关于Dependency Injection及各类注入方式中“Injection”含义与差异的问询

Great question—let’s break this down clearly, since dependency injection (DI) can feel like a bag of overlapping terms when you’re first getting deep into it.

First off: Dependency Injection is the overarching design pattern, and the four "Injection" types you listed are specific implementation strategies for that pattern. They’re not identical to DI—they’re just different ways to achieve the core goal of DI: decoupling a class from the dependencies it needs to function.

Let’s walk through each one to see how they differ:

• Constructor Injection（构造注入）: Exactly what you guessed—this is when you pass dependencies as parameters to a class’s constructor. It’s great for required dependencies because it guarantees the class is fully initialized and usable as soon as it’s created (no chance of null dependencies later). Example:

  public class OrderService {
      private final PaymentGateway paymentGateway;
  
      // 构造注入：依赖在实例化时强制传入
      public OrderService(PaymentGateway paymentGateway) {
          this.paymentGateway = paymentGateway;
      }
  }
  

• Setter Injection（Setter注入）: Here, you use public setter methods to pass dependencies to the class after it’s been instantiated. This works well for optional dependencies or cases where you might need to swap out a dependency at runtime. Example:

  public class OrderService {
      private PaymentGateway paymentGateway;
  
      // Setter注入：依赖可在实例化后设置
      public void setPaymentGateway(PaymentGateway paymentGateway) {
          this.paymentGateway = paymentGateway;
      }
  }
  

• Getter Injection（Getter注入）: This is a rare, less common variant of setter injection. In some older frameworks, the DI container would use a class’s getter method to either retrieve a dependency to inject or to trigger the injection process. It’s not widely used today because it adds unnecessary complexity compared to setter or constructor injection.
• Interface Injection（接口注入）: This approach requires your class to implement a specific interface that defines an injection method. The DI container then calls that method to inject the dependency. It’s fallen out of favor because it forces your class to implement a framework-specific interface, adding unnecessary coupling. Example:

  // 框架定义的注入接口
  public interface PaymentGatewayInjectable {
      void injectPaymentGateway(PaymentGateway gateway);
  }
  
  // 业务类实现接口以接收依赖
  public class OrderService implements PaymentGatewayInjectable {
      private PaymentGateway paymentGateway;
  
      @Override
      public void injectPaymentGateway(PaymentGateway gateway) {
          this.paymentGateway = gateway;
      }
  }
  

You’re on the right track with "providing the required objects/parameters to a class"—but let’s refine that:
Injection means transferring control of dependency creation from the class itself to an external entity (either a DI container or a manual caller). Instead of the class doing new PaymentGateway() to create its own dependency, that dependency is "injected" into it from outside.

It’s not just objects, either—you can inject primitive values (like API keys, port numbers), configuration objects, or even other services. The core idea is that the class doesn’t need to know how its dependencies are created, only that it needs them to work.

• DI’s real value is decoupling: The injection methods are just tools—what matters is that your class depends on abstractions (like interfaces) instead of concrete implementations. For example, OrderService depends on PaymentGateway (an interface), not StripePaymentGateway (a concrete class), making it easy to switch payment providers without changing OrderService code.
• DI doesn’t require a framework: You can practice DI manually! If you create a StripePaymentGateway instance and pass it to OrderService via its constructor, that’s still DI—no Spring or Guice needed. Frameworks just automate the process of resolving and injecting dependencies.
• Lifecycle implications: Constructor injection often ties the dependency’s lifecycle to the class instance (especially if the dependency is marked final), while setter injection lets you swap dependencies at runtime (useful for things like mock dependencies in testing).
• Required vs optional dependencies: Constructor injection enforces required dependencies (you can’t create the class without them), while setter injection is better for optional ones (the class can function without them, but gains extra features when they’re provided).

内容的提问来源于stack exchange，提问作者Shobi