the puzzle was that your component provided an inject method for the Activity base class, but not for your actual concrete activity.

Unfortunately Dagger 2 requires an inject method for each activity or other class you want to inject into.

As you mentioned, this will be annoying when you have many different activities in your app. There some possible workarounds for this, search for "dagger 2 inject base class", for example this suggestion by @EpicPandaForce: Dagger 2 base class injections

Also note, as pointed out by @EpicPandaForce in the comments, that in my simplistic example I called DaggerLibraryComponent.create() every time which is probably not what you want, since that component is supposed to provide your singletons, so you're probably better off getting the existing instance from somewhere else such as from your Application instance.