匕首2-场注入中的空指针

Question

I am using dagger 2 for dependency injection and got stuck with field injection . Below is the complete scenario with code sample:

Let's say we have a class A which depends on a library B

class A {
    @Inject
    B b;
}

Module for B:

@Module
public class BModule {

    @Provides
    @Singleton
    public B provideB() {
        return new C.methodA();
        // C - static class; C.methodA returns B

    }
}

But when I try to use b in class A then I get null pointer exception but if I do the same using constructor injection then it works perfectly. I can assure that component and other dependencies are fine as the constructor part works correctly.

A is a dependency of some other class (let's call X) and A is being initialized using constructor injection (tested). Also, X is being injected as void inject(X x);

I have 2 questions:

1. Is there anything I am missing out for field injection due to which it is not being injected?
2. I am able to successfully compile the code and get runtime exception, but dagger2 is compile-time DI then why is it unable to catch this while compiling?

PS: I have just shared a part of the code as there are multiple dependencies, so just trying to explain the scenario. Let me know if the question/scenario is still unclear or needs more info.

Thanks.

Answer 1

Field injection in dagger is a bit more complicated than constructor injection. When you use constructor injection like this

class A {
    @Inject
    public A(B b) {}
}

and you have provider for class B

@Module
class DaggerModule {
    @Provides
    B provideB() {}
}

now dagger will know how to create instance of A and pass it required constructor parameter. So everything is fine, compile successfully and works perfect.

But if we speak about field injection

class A {
    @Inject
    B b;
}

and have somewhere provider for B , dagger can't know how to create A instance and when inject b property (in case of manually creating instance of A by hand). To make it work you need write special method in component

@Component(DaggetModule.class)
interface DaggerComponent {
    void inject(A a);
}

and somewhere in code

A a = new A();
DaggerComponent component = //TODO getDaggerComponent()
component.inject(a);

After that b property will be initialized and available for later usage. Hope, it's clear now how to make field injection work.