c5a4068e84
* add state and callback pattern * add command and template-method pattern * add iterator pattern * add bridege and DI pattern * fix issue #1600 * add converter,proxy,visitor pattern * add caching,composite,delegation,dirty-flag,interpreter patterns * add dao and producer-consumer * add dto and provate class data pattern * fix #1646 png path problems * fix #1646 composite png path case problem * add abstract document pattern and version-number pattern * add ambassador pattern * add acyclic-visitor and api-gateway pattern * add abstract-factory pattern * add active-object pattern * add aggregator-microservices and arrange-act-assert pattern * update async-method-invocation pattern * add balking and business-delegate pattern * add bytecode and circuit-break pattern * update arrange/act/assert pattern problems * add csch pattern * add language code, correct pic path * #1805 update permalink Co-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com> Co-authored-by: Mike <admin@xiaod.info> Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>
143 lines
4.2 KiB
Markdown
143 lines
4.2 KiB
Markdown
---
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layout: pattern
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title: Factory
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folder: factory
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permalink: /patterns/factory/
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categories: Creational
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language: zh
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tags:
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- Gang of Four
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---
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## 也被称为
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* 简单工厂
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* 静态工厂方法
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## 含义
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在工厂类中提供一个封装的静态工厂方法，用于隐藏对象初始化细节，使客户端代码可以专注于使用，而不用关心类的初始化过程。
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## 解释
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现实例子
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>
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> 假设我们有一个需要连接到 SQL Server 的 Web 应用，但现在我们需要切换到连接 Oracle。为了不修改现有代码的情况下做到这一点，我们需要实现简单工厂模式。在这种模式下，可以通过调用一个静态方法来创建与给定数据库的连接。
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维基百科
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> 工厂类是一个用于创建其他对象的对象 -- 从形式上看，工厂方法是一个用于返回不同原型或类型的函数或方法。
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**编程示例**
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我们有一个 `Car` 接口，以及实现类 `Ford`, `Ferrari`。
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```java
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public interface Car {
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String getDescription();
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}
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public class Ford implements Car {
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static final String DESCRIPTION = "This is Ford.";
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@Override
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public String getDescription() {
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return DESCRIPTION;
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}
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}
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public class Ferrari implements Car {
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static final String DESCRIPTION = "This is Ferrari.";
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@Override
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public String getDescription() {
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return DESCRIPTION;
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}
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}
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```
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Enumeration above represents types of cars that we support (`Ford` and `Ferrari`).
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以下的枚举用于表示支持的 `Car` 类型（`Ford` 和 `Ferrari`）
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```java
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public enum CarType {
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FORD(Ford::new),
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FERRARI(Ferrari::new);
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private final Supplier<Car> constructor;
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CarType(Supplier<Car> constructor) {
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this.constructor = constructor;
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}
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public Supplier<Car> getConstructor() {
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return this.constructor;
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}
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}
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```
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接着我们实现了一个静态方法 `getCar` 用于封装工厂类 `CarsFactory` 创建 `Car` 具体对象实例的细节。
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```java
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public class CarsFactory {
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public static Car getCar(CarType type) {
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return type.getConstructor().get();
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}
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}
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```
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现在我们可以在客户端代码中通过工厂类创建不同类型的 `Car` 对象实例。
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```java
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var car1 = CarsFactory.getCar(CarType.FORD);
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var car2 = CarsFactory.getCar(CarType.FERRARI);
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LOGGER.info(car1.getDescription());
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LOGGER.info(car2.getDescription());
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```
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程序输出：
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```java
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This is Ford.
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This is Ferrari.
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```
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## 类图
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## 适用场景
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在你只关心对象的创建，但不关心如何创建、管理它的时候，请使用简单工厂模式。
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**优点**
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* 可以把对象创建代码集中在一个地方，避免在代码库存散布 "new" 关键字。
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* 可以让代码更加低耦合。它的一些主要优点包括更好的可测试性、更好的可读性、组件可替换性、可拓展性、更好的隔离性。
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**缺点**
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* 会使代码变得比原来的更加复杂一些。
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## 现实案例
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* [java.util.Calendar#getInstance()](https://docs.oracle.com/javase/8/docs/api/java/util/Calendar.html#getInstance--)
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* [java.util.ResourceBundle#getBundle()](https://docs.oracle.com/javase/8/docs/api/java/util/ResourceBundle.html#getBundle-java.lang.String-)
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* [java.text.NumberFormat#getInstance()](https://docs.oracle.com/javase/8/docs/api/java/text/NumberFormat.html#getInstance--)
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* [java.nio.charset.Charset#forName()](https://docs.oracle.com/javase/8/docs/api/java/nio/charset/Charset.html#forName-java.lang.String-)
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* [java.net.URLStreamHandlerFactory#createURLStreamHandler(String)](https://docs.oracle.com/javase/8/docs/api/java/net/URLStreamHandlerFactory.html) (Returns different singleton objects, depending on a protocol)
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* [java.util.EnumSet#of()](https://docs.oracle.com/javase/8/docs/api/java/util/EnumSet.html#of(E))
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* [javax.xml.bind.JAXBContext#createMarshaller()](https://docs.oracle.com/javase/8/docs/api/javax/xml/bind/JAXBContext.html#createMarshaller--) and other similar methods.
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## 相关模式
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* [Factory Method](https://java-design-patterns.com/patterns/factory-method/)
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* [Factory Kit](https://java-design-patterns.com/patterns/factory-kit/)
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* [Abstract Factory](https://java-design-patterns.com/patterns/abstract-factory/)
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