字段注入在Dagger2 Android中不起作用

Question

I am trying out my hands on dagger.Till now what i have understood is to inject dependency in one class we need to first dagger how that dependency is created through Modules or Constructor injection.Then use that dependency using Components methods.

Here is the sample code i am trying
War class depends on House1 and House2

War Class

public class War {

    @Inject House1 house1;
    @Inject House2 house2;

    public void startWar(){
        house1.prepareForWar();
        house1.reportForWar();

        house2.prepareForWar();
        house2.reportForWar();

        System.out.println("House1 is "+house1);
    }
}

House1 Class

public class House1 implements House {

    @Inject
    public House1() {
    }

    @Override
    public void prepareForWar() {
        System.out.println("House1 Prepared for War");
    }

    @Override
    public void reportForWar() {
        System.out.println("House1 reported for War");
    }
}

House2 Class

public class House2 implements House {

    @Inject
    public House2() {
    }

    @Override
    public void prepareForWar() {
        System.out.println("House2 prepared for war");
    }

    @Override
    public void reportForWar() {
        System.out.println("House2 reported for War");
    }
}

WarModule Class

@Module
public class WarModule {

    @Provides
    public War provideWarObject(){
        return new War();
    }
}

Component Class

@Component(modules = {WarModule.class})
public interface HouseComponent {
    War getWar();
}

Main Class

public class TempMain {

    public War war;

    public void startWar(){
        war = DaggerHouseComponent.builder().warModule(new WarModule()).build().getWar();
        war.startWar();
    }
}

Here Dagger knows how House1 and House2 are Created using Constructor and War Object through WarModule.

So In war class when @Inject House1 and @Inject House2 are used dagger should inject it as it knows how to create these dependencies. But I am getting Null Pointer Exception here

Error : Exception in thread "main" java.lang.NullPointerException at com.gosemathraj.dagger2.java.models.war.War.startWar(War.java:14)

Answer 1

So In war class when @Inject House1 and @Inject House2 are used dagger should inject it as it knows how to create these dependencies. But I am getting Null Pointer Exception here

Dagger won't do anything because you create the object here:

@Provides
public War provideWarObject(){
    return new War();
}

If you use an @Provides annotated method you need to setup the object before returning it. Dagger will only inject objects that it creates or where you call component.inject(object) .

The simples solution is to remove the @Provides annotate method and add a @Inject public War() {} to your class. Then Dagger can create the object and will inject it afterwards, the same thing you're already doing for your House* classes.

On an additional note, I don't know why you would use field injection over constructor injection, but you can move your dependencies to the constructor as well, allowing the fields to be private.