理解工厂方法模式（理解.工厂.模式.方法...）
本文将深入探讨工厂方法模式，一种在实际应用中广泛使用的高效设计模式。文中如有任何错误，欢迎指出，我将及时更正。
工厂方法模式的核心在于定义一个用于创建对象的接口，但允许子类决定实例化哪个具体的类。
问题场景假设您正在开发一个银行转账应用，需要支持多种转账方式，例如银行转账和PayPal转账。
在未应用工厂方法模式之前，代码可能存在以下问题：
首先，我们来看一个Java示例。场景：用户A向用户B转账（通过银行转账或PayPal）。
项目结构：
problem/ ├─ BankApp.java ├─ service/ │ ├─ PaypalTransferPayment.java │ ├─ BankTransferPayment.java ├─ data/ │ ├─ Person.java
主程序创建两个用户对象，并设定初始金额：
package problem;
import problem.data.Person;
public class BankApp {
public static void main(String[] args) {
Person person1 = new Person("John", 1000);
Person person2 = new Person("Jane", 500);
}
}
分别实现银行转账和PayPal转账类：
package problem.service;
import problem.data.Person;
public class BankTransferPayment {
public void processPayment(Person fromAccount, Person toAccount, float amount) {
fromAccount.withdraw(amount);
toAccount.deposit(amount);
System.out.println("Bank transfer payment success.");
}
}
package problem.service;
import problem.data.Person;
public class PaypalPayment {
public void processPayment(Person fromAccount, Person toAccount, float amount) {
fromAccount.withdraw(amount);
toAccount.deposit(amount);
System.out.println("Paypal transfer payment success.");
}
}
在主函数中添加转账逻辑：
package problem;
import problem.data.Person;
import problem.service.BankTransferPayment;
import problem.service.PaypalPayment;
public class BankApp {
public static void main(String[] args) {
Person person1 = new Person("John", 1000);
Person person2 = new Person("Jane", 500);
String paymentMethod = "bank_transfer";
if (paymentMethod.equals("bank_transfer")) {
BankTransferPayment bankTransferPayment = new BankTransferPayment();
bankTransferPayment.processPayment(person1, person2, 100);
System.out.println("===Method bank_transfer===");
System.out.println(person1.getName() + " has " + person1.getAmount());
System.out.println(person2.getName() + " has " + person2.getAmount());
} else if (paymentMethod.equals("paypal")) {
PaypalPayment paypalPayment = new PaypalPayment();
paypalPayment.processPayment(person1, person2, 100);
System.out.println("===Method paypal===");
System.out.println(person1.getName() + " has " + person1.getAmount());
System.out.println(person2.getName() + " has " + person2.getAmount());
}
}
}
现有方案的问题：
- 代码冗余： processPayment 方法在不同支付方式中重复。
- 耦合度高： 应用程序直接创建支付方式对象，扩展性差。
- 可扩展性差： 添加新的支付方式需要修改源代码。
为了解决上述问题，我们将采用工厂方法模式。
项目结构（解决方案）：
solution/ ├─ BankApp.java ├─ service/ │ ├─ payments/ │ │ ├─ Payment.java │ │ ├─ PaymentFactory.java │ │ ├─ BankTransferPayment.java │ │ ├─ PaypalTransferPayment.java ├─ data/ │ ├─ Person.java
步骤一：创建 Payment 接口，定义 processPayment 方法：
package solution.service.payments;
import solution.data.Person;
public interface Payment {
void processPayment(Person fromAccount, Person toAccount, float amount);
}
步骤二：BankTransferPayment 和 PaypalPayment 类实现 Payment 接口：
package solution.service.payments;
import solution.data.Person;
public class BankTransferPayment implements Payment {
@Override
public void processPayment(Person fromAccount, Person toAccount, float amount) {
fromAccount.withdraw(amount);
toAccount.deposit(amount);
System.out.println("Bank transfer payment success.");
}
}
package solution.service.payments;
import solution.data.Person;
public class PaypalPayment implements Payment {
@Override
public void processPayment(Person fromAccount, Person toAccount, float amount) {
fromAccount.withdraw(amount);
toAccount.deposit(amount);
System.out.println("Paypal transfer payment success.");
}
}
步骤三：创建 PaymentFactory 类，根据支付类型创建对应的支付对象：
package solution.service.payments;
public class PaymentFactory {
public Payment createPayment(String paymentType) {
if (paymentType == null) {
return null;
}
if (paymentType.equalsIgnoreCase("bank_transfer")) {
return new BankTransferPayment();
} else if (paymentType.equalsIgnoreCase("paypal")) {
return new PaypalPayment();
}
return null;
}
}
步骤四：在主程序中使用工厂：
修改主函数，使用工厂方法模式：
package solution;
import solution.data.Person;
import solution.service.payments.Payment;
import solution.service.payments.PaymentFactory;
public class BankApp {
public static void main(String[] args) {
Person person1 = new Person("John", 1000);
Person person2 = new Person("Jane", 500);
String paymentMethod = "paypal";
Payment payment = new PaymentFactory().createPayment(paymentMethod);
payment.processPayment(person1, person2, 100);
}
}
工厂方法模式的优势
- 代码更简洁： 消除了冗余代码。
- 降低耦合度： 对象创建委托给工厂，提高了可维护性。
- 增强可扩展性： 添加新的支付方式无需修改原有代码，只需创建新的支付类和注册到工厂即可。
为了进一步提高可扩展性，并遵循开闭原则，我们可以结合策略模式，实现动态注册支付方式。
修改 PaymentFactory.java：
package solution.service.payments;
import java.util.HashMap;
import java.util.Map;
public class PaymentFactory {
private Map<String, Payment> paymentMap;
public PaymentFactory() {
this.paymentMap = new HashMap<>();
}
public Payment createPayment(String paymentType) {
return paymentMap.get(paymentType);
}
public void registerPayment(String paymentType, Payment payment) {
paymentMap.put(paymentType, payment);
}
public PaymentFactory initializePaymentMethods() {
Payment bankTransferPayment = new BankTransferPayment();
Payment paypalPayment = new PaypalPayment();
this.registerPayment("bank_transfer", bankTransferPayment);
this.registerPayment("paypal", paypalPayment);
return this;
}
}
修改主程序：
package solution;
import solution.data.Person;
import solution.service.payments.Payment;
import solution.service.payments.PaymentFactory;
public class BankApp {
public static void main(String[] args) {
Person person1 = new Person("John", 1000);
Person person2 = new Person("Jane", 500);
String paymentMethod = "BANK_TRANSFER";
Payment payment = new PaymentFactory()
.initializePaymentMethods()
.createPayment(paymentMethod);
payment.processPayment(person1, person2, 100);
}
}
通过这种方式，添加新的支付方式只需创建新的类并注册到工厂即可，无需修改工厂本身的代码，完美遵循开闭原则。
希望本文对您理解和应用工厂方法模式有所帮助。
参考资料：设计模式大全
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