Clean Architecture & Design Patterns in Practice · 课时

输入与输出端口

理解输入端口和输出端口（接口）如何定义用例与外部层之间的通信边界。

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输入与输出端口 是 CoddyKit 上的免费 Clean Architecture & Design Patterns in Practice 课时。这是第 3 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Clean Architecture & Design Patterns in Practice 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Clean Architecture & Design Patterns in Practice 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What are Input & Output Ports?

In Clean Architecture, "Ports" are crucial interfaces that define how different layers communicate. They act like contracts.

Think of them as the "sockets" of your application's core logic. They specify what goes in (Input) and what comes out (Output).

Defining Input Ports

An Input Port is an interface defined by the Use Case layer. It declares the methods that an external layer (like a UI controller) can call to interact with the Use Case.

It's the "what you can do" contract for the Use Case.
It ensures the Use Case doesn't know about the UI or external callers.

Input Port: User Login

Here's a simple Input Port for a user login feature. It defines the operations the Use Case expects.

public interface UserLoginInputPort {
  void login(String username, String password);
}

public class UserLoginUseCase implements UserLoginInputPort {
  @Override
  public void login(String username, String password) {
    // Business logic for login
    System.out.println("Attempting login for: " + username);
    if (username.equals("admin") && password.equals("pass")) {
      System.out.println("Login successful!");
    } else {
      System.out.println("Login failed.");
    }
  }

  public static void main(String[] args) {
    UserLoginInputPort loginService = new UserLoginUseCase();
    loginService.login("admin", "pass");
    loginService.login("user", "123");
  }
}

Defining Output Ports

An Output Port is also an interface, defined by the Use Case layer. It declares the methods that the Use Case will call to deliver results or interact with external services (like a database or UI presenter).

It's the "what I need to tell you" contract.
It keeps the Use Case unaware of specific implementations (e.g., how results are displayed).

Output Port: Login Presenter

This Output Port defines how the login result should be presented or handled. The Use Case calls these methods, but doesn't know how they are implemented.

public interface UserLoginOutputPort {
  void presentLoginSuccess(String username);
  void presentLoginFailure(String message);
}

public class ConsolePresenter implements UserLoginOutputPort {
  @Override
  public void presentLoginSuccess(String username) {
    System.out.println("Welcome, " + username + "!");
  }

  @Override
  public void presentLoginFailure(String message) {
    System.out.println("Error: " + message);
  }

  public static void main(String[] args) {
    UserLoginOutputPort presenter = new ConsolePresenter();
    presenter.presentLoginSuccess("Alice");
    presenter.presentLoginFailure("Invalid credentials.");
  }
}

How Use Cases Use Ports

The Use Case (or Interactor) depends on these port interfaces. It takes an Input Port as its public method and receives an Output Port through its constructor or method parameters.

This means the Use Case only knows about abstractions, not concrete implementations.

Use Case Connecting Ports

Here, our UserLoginUseCase now uses both ports. Notice it doesn't know if it's talking to a web UI, a mobile app, or a console!

public interface UserLoginInputPort {
  void login(String username, String password);
}

public interface UserLoginOutputPort {
  void presentLoginSuccess(String username);
  void presentLoginFailure(String message);
}

public class UserLoginUseCase implements UserLoginInputPort {
  private final UserLoginOutputPort outputPort;

  public UserLoginUseCase(UserLoginOutputPort outputPort) {
    this.outputPort = outputPort;
  }

  @Override
  public void login(String username, String password) {
    if (username.equals("admin") && password.equals("pass")) {
      outputPort.presentLoginSuccess(username);
    } else {
      outputPort.presentLoginFailure("Invalid credentials.");
    }
  }

  public static void main(String[] args) {
    // In real app, this would be wired by a framework
    UserLoginOutputPort presenter = new ConsolePresenter(); // From Scene 5
    UserLoginInputPort loginService = new UserLoginUseCase(presenter);

    loginService.login("admin", "pass");
    loginService.login("user", "wrong");
  }
}

class ConsolePresenter implements UserLoginOutputPort {
  @Override
  public void presentLoginSuccess(String username) {
    System.out.println("Welcome, " + username + "!");
  }

  @Override
  public void presentLoginFailure(String message) {
    System.out.println("Error: " + message);
  }
}

Ports and the Dependency Rule

The use of Ports perfectly adheres to the Dependency Rule. The Use Case layer (core business logic) depends on interfaces (the Ports).

The outer layers (like UI or databases) then implement these interfaces, providing the concrete details without the core knowing about them.

Why Use Ports?

Ports offer powerful advantages:

Decoupling: The Use Case is isolated from UI, database, or external services.
Testability: You can easily mock or stub these interfaces for robust unit testing of Use Cases.
Flexibility: You can swap out implementations (e.g., change database) without touching core logic.
Clarity: They clearly define the boundaries and interactions of your application's core.

Quick Check: Port Roles

Consider the roles of Input and Output Ports in Clean Architecture.

在交互式课程中回答

Recap: Ports as Boundaries

We learned that Input Ports are interfaces defining what a Use Case expects, acting as entry points.

Output Ports are interfaces defining what a Use Case delivers or needs from external services.

Together, they enforce the Dependency Rule, ensuring our core business logic remains independent, testable, and flexible.

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常见问题解答

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理解输入端口和输出端口（接口）如何定义用例与外部层之间的通信边界。你通过在浏览器中直接运行的动手代码来练习 Clean Architecture & Design Patterns in Practice，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

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