在面向服务的体系结构中，映射器调用服务是一种好的做法吗？

Question

我想知道在自定义对象映射器中调用/注入（微）服务是否是一种好习惯。

此映射器将实体转换为 DTO。在这个 DTO 中有一个字段包含一些值的总和（需要一些业务逻辑）。我定义了一个服务来集中逻辑，因为它在许多其他地方使用。

Answer 1

虽然严格来说从映射器调用服务并没有什么不对，但它确实给映射器添加了依赖，映射器通常是一个纯对象（Plain Old Java Object，POJO），只是简单地将一种类型的对象转换为另一种类型。另一种方法是将所需信息作为参数传递到映射器中。例如，假设您当前的设计类似于以下内容：public class Foo { /* ... */}public class FooDto { /* ... */}public class FooService {    public FooResults getSomeResults() { /* ... */ }}public class FooMapper {    private final FooService service;    public FooMapper(FooService service) {        this.service = service;    }    public FooDto convert(Foo foo) {         FooResults results = service.getSomeResults();        // ... use results ...    }}您可以将in 作为参数传递给方法，而不是依赖FooMapper于：FooServiceFooResultsconvertpublic class FooMapper {    public FooDto convert(Foo foo, FooResults results) {            // ... use results ...    }}这种方法的优点是FooMapper不再依赖于FooService. 相反，调用该方法的客户端convert必须具有对 的引用才能FooService获取FooResults对象。

Answer 2

映射器将一种数据结构转换为另一种数据结构。因此它应该只依赖于这两个数据结构之一。&nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +----------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp; &nbsp;&nbsp;| data structure 1 |&nbsp; &nbsp;<-----&nbsp; &nbsp; &nbsp;|&nbsp; mapper&nbsp; |&nbsp; ----->&nbsp; &nbsp;| data structure 2 |&nbsp;&nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +----------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp;&nbsp;如果映射器使用服务，它也依赖于该服务。这意味着它知道数据是如何获得的。我猜你的映射器依赖于具体的服务。因此，映射器将受到其中一个数据结构的更改和该服务的更改的影响。&nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +----------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp; &nbsp;&nbsp;| data structure 1 |&nbsp; &nbsp;<-----&nbsp; &nbsp; &nbsp;|&nbsp; mapper&nbsp; |&nbsp; ----->&nbsp; &nbsp;| data structure 2 |&nbsp;&nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +----------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; V&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+----------+&nbsp; &nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; service |&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+----------+&nbsp;可能很难模拟该服务以进行测试。也许该服务依赖于某种存储库或者它自己执行 SQL。通过引入一个从数据获取方式中抽象出来的接口，使映射器独立于数据结构的来源可能会更好。然后您的服务可以实现该接口或创建一个简单的适配器。&nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +----------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp; &nbsp;&nbsp;| data structure 1 |&nbsp; &nbsp;<-----&nbsp; &nbsp; &nbsp;|&nbsp; mapper&nbsp; |&nbsp; ----->&nbsp; &nbsp;| data structure 2 |&nbsp;&nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +----------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; V&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +--------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp;Data1Provider&nbsp; | <------&nbsp; |&nbsp; &nbsp;service&nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+------------------+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +--------------+使用这种方法，您可以通过引入如何获取数据的稳定抽象来反转对服务的依赖性。这就是所谓的依赖倒置。它可能是一个看起来像这样的简单界面&nbsp;public interface Data1Provider {&nbsp; &nbsp; &nbsp; public FooResults getResults();&nbsp;}这个接口可以很容易地模拟测试。另一个好方法是在调用映射器之前简单地调用服务。只需将服务返回的数据传递给映射器即可。我不会在这里详细介绍，因为Justin Albano 的回答已经表明了这一点。