What is the difference between Dagger 1 and Dagger 2?

The main difference is that Dagger 2 generates code at compile time, whereas Dagger 1 relied on reflection at runtime.

Advantages of Dagger 2 over Dagger 1:

• Performance: Absence of reflection makes runtime faster.
• Debugging: Configuration errors are visible at compile time, not at runtime.
• Reliability: Generated code is more predictable and testable.
• No hidden errors: Reflection can hide errors that only manifest under certain conditions.

In Dagger 1, @Module and @Inject annotations were used, along with @ObjectGraph classes for dependency management.

In Dagger 2, @Module and @Inject are also used, but the main mechanism is @Component, which acts as a factory for injectable objects and is generated by Dagger.

Example of a module in Dagger 1:

// Dagger 1 Module
@Module(injects = MyActivity.class)
public class MyModule {

    @Provides
    public MyDependency provideMyDependency() {
        return new MyDependency();
    }
}

Example of a module in Dagger 2:

// Dagger 2 Module
@Module
public class MyModule {

    @Provides
    public MyDependency provideMyDependency() {
        return new MyDependency();
    }
}

Example of a component in Dagger 2:

// Dagger 2 Component
@Component(modules = MyModule.class)
public interface MyComponent {

    void inject(MyActivity activity);
}

The main principle of Dagger 2: you describe the dependency graph using modules and annotations, and Dagger generates classes that implement this graph, providing the ability to obtain instances of dependencies. This is similar to manually writing factories, but automatically and efficiently.