In service oriented architecture, Is it a good practise that a mapper calls a service?

Question

I would like to know whether it is a good practise to call/inject a (micro) service inside a custom object mapper.

This mapper transform an Entity to a DTO. In this DTO there is a field that contains a sum up of some values (need some business logic). I defined a service to centralice the logic as it is used in many other places.

Answer 1

Although there is strictly speaking nothing incorrect about calling a service from a mapper, it does add dependencies to the mapper, which is usually a pure object (Plain Old Java Object, POJO) that simply converts an object of one type to another. An alternative is to pass the required information into the mapper as a parameter. For example, suppose your current design resembles the following:

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 ...
    }
}

Instead of having the FooMapper dependent on the FooService, you can pass the FooResults in as a parameter to the convert method:

public class FooMapper {

    public FooDto convert(Foo foo, FooResults results) {    
        // ... use results ...
    }
}

The advantage to this approach is that FooMapper is no longer dependent on FooService. Instead, the client that calls the convert method must have a reference to the FooService to obtain a FooResults object.