102 lines
2.8 KiB
Markdown
102 lines
2.8 KiB
Markdown
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---
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layout: pattern
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title: Converter
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folder: converter
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permalink: /patterns/converter/
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categories: Creational
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tags:
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- Decoupling
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---
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## 目的
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转换器模式的目的是提供相应类型之间双向转换的通用方法，允许进行干净的实现，而类型之间无需相互了解。此外，Converter模式引入了双向集合映射，从而将样板代码减少到最少。
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## 解释
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真实世界例子
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> 在真实的应用中经常有这种情况，数据库层包含需要被转换成业务逻辑层DTO来使用的实体。对于潜在的大量类进行类似的映射，我们需要一种通用的方法来实现这一点。
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通俗的说
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> 转换器模式让一个类的实例映射成另一个类的实例变得简单
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**程序示例**
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我们需要一个通用的方案来解决映射问题。让我们来介绍一个通用的转换器。
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```java
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public class Converter<T, U> {
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private final Function<T, U> fromDto;
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private final Function<U, T> fromEntity;
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public Converter(final Function<T, U> fromDto, final Function<U, T> fromEntity) {
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this.fromDto = fromDto;
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this.fromEntity = fromEntity;
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}
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public final U convertFromDto(final T dto) {
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return fromDto.apply(dto);
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}
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public final T convertFromEntity(final U entity) {
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return fromEntity.apply(entity);
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}
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public final List<U> createFromDtos(final Collection<T> dtos) {
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return dtos.stream().map(this::convertFromDto).collect(Collectors.toList());
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}
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public final List<T> createFromEntities(final Collection<U> entities) {
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return entities.stream().map(this::convertFromEntity).collect(Collectors.toList());
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}
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}
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```
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专属的转换器像下面一样从基类继承。
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```java
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public class UserConverter extends Converter<UserDto, User> {
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public UserConverter() {
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super(UserConverter::convertToEntity, UserConverter::convertToDto);
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}
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private static UserDto convertToDto(User user) {
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return new UserDto(user.getFirstName(), user.getLastName(), user.isActive(), user.getUserId());
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}
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private static User convertToEntity(UserDto dto) {
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return new User(dto.getFirstName(), dto.getLastName(), dto.isActive(), dto.getEmail());
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}
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}
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```
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现在，在User和UserDto之间的映射变得轻而易举。
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```java
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var userConverter = new UserConverter();
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var dtoUser = new UserDto("John", "Doe", true, "whatever[at]wherever.com");
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var user = userConverter.convertFromDto(dtoUser);
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```
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## 类图
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## 适用性
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在下面这些情况下使用转换器模式：
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* 如果你的类型在逻辑上相互对应，并需要在它们之间转换实体
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* 当你想根据上下文提供不同的类型转换方式时
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* 每当你引入DTO（数据传输对象）时你可能都需要将其转换为
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DO
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## 鸣谢
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* [Converter](http://www.xsolve.pl/blog/converter-pattern-in-java-8/)
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