外观与代理模式
使用外观模式简化复杂子系统，并使用代理模式控制对对象的访问。
外观与代理模式 是 CoddyKit 上的免费 Clean Architecture & Design Patterns in Practice 课时。 这是第 2 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。
Welcome to Facade & Proxy!
In this lesson, we'll dive into two powerful Structural Design Patterns: the Facade and Proxy patterns.
Structural patterns help you compose objects and classes into larger structures, making systems more flexible and efficient.
We'll learn how Facade simplifies complex systems and how Proxy controls access to objects.
Meet the Facade Pattern
Imagine a complex machine with many buttons and levers. The Facade pattern provides a simple, unified interface to a set of interfaces in a subsystem.
- It defines a higher-level interface that makes the subsystem easier to use.
- Think of it as a simplified front panel for a complicated system.
Its main goal is to reduce complexity and decouple the client from the subsystem's inner workings.
Dealing with Complexity
Without a Facade, a client might need to interact with many different classes and objects within a subsystem to perform a single task.
This leads to:
- Tight coupling: Client code becomes dependent on all subsystem components.
- Increased complexity: Clients need to know too much about the subsystem's internal structure.
- Harder maintenance: Changes in the subsystem's internal parts might break client code.
How Facade Simplifies
The Facade pattern introduces a single Facade class that wraps the complex subsystem.
The Facade class:
- Knows which subsystem classes are responsible for a request.
- Delegates client requests to the appropriate subsystem objects.
- Converts complex interactions into a simple method call.
Clients interact only with the Facade, not the individual subsystem classes.
Facade in Action: Home Theater
Let's see a simple example with a home theater system. Instead of turning on the TV, amplifier, and DVD player separately, a Facade does it all.
public class Amplifier {
public void on() { System.out.println("Amplifier On"); }
public void setDvd(DvdPlayer dvd) { System.out.println("Amplifier setting DVD"); }
public void setVolume(int volume) { System.out.println("Amplifier volume " + volume); }
public void off() { System.out.println("Amplifier Off"); }
}
public class DvdPlayer {
public void on() { System.out.println("DVD Player On"); }
public void play(String movie) { System.out.println("Playing movie: " + movie); }
public void off() { System.out.println("DVD Player Off"); }
}
public class Television {
public void on() { System.out.println("TV On"); }
public void off() { System.out.println("TV Off"); }
}
// The Facade
public class HomeTheaterFacade {
Amplifier amp;
DvdPlayer dvd;
Television tv;
public HomeTheaterFacade(Amplifier amp, DvdPlayer dvd, Television tv) {
this.amp = amp;
this.dvd = dvd;
this.tv = tv;
}
public void watchMovie(String movie) {
System.out.println("Get ready to watch a movie...");
tv.on();
amp.on();
amp.setDvd(dvd);
amp.setVolume(5);
dvd.on();
dvd.play(movie);
}
public void endMovie() {
System.out.println("Shutting down home theater...");
dvd.off();
amp.off();
tv.off();
}
}
public class Main {
public static void main(String[] args) {
Amplifier amp = new Amplifier();
DvdPlayer dvd = new DvdPlayer();
Television tv = new Television();
HomeTheaterFacade homeTheater = new HomeTheaterFacade(amp, dvd, tv);
homeTheater.watchMovie("Inception");
System.out.println("---");
homeTheater.endMovie();
}
}Why Use Facade?
The Facade pattern offers several key advantages:
- Simplifies client code: Clients don't need to learn the complex API of the subsystem.
- Decouples client from subsystem: Reduces dependencies, making the system more robust to changes.
- Improves testability: You can mock the Facade for easier testing of client code.
- Promotes layering: Helps structure a system into layers with clear responsibilities.
Introducing the Proxy Pattern
The Proxy pattern provides a surrogate or placeholder for another object to control access to it.
Think of it as a middleman. Instead of talking directly to the real object, you talk to its proxy.
The proxy can add extra logic before or after accessing the real object, like security checks, lazy loading, or logging.
Common Proxy Applications
Proxies are versatile and used in many scenarios:
- Protection Proxy: Controls access based on permissions (e.g., only authenticated users can access).
- Virtual Proxy: Creates expensive objects on demand (lazy loading), improving performance.
- Remote Proxy: Represents an object located in a different address space (e.g., network proxy for a remote service).
- Logging Proxy: Logs method calls to the real object.
Proxy in Action: Secure Access
Here's an example of a Protection Proxy. A user tries to access a sensitive database. The proxy checks if the user has admin rights first.
// Subject Interface
interface Database {
void executeQuery(String query);
}
// Real Subject
class RealDatabase implements Database {
public void executeQuery(String query) {
System.out.println("Executing query: " + query);
}
}
// Proxy
class DatabaseProxy implements Database {
private RealDatabase realDatabase;
private String userRole;
public DatabaseProxy(String userRole) {
this.userRole = userRole;
this.realDatabase = new RealDatabase(); // Create real object if needed, or lazily
}
public void executeQuery(String query) {
if (userRole.equals("ADMIN")) {
System.out.println("Proxy: Admin access granted.");
realDatabase.executeQuery(query);
} else {
System.out.println("Proxy: Access denied! Only ADMIN can execute queries.");
}
}
}
public class Main {
public static void main(String[] args) {
Database adminUser = new DatabaseProxy("ADMIN");
adminUser.executeQuery("SELECT * FROM sensitive_data");
System.out.println("---");
Database regularUser = new DatabaseProxy("USER");
regularUser.executeQuery("DELETE FROM sensitive_data");
}
}Why Use Proxy?
The Proxy pattern offers significant benefits:
- Controlled Access: Can restrict or enhance access to the real object.
- Lazy Initialization: Delays object creation until it's actually needed, saving resources.
- Remote Access Abstraction: Hides the complexities of accessing objects in different locations.
- Added Functionality: Can inject logging, caching, or security checks without modifying the real object.
Quick Check: Facade vs. Proxy
Consider a scenario where you have a complex image processing library with dozens of classes and methods. You want to provide a simple function like processImage(imageFile) to your users.
Which design pattern would be most suitable to achieve this simplification?
Facade & Proxy: Key Takeaways
We've explored two powerful structural patterns:
- The Facade pattern provides a simplified, high-level interface to a complex subsystem, making it easier to use and reducing coupling.
- The Proxy pattern provides a surrogate or placeholder for another object to control access to it, adding logic like security, lazy loading, or logging.
Both patterns enhance system structure and maintainability by managing complexity and controlling interactions between objects.
常见问题解答
「外观与代理模式」课时是免费的吗？
是的 — 「外观与代理模式」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Clean Architecture & Design Patterns in Practice 课程的其余内容，请升级到 CoddyKit PRO。 Clean Architecture & Design Patterns in Practice 课程共包含 4 节课。
「外观与代理模式」这节课中我会学到什么？
使用外观模式简化复杂子系统，并使用代理模式控制对对象的访问。 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。
学习 Clean Architecture & Design Patterns in Practice 需要有经验吗？
无需任何先前经验。CoddyKit 上的 Clean Architecture & Design Patterns in Practice 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 2 节课，共 4 节。
「外观与代理模式」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 Clean Architecture & Design Patterns in Practice 课中编写并运行代码吗？
能。每节 Clean Architecture & Design Patterns in Practice 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。