使用Dagger 2是否需注入所有对象？基础Java类如String等需注入吗？

Hey there! As someone who’s navigated the Dagger learning curve, I totally get this confusion—let’s break down when to use injection and when to skip it, clearly.

Objects You Never Need to Inject #

• Basic Java/Kotlin value types & lightweight objects: Things like String, StringBuilder, ArrayList, primitive types (int, boolean)—these are simple, short-lived, and require no external dependencies to create. In your example, creating a StringBuilder directly inside the create method is exactly what you should do. Injecting these would only add unnecessary boilerplate and complicate your Dagger setup for zero practical benefit.

Objects That Should Be Injected #

Focus on objects that fit one or more of these criteria:

• Android framework components with dependencies: Like Context (stick to Application context for singletons to avoid memory leaks), ViewModel (especially with Dagger Hilt), Room Database instances, or Retrofit services. These often rely on other objects to be initialized correctly.
• Custom business logic classes: Think Repository classes, NetworkManager, UserSessionManager—any class that has its own dependencies (e.g., a UserRepository needing a UserDao and ApiService). Injecting these decouples your code, makes testing easier (you can swap in mock implementations), and centralizes object creation logic.
• Singleton or scoped objects: If you need a single instance of an object across your app (like a global cache) or within a specific lifecycle scope (e.g., an Activity’s lifespan), Dagger’s scoping annotations (@Singleton, @ActivityScoped) handle this cleanly without you having to manage singletons manually.

Quick Rule of Thumb to Decide #

Ask yourself three simple questions:

1. Does this object have a chain of dependencies (e.g., it needs other objects to function)? If yes, inject it.
2. Will this object be reused across multiple parts of the app? If yes, inject it.
3. Do I need to easily swap its implementation (e.g., for testing with mocks)? If yes, inject it.

If none of these apply—like your StringBuilder example—just create it directly with new.

内容的提问来源于stack exchange，提问作者Mohammed Daoud